Construction Technology: An Illustrated Introduction · Construction Technology an illustrated...

ConstructionTechnologyan illustrated introduction

Eric Fleming

This primer on construction technology is aimed at peoplewho have no background in construction or who havejust joined the industry from college. It gives acomprehensive introduction to every aspect of low-riseconstruction and will be indispensable on all courseswhere construction technology is taught.The authordemonstrates good practice and common faults usingdetailed drawings paired with clear photos and briefexplanations of all the elements of the building process.

The book follows the sequence of construction:

and illustrates the most common technical problems thatyou will meet on site. A set of simple solutions is givenfor each.These are sound, tried-and-tested solutions; theywill meet the current Building Regulations across the UK.

Site visits have become rare (due to Health & Safety andinsurance considerations) and newcomers to the industryface the difficulty of interpreting a 2-D representation ofvery 3-D realities.The book addresses this problem ofvisualisation of construction scenarios and helps studentsrelate the detail drawings to the actual construction bysupporting as many of the drawings as possible withphotographs of the pieces of work – both in a part-finished state and as the completed work.

● foundations● walls● floors● doors and windows

● stairs● roofs● plumbing● electrical

The front cover illustrates a solid blockwork wall and brick/blockcavity wall as a substructure for a timber frame panel construction.

Eric Fleming is former lecturer inconstruction economics and buildingconstruction at Heriot-Watt University

If you’ve never set foot on abuilding site but need to knowwhat works and what doesn’t,this book is for you!

ConstructionTechnologyan illustrated introduction

Cover design by Garth Stewart

Construction Technology an illustrated introduction•

Eric Fleming

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Construction Technology

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Construction Technology:an illustrated introductionEric FlemingFormer LecturerConstruction Economics and Building ConstructionDepartment of Building Engineering and SurveyingHeriot-Watt University

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c© 2005 by Blackwell Publishing Ltd

Editorial offices:Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK

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First published 2005 by Blackwell Publishing Ltd

Library of Congress Cataloging-in-Publication Data

Fleming, Eric.Construction technology / Eric Fleming.

p. cm.Includes index.ISBN 1-4051-0210-1 (pbk. : alk. paper)1. Building. I. Title.

TH146 .F58 2004690–dc22 2004008229

ISBN 1-4051-0210-1

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For further information on Blackwell Publishing, visit our website:www.thatconstructionsite.com

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Contents

Introduction xiAcknowledgements and Dedication xiiiAbbreviations xiv

1 Masonry Construction in Bricks and Blocks 1

Bricks and blocks standards and dimensions 2Bricks 2Terminology 2Brick sizes 2

Nominal sizing 3Durability of bricks 3Mortar joints 3Coordinating sizes 3Types of brick by shape 4Kinds of brick by function 4Brick materials 5Testing of bricks 5The bonding of bricks to form walls 5Convention on thicknesses of walls 8Types of bond 9

Vertical alignment 14Honeycomb brickwork 16Quoins – an alternative definition 16Half brick thick walls 16Frog up or frog down 17

‘Tipping’ 17Common and facing brickwork 18Facing brickwork 18Pointing and jointing 19General principles of bonding 21Blocks 22Block materials 22Concrete blocks 22

Dense and lightweight concretes 23Autoclaved aerated concrete 23

Dimensions of standard metric block 23Whys and wherefores of mortar 25

Cement 25Lime 26Sand 27Water 27

Which mortar mix? 27‘Fat’ mixes 28General rules for selection of mortar 29

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vi Contents

Mortar additives 30Mixing in additives 30Mixing mortar 31

Good or bad weather 32

2 Substructures 34

Excavation generally 34Topsoil 35Subsoils 36

General categorisation of subsoils and their loadbearing capacities 37Foundations 37

The principal considerations 38Simple foundation calculations 39The mass of buildings 39Mass, load and bearing capacity 40Foundation width and thickness 41Reinforced concrete foundations 44Failure of wide, thin, strip foundations 44Trench fill foundations 45Critical levels and depths 46Level 46Finished ground level 47Bearing strata 48Depths and levels 48Step in foundation 49

Setting out 49The site plan 49Where do we put the building? 49Equipment required for basic setting out 49Setting out procedure 50

Excavation 53Marking out the excavation 53Excavation for and placing concrete foundations – and not

wasting money doing it 53Building masonry walls from foundation up to DPC level 57Ground floor construction 59

Detail drawings 59Wall–floor interfaces generally 62

Precautions 62Solid concrete floors 62

Single and double layer concrete floors with hollow masonry wall 62Hung floors 64

Hung timber floors 64Hung timber floor alternatives 66Hung concrete floors 67

Blockwork substructure 71

3 Walls and Partitions 73

General 73Requirements 74

Walls – environmental control 75Heat loss and thermal capacity 75

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Resistance to weather – precipitation 75Air infiltration 77Noise control 79Fire 79

Dimensional stability 79Walls of brick and blockwork 81Insulation of external walls 84Timber frame construction 88

Traditional timber frame 88Modern timber frame construction 91

Loadbearing and non-loadbearing internal partitions 96Expansion joints 99

4 Timber Upper Floors 103

Upper floor joists 103Linear and point loadings on upper floors 112Openings in upper floors 113

For pipes 113For flues 114For stairs 116

Alternative materials for joisting 118Sound proofing 120

Modern sound and fire proofing 121Support of masonry walls 123Floor finishes 124Ceiling finishes 124

5 Openings in Masonry Walls 126

For small pipes and cables 126For larger pipes and ventilators 127Large openings in masonry walls 127Alternative sill arrangements 136Threshold arrangements 137Partitions of masonry 139Openings in timber frame walls 141

6 Roof Structure 148

Roof classifications 148Prefabrication 149Trussed roofs 150

The trussed rafter 153Verges meet eaves 159Roof bracing 160Flat roofs in timber 162

Insulation, vapour control layers and voids and ventilation 164Traditional roofs 167Roof insulation 169

7 Roof Coverings 171

Tile and slate materials 171Slates 175

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Plain tiles 175Interlocking tiles 176Timber shingles 176Bituminous shingles 176Pantiles 177Spanish and Roman tiles 177Edges and abutments 178

8 Doors 182

Functions of doors and windows – obvious and not so obvious 182Types of door 184

Ledged and braced doors 185Bound lining doors 185Flush panel doors 187Panelled doors 188Pressed panel doors 18915 pane doors 190

Hanging a door 190Fire resistant doors 193

Smoke seals 195Glazing 196

Ironmongery 196

9 Windows 204

Timber casement windows 205Depth and height of glazing rebates 206Timber for casement windows 206Draught stripping materials 206Hanging the casements 207Joining the frame and casement members 209

Timber sash and case windows 211The case 212The sashes and case together 214

Vertical sliding sash windows 214Glazing 218

For ordinary glazing work 218

10 Stairs 221

Landings 222Steps 222Balustrades 223Measurements 224Joining steps to stringer 225Winders 227

11 Mutual Walls 228

Transmission of sound 228Calculation of surface density 228Wall types 229Fire resistance 231

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Contents ix

12 Plumbing and Heating 233

Pipework 233Pipe fittings – couplings and connections 234

Range of fittings 239Valves and cocks 241Services generally 243Hot and cold water services 243Soil and ventilation stacks 246Overflows 246Water supply from the main 246Equipment 247

Cold water storage cisterns 248Hot water storage cylinders 248Feed and expansion tanks 251

Central heating 252Piping for central heating systems 253Emitters 255

Appliances 255Waste disposal piping and systems 259Insulation 262Corrosion 263Air locking and water hammer 263First fixings 264

13 Electrical Work 266

Power generation 266Wiring installation types 267Sub-mains and consumer control units 268Sub-circuits 270Work stages 272

Electrician’s roughing 272Earth bonding 273Final fix 275

Testing and certification 275More on protective devices 275Wiring diagrams 276Accessories 277

Appendices:

A Maps and Plans 279

B Levelling Using the Dumpy Level 285

C Timber, Stress Grading, Jointing, Floor Boarding 291

D Plain and Reinforced In-situ Concrete 316

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E Mortar and Fine Concrete Screeds laid over Concrete Sub-floors or Structures 322

F Shoring, Strutting and Waling 325

G Nails, Screws, Bolts and Proprietary Fixings 328

H Gypsum Wall Board 341

I DPCs, DPMs, Ventilation of Ground Floor Voids, Weeps 344

J Drawing Symbols and Conventions 353

K Conservation of Energy 355

L Short Precis of Selected British Standards 356

Index 380

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Introduction

One of the many reasons for writing this bookwas the need to introduce students to a levelof detail which they would gain only withpractical experience on site or in workshops.The accusation that the text includes too much‘trade’ material could be levelled, but bearingin mind that many of the students who mightuse this text will be potential builders, quan-tity surveyors and building surveyors, thenthe inclusion of the trade material is very nec-essary. One of the primary functions of cer-tainly the builders and quantity surveyors isthe need to be able to assess the cost of anybuilding operation. Unless they understandthe processes to be gone through it is impossi-ble for these professionals to give an accuratecost. They don’t have to be able to physicallydo the work but they must know exactly whatis involved. So this text is for the ‘early learner’who has no background in the construction in-dustry. It is not intended to be an all embrac-ing text; the physical size of the book couldnot allow that. So the author has been quiteselective in what has been included, the rea-soning behind the selection being the need tointroduce the early learner to sufficient infor-mation to allow a general appreciation of themore common techniques used in domesticconstruction today.

Emphasis has been given to technical termsand terminology by having them printed inbold on at least the first occasion they are used.Where these terms are generally confined toone part of the UK, some alternative formsare given as well. References to Building Reg-ulations should be understood to mean all theRegulations which are used in England, Walesand Scotland at the time of writing. Referencesto particular Regulations will have the suffixes(England and Wales) or (Scotland) appended.Where the reference is to earlier editions of

any particular Regulations, the date will begiven, e.g. (1981).

A word about the drawings scatteredthrough the text. None is to scale although, inthe majority of instances, all component partsand components shown in any one draw-ing are in the correct proportion, with theexception of thin layers or membranes suchas damp proof courses, felts, etc. which areexaggerated in thickness, following the con-vention in architectural drawing practice. Ap-pendix J shows some of the conventional sym-bols used. The reader should get to knowthese; they are common currency when drawninformation has to be read.

For the student who has recently left schoolthere may be confusion, for the teaching ofthe use of centimetres in schools does notmatch up with the agreement by the construc-tion industry to use only SI (Systeme Interna-tional) units where only the millimetre, metreand kilometre are used to measure length. Onarchitectural drawings dimensions are givenonly in millimetres and levels in metres totwo places of decimals. Students will be ex-pected to produce drawings in this mannerduring their courses. Following the conven-tion on drawings etc., no mention of the unitof measurement will be made in the text whenthese are in millimetres. Any dimension givensimply as a number must be assumed to bein millimetres. Any other measurements willhave the unit of measurement following thenumber, e.g. 14.30 m meaning metres; 10 600kN meaning kilonewtons and so on.

There are already hundreds of books onbuilding construction or on just one aspectof it, be it a trade, material or technique(s).There must be many more technical papersand leaflets and books produced by variousorganisations with an interest in the industry.

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xii Introduction

They include the Building Research Establish-ment (BRE), Construction Industry Researchand Information Association (CIRIA), Tim-ber Research and Development Association(TRADA), the British Standards Institution,all the trade and manufacturing associations –the list is endless, but those mentioned arereckoned to be the experts. So why has this au-thor chosen not to quote them at every oppor-tunity? Well, I have quoted bits of the BritishStandards where they were appropriate, butso much of the rest of the material is on ahigher plane as to confuse the early learner inthe art of construction. There is enough in hereto get someone started on domestic construc-tion as it is today. Get that correct and thengo on to read the more esoteric material, espe-cially when so much is about what has gonewrong in the past and how it was put right.

A couple of areas which are sorely neglectedby too many students are:

� Manufacturer’s literature – now widelyavailable on the Internet

� Using their own eyes.

On the first point above, there was a timenot so long ago when manufacturers tendedto have their literature about a product pre-pared by graphic artists who knew diddley-squat about building and so perpetrated somereal howlers and horrors, so much so thatmany lecturers had to tell students to ignorethat source of information until they couldsort out the good from the ugly. There werenotable exceptions and many will rememberthe competition to get hands on a copy ofBritish Gypsum’s White Book or the receptiongiven to Redland’s award winning catalogueon roofing materials – goodness, was it thatlong ago? Nowadays catalogues have to beconsidered as a serious source of informationand they come out faster than any other formof information and so become almost the onlyway to keep up to date.

On the second point above, what better wayto see how a wash hand basin is installed thanto get underneath it with a good torch andhave a good look. Look into the attic withthat torch, probe into all the corners and see

how the roof is put together. Look at the doorsand windows and how they interface with thewalls and floors and the ceilings.

Experience in teaching the subject to schoolleavers has brought one difficulty to the forewhich many students have – the inability to vi-sualise. Test this for yourself – describe some-thing to a friend and ask them to draw it asyou speak. I’m sure you’ll get some funnyresults and some funny comments. It is adaunting task to be faced with technical con-struction drawings, especially detail draw-ings, and be expected to ‘see’ what is goingon in terms of bricks, concrete in holes in theground, joists and plasterboard, especially asyou don’t know what these are in their rawstate. Hence the inclusion in this book of pho-tographs of bits and pieces and of construc-tion. Fewer and fewer students get the oppor-tunity to see a building site, mainly due to thesafety aspects of a site visit and ever increas-ing insurance premiums. And yet seeing forthemselves is what so many desperately re-quire.

When starting this book a year or two back,the idea was to include a detail drawing along-side a photograph of what it looked like onsite, hoping that this would in some small waymake up for lack of on-site experience. Whilethere are a lot of photographs, the result is notas good as had been hoped. The author couldeasily spend another year just getting the pho-tography up to scratch and would certainly dothings differently. For this text I was unable tofind herringbone strutting anywhere close tome so I made a mock-up of a pair of joistsand put in timber and steel strutting. It makesthe point adequately when viewed alongsidethe details. So many other photographs couldhave been of that type had I realised the valueof mock-ups earlier.

If you think the book lacks something or hastoo much of one thing, or is a bit of a curate’segg or whatever, please write to me care of thepublishers. If there is ever another edition itwould be good – indeed vital – to have con-structive feedback.

Eric Fleming

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Acknowledgements and Dedication

I must formally thank Mitek Industries of Dudley and Eaton MEM of Oldham for giving mepermission to reproduce images and providing the images on disc to include in this book. AlsoSimpson Strong Tie and their branch at Stepps near Glasgow who very kindly supplied me withsamples. I must thank John Fleming & Co Ltd, timber and builders merchants of Elgin, KeithBuilders Merchants and Mackenzie and Cruickshank, hardware retailers, both of Forres, whoall allowed me to take photographs of materials, components and ironmongery.

I must also thank the many family, colleagues and friends from the half century I spent in theconstruction industry who have contributed to the information on which I have drawn so freelyin writing this text.

Finally, I would like to dedicate this book to Myra, who has given me great encouragement withthe writing and who has not complained when the book came between me and the renovationwork we are attempting on our battered Georgian home.

F. W. ‘Eric’ Fleming FRICSForresScotlandMarch 2004

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Abbreviations

aac autoclaved aerated concreteABS acrylonitrile butadiene styreneach air changes per hourbj black japannedBM benchmarkBMA bronze metal antiqueBOE brick on edgeBRE Building Research EstablishmentBS British StandardBSI British Standards InstitutionCAAD computer aided architectural designCAD computer aided designCCU consumer’s control unitCH central heatingCIRIA Construction Industry Research and

Information Associationcs centrescsk countersunkCW cold waterDLO direct labour organisationDPC damp proof courseDPM damp proof membraneELCB earth leakage circuit breakerEPDM electronic position and

distance measurementEVA ethyl vinyl acetateFFL finished floor levelFGL finished ground levelFS full sheetgalv. (hot dipped) galvanisedHBC high breaking capacityH&C hot and coldHRC high rupturing capacityHW hot waterIEE Institution of Electrical EngineersLH left-handLPG liquefied petroleum gas

MC moisture contentMCB miniature circuit breakerMR moisture resistantm.s. mild steelm&t mortice and tenonOPC ordinary Portland cementOS Ordnance SurveyOSB oriented strand boardPCC pre-cast concretePFA pulverised fuel ashPS pressed steelPTFE polytetrafluorethylenePVA polyvinyl acetateRCCB residual current circuit breakerRH right-handrh round headRSJ rolled steel joistRWP rainwater pipeSAA satin anodised aluminiumSLC safe loadbearing capacitySS stainless steelSSHA Scottish Special

Housing AssociationSVP soil and ventilation pipeS/w softwoodSWVP soil, waste and ventilation pipet&g tongue and grooveTC tungsten carbideTRADA Timber Research and

Development AssociationTRV thermostatic radiator valveUB universal beamUV ultravioletVCL vapour control layerWHB wash-hand basinWBP water and boil proofzp zinc plated

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1 Masonry Constructionin Bricks and Blocks

Bricks and blocks standards and dimensions 2Bricks 2Terminology 2Brick sizes 2

Nominal sizing 3Durability of bricks 3Mortar joints 3Coordinating sizes 3Types of brick by shape 4Kinds of brick by function 4Brick materials 5Testing of bricks 5The bonding of bricks to form walls 5Convention on thicknesses of walls 8Types of bond 9

Vertical alignment 14Honeycomb brickwork 16Quoins – an alternative definition 16Half brick thick walls 16Frog up or frog down 17

‘Tipping’ 17Common and facing brickwork 18

Facing brickwork 18Pointing and jointing 19General principles of bonding 21Blocks 22Block materials 22Concrete blocks 22

Dense and lightweight concretes 23Autoclaved aerated concrete 23

Dimensions of standard metric block 23Whys and wherefores of mortar 25

Cement 25Lime 26Sand 27Water 27

Which mortar mix? 27‘Fat’ mixes 28General rules for selection of mortar 29

Mortar additives 30Mixing in additives 30Mixing mortar 31

Good or bad weather 32

Why should we be starting a book on buildingconstruction with a discussion of bricks andblocks? Quite simply because bricks are one ofthe major construction materials instantly as-sociated with construction in the mind of thenovice or lay person, but more importantlybecause the sizes chosen for the manufactureof bricks and blocks affect practically every-thing in a building except the thickness of thecoats of paint or the coats of plaster. This willbe discussed in more detail as we proceed.

Bricks and blocks are entirely ‘man-made’masonry units. A variety of materials arequarried, mined or salvaged from manufac-turing processes and made into bricks orblocks.

Stone is quarried and shaped but occursnaturally and was often used as it was foundbelow cliffs or outcrops or on beaches, or fromthe general stones on or in the ground.

Artificial stone and reconstructed stoneare ‘man-made’. Artificial stone is made bymixing particles of stone with a cement binder,water and occasionally a colouring materialand then casting it into shapes. The idea is tocreate a ‘look’ of a particular kind of stone,even though none of that stone is used in theproduction. Reconstructed stone follows thesame idea but generally omits the colouringagents since the stone particles used are thestone which is required at the end of the cast-ing process. This is sometimes cheaper than

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2 Construction Technology

Fig. 1.1(a) Solid and perforated bricks.

Fig. 1.1(b) Single shallow frogged bricks.

the original stone and can sometimes be theonly way to produce any quantity of some-thing closely resembling the original stonewhere quarries are run down or closed.

We will consider only bricks and blocks.In Figure 1.1(a) there are two solid bricks onthe left and two perforated bricks on the right.In Figure 1.1(b) there are two single shallowfrogged bricks. There is obviously need for ex-planation so we will start by looking at mater-ials, sizes and shapes and so on:

� Bricks and blocks can be made from avariety of materials other than fired clayor brick earth, e.g. calcium silicate and con-cretes.

� Bricks and blocks can be obtained in a vari-ety of sizes and types and kinds.

� Bricks and blocks can be made in a varietyof shapes other than the standard rectilin-ear shape discussed in this text but specialshapes are the subject of British Standard4729, Dimensions of bricks of special shapes andsizes.

� Bricks and blocks can be cut into differentshapes and these we will discuss later in thechapter.

Bricks and blocks standardsand dimensions

� Bricks and blocks of fired clay are the subjectof British Standard BS 3921 (see precis inAppendix L).

� Brick is defined as a unit having all dimen-sions less than 337.5 × 225 × 112.5.

� Block is defined as a unit having one or moredimensions greater than those of the largestpossible brick.

BRICKS

Terminology

The surfaces of a brick have names:

� Top and bottom surfaces are beds� Ends are headers or header faces� Sides are stretchers or stretcher faces.

Bricks and blocks are made using mortar; theyare not made in cement. Cement, usually a drypowder, may or may not be an ingredient ina mortar depending on the type of wall, itssituation, etc. Mortars are mixed with waterinto a plastic mass just stiff enough to supportany masonry unit pressed into them. This isan important and fundamental issue which isdiscussed in detail a little later in the chapter.

Brick sizes

Bricks are made in many sizes; however, wewill use only one size in this text – the standardmetric brick. A standard metric brick hascoordinating dimensions of 225 × 112.5 ×75 mm and working dimensions of 215 ×102.5 × 65.

Why two sizes? The coordinating dimen-sions are a measure of the physical spacetaken up by a brick together with the mortar



Masonry Construction in Bricks and Blocks 3

required on one bed, one header face and onestretcher face. The working dimensions arethe sizes to which manufacturers will try tomake the bricks. Methods of manufacture formany units and components are such that thefinal piece is not quite the size expected but itcan fall within defined limits. This can be dueto things like shrinkage or distortion whendrying out, firing, etc.

The difference between the working and co-ordinating dimensions of a brick is 10 mmand this difference is taken up with the layerof mortar into which the bricks are pressedwhen laying. The working dimensions arealso known as the nominal size of a brick.

Nominal sizing

The term nominal sizing is used to describe asize which is subject to slight variation duringthe manufacture of a component or unit. Thevariation – larger or smaller – allowed is gen-erally given in a British Standard. The differ-ences – plus and/or minus – can be different.

The slight variation in size of individualbricks is allowed for by pressing the brick intothe mortar layer a greater or lesser amount butalways using up to the coordinating dimen-sion or space of 225 × 112.5 × 75.

Durability of bricks

Durability of bricks is very important whenbuilding in situations where freezing wouldbe a problem and where the soluble salt con-tent of the bricks would cause problems withthe mortar – see sulphate attack later in thetext. British Standard 3921 gives classifica-tions of durability in terms of frost resistanceand salt content, and an extract from the precisof BS 3921 given in Appendix L of the book isgiven here:

Durability of brickwork depends on two fac-tors which arise from the use of any partic-ular brick: resistance to frost and the solu-ble salts content. Frost resistance falls intothree classes: Frost resistant (F), Moderately

Frost Resistant (M) and Not Frost Resistant(O). Soluble salts content is classed as eitherLow (L) or Normal (N). So, one could havea brick which is frost resistant with normalsoluble salt content and this would be clas-sified as FN. Similarly a brick which hadno frost resistance and had low soluble saltcontent would be classed as OL.

Mortar joints

Mortar placed horizontally below or on top ofa brick is called a bed. Mortar placed verticallybetween bricks is called a perpend.

Coordinating sizes

� The coordinating sizes allow the bricks to bebuilt together in a number of different ways,illustrated in Figure 1.2. It is important tobuild brickwork to the correct coordinatingsize for the particular working size of brickspecified.

Other components such as cills1, lintels2,door and window frames, etc. are manufac-tured to fit into openings whose size is cal-culated on the basis of whole or cut bricksdisplaced. This is illustrated in Figure 1.3.If non-metric sizes of brick are to be usedthen the components built into the brickworkshould coordinate with that size.

� The height of the lintel is shown as threecourses plus the joints between them mea-suring 3 × 65 + 2 × 10 = 215 mm.

� The width of the window opening must bea multiple of half a brick plus the perpends,e.g. 8 × 102.5 + 9 × 10 = 880 mm.

� The length of the lintel has to be the width ofthe opening plus the pieces which are builtinto the wall – the rests.

1 Cill: Alternative spelling, sill, is a unit or constructionat the bottom of a window opening in a wall designedto deflect water running off a window away from theface of the wall below.

2 Lintel: A unit or construction over an opening in a walldesigned to carry the loadings of the wall over the open-ing.




Bed

Header orheader face

Stretcher or

stretcher

face

65

102.5215

Dimensions shown areworking dimensionswith a 10 mortar bed

21565

10

65

102.510102.5

102.5

656565 10 10

10

65215

Headerface

Headerface

Stretcher face

Stretcher face

Header face

Header face

Header face

Both coursesbuilt on bed

Course builton edge

Course builton bed

Fig. 1.2 Coordinating and working dimensions of thestandard metric brick.

� Wall rests vary according to the load butassume in this case they are half the lengthof a brick each, less the mortar required inthe perpends between the lintel ends andthe adjacent brickwork.

� The length of the lintel is therefore 880 +2 × 102.5 = 1085

Lintel

Windowopening

Wall rest

Brick courses

Fig. 1.3 A lintel over a window opening in a brickwall.

Types of brick by shape

Bricks can just be rectilinear pieces of mater-ial, and these are described as solid, but theymight instead have a depression in one or bothbeds called a frog. Frogs can be quite shallowor quite deep but they will not exceed 20% ofthe volume in total.

Instead of being solid or having a frog(s), abrick might be:

� Cellular – having cavities or depressions ex-ceeding 20% of the volume in total, or

� Perforated – holes not exceeding 20% of thevolume in total; minimum 30% solid acrosswidth of brick.

All of these types by shape are illustrated inFigure 1.4.

Kinds of brick by function

Bricks can be manufactured to fulfil differ-ent functions, i.e. strength, resistance to water

Brick with shallowdouble frogs--long section

Brick with a deep frog--long section

Cellular brick--long section

Cellular brick--view ofupper bed

Perforated brick--longsection

Perforated brick--viewof either bed

Cross-section

Cross-section

Fig. 1.4 Types of brick by shape.




Table 1.1 Brick types by end use – compressive strength and water absorption.

Type by end use Compressive strength N/mm2 Water absorption %

Engineering A ≤ 70 ≤ 4.5Engineering B ≤ 50 ≤ 7.0Damp proof course 1 ≤ 5 ≤ 4.5Damp proof course 2 ≤ 5 ≤ 7.0All other ≤ 5 No limits

absorption, decoration or for no particularfunction other than to build a wall and be cov-ered over with plaster or render.

The British Standard recognises five typesclassified by function or end use, as shown inTable 1.1.

The vast majority of bricks used are of the‘All other’ category, the compressive strengthbeing perfectly adequate for all but the mostsevere loadings. The ‘No limits’ category forwater absorption must of course be temperedwith any requirement to resist weather pene-tration of a wall. It would be foolish to buildan external wall of facing brick if these brickswere very absorbent. On the other hand a wa-ter absorption less than required by dampproof course (DPC)3 or engineering brickswould be an unnecessary expense and mighteven be counter productive in the long term.We will look at the effect of water absorptionon wall faces in Chapter 3.

Brick materials

As well as fired clay or brick earth, bricksare also manufactured from calcium silicate(BS 187 and 6649) and concrete (BS 6073). Stan-dard metric-sized bricks are manufactured inboth kinds. Shapes of concrete bricks differfrom those in clay and calcium silicate. Differ-ent strengths apply to all kinds of bricks. Inparts of the UK a naturally occurring mixture

3 The damp proof course is most commonly known bythe initials DPC. It is a layer impervious to water builtinto walls and floors to prevent moisture in the groundrising into the structure of a building. It will be dis-cussed in greater detail in Chapter 2 and Appendix I.

of clay and coal has been quarried or mined.This clay can be formed into bricks and firedin traditional brick kilns but uses less fuel be-cause of the entrained coal particles. In Scot-land the resulting bricks are known as com-position bricks. They display a rather burntlook on the outer faces and when cut the core isfrequently quite black. They are only used forcommon brickwork (see later is this chapter).

Testing of bricks

All brick kinds and types are subject to test inorder to comply with the appropriate BritishStandard. Tests include dimensions, solublesalt content, efflorescence, compressivestrength and water absorption. Please refer tothe precis of BS 3921 in Appendix L where thetests are listed but not discussed in detail.The reasons for these tests will be discussedin Chapter 3.

The bonding of bricks to form walls

Bonding of bricks refers to the practice of lay-ing the bricks in layers or courses and in anyof a number of patterns or bonds to form awall of a homogeneous construction, i.e. theindividual bricks overlap each other in adja-cent layers, the pattern alternating in adjacentlayers or after a number of similar layers. Thepatterns in these layers are formed with wholeand cut bricks as well as with bricks manufac-tured to a ‘special shape’ other than the stan-dard rectilinear one.

We must first examine why we need to cutbricks in specific ways. The most simple cut




215one brick

102.5half brick

65

102.5half brick

102.5half brick

A half batt The

cut face is shownhatched.

It should be noted that making a half

batt is not a question of cutting cleanly

through the centre of a whole brick length

but of cutting through at a point approx.

5 mm short of the centre so that the piece required

is 102.5 mm long.In practice this can

only be achieved every time by using a saw to make the cut. In many instances cutting with a chisel or a brick hammer

results in one half batt and a pile of broken brick.

Whole brick

65

Fig. 1.5 Whole brick and half batt dimensions.

is the half brick, which can be described ascutting a brick along a plane vertical to its bed,along the centre of its short axis. Figure 1.5shows the dimensions of a whole brick andbelow it the dimensions of a brick cut in half –a half batt. Note that the half batt is not a truehalf of a brick length as there must always bean allowance made for the thickness of themortar used in building, 10 mm.

This most simple of cuts makes the buildingof walls with straight, vertical ends possible.Figure 1.6 shows this quite clearly and moresimply than words can describe. The wall isbuilt in stretcher bond, i.e. the bricks are laidwith their long axis along the length of thewall and the bricks in adjacent courses overlapeach other by half a brick. The views shownare, (a) without using cut bricks and (b) usingcut bricks. Not being able to use cut bricks

(a) Face view of the end of a wall where cutting of bricks is

not available.The wall end has ajagged appearance

(b) Face view of the end of a wall where cuts are available.

The jagged edge has been filled with half brick pieces -- shown

hatched.

The walls are built in stretcher or common bond, i.e. all the main bricks show a stretcher face and overlap

by half a brick length

Fig. 1.6 Why bricks are cut.

means that the end of the wall takes on a saw-toothed appearance, which is not the case ifhalf brick cuts are available.

Another bond which is commonly used isEnglish bond, more complex than stretcherbond and only used where the wall thicknessis 215 mm thick – one brick or over. If welook at Figure 1.7, a drawing of two adjacentcourses of this bond in a particular situation,we can see how another of the standard cutscan be put to good effect in maintaining thebonding of the bricks as well as allowing ver-tical ends to the walls. When we start to lookat walls of one brick thick and upwards, a fur-ther development of bonding comes into play.Every bond in this category displays a group-ing of bricks which repeats across a course. Insome instances the pattern repeats across ev-ery course, in others adjacent courses displaya mirror of that pattern. In one-brick walls inEnglish bond the pattern is of two bricks, sideby side and which are turned through 180◦.This is called sectional bond and is shownhatched in all the figures which follow.




qh

qh -- quoin header

qh

qh

qh

qh

Sectional bond shownhatched

Queen closer shown cross

hatched

Fig. 1.7 The use of a closer in a one brick thick wallbuilt in English bond.

The drawing shows the two adjacentcourses at the end of a wall as well as the faceview of the wall over a number of courses. Thecut illustrated is called a queen closer and isformed by cutting a brick in a plane vertical tothe bed and along the centre line of the longaxis of the brick. Note that there is a wholebrick in alternate courses at the end of the wallnext to the queen closer. This brick is referredto as the quoin header. We will explain theterminology properly as we look at particularbonds and bonding in more detail.

Having established the need for cut bricks,let us look at the first of the standard cuts takenfrom a whole metric brick, beginning with thatshown in Figure 1.8.

Having cut the brick in half, the bricklayercan also now cut it into quarters. Note that

Quarterbatt

102.5

65

Three-quarters of brickcut away

Fig. 1.8 Quarter batt.

pieces of brick cut into quarter, half or three-quarter lengths are referred to as batts, andother ‘named’ cuts as closers. They are all il-lustrated in Figures 1.8 to 1.13.

We will find out shortly in this chapterwhere and when to use these cuts when welook at how the various bonds are laid outcourse-by-course and situation-by-situation.What we must ask ourselves now is ‘whydo we bother to bond in any particular wayat all?’ Anyone who has played with oldfashioned wooden blocks or their modern

Half batt

102.5

65

Half brickcut away

Fig. 1.9 Half batt.




65

102.5Three-quarter

batt

Quarter brick cutaway

Fig. 1.10 Three-quarter batt.

equivalent, Lego, will understand the need toconnect vertical layers of these units togetherto increase the stability of an increasing heightof built work. The broader the base and themore comprehensive the bonding of the layer,the higher the structure can be made. So it iswith brick and blockwork walls. As we buildwe overlap the bricks in adjacent layers, thusensuring that there is never a complete verti-cal layer joined only to the remainder of thestructure with a layer of mortar.

The second criterion is to spread any verti-cal loading on a part of the wall to an ever-

65

Half brick face

Half brick facecut away

Quarterbatt face

Quarter brick

cut away

Fig. 1.11 King closer.

One brick long

65

Quarterbatt

Quarter brick

cut away

Stretcher facecut away

Fig. 1.12 Queen closer.

widening area of brickwork, thus dissipatingthe load. This is illustrated in Figure 1.14.

Convention on thicknesses of walls

Thicknesses of walls are not generally given inmillimetres but in multiples of ‘half a brick’(which of course equals 102.5 mm) plus the in-tervening thickness of mortar (10 mm) whereappropriate. So a ‘half brick’ thick wall wouldbe 102.5 mm thick, a ‘one brick’ thick wall

One brickHalf brick

65

Stretcher facecut away

Quarter brickcut away

Quarterbatt face

Fig. 1.13 Bevelled closer.




vv v

v vvv v vv

v v vv vvvv vv v

v v v vv v v

Column of bricks not bonded to wall either side

Transfer of load to more bricks in courses whenwall is fully bonded

Fig. 1.14 Loadings on walls which are not bonded andthose which are bonded.

215 mm thick, a ‘one and a half brick’ thickwall would be 327.5 mm thick, and so on4.

This is a particularly helpful device whendrawing details, plans and sections, etc. aswell as in the preparation of contract docu-mentation such as specifications of workman-ship and material and bills of quantities. Manyof those documents can be pre-prepared instandard form if this terminology is adoptedwhen describing brickwork, and then by thesimple addition of a statement regarding thesize of the bricks to be used the whole becomesrelated to that simple, short statement of thesize.

Types of bond

We will illustrate bonds by showing whattakes place at three important points in anywall construction:

4 Note that a mortar joint of 10 mm is always added ex-cept with half brick thick walls.

Scuntion orreveal

Intersection

Quoin

Fig. 1.15 Parts of a wall – scuntion, intersection andquoin.

� The end of the wall, called a scuntion orreveal5. You may see an alternative spellingof scuncheon.

� The junction of two walls at right angles,called an intersection.

� The right-angled corner of a wall, called aquoin.

Figure 1.15 illustrates this terminology.For each bond we show two adjacent

courses. Where necessary explanatory noteswill follow each figure. Before beginning adescription of bonds it should be noted thatthis is not a comprehensive list of bonds orof situations. When looking at other texts thereader will find numerous variations on thebonds discussed here, additional bonds andother situations which include intersectionsand quoins at other than right angles and ofwalls of different thicknesses.

Stretcher or common bond

The first and most simple bond, stretcher orcommon bond, is illustrated in Figure 1.16.

5 Mitchell’s Construction series refers to this as a ‘stoppedend’ but this is an incorrect use of that term. A mould-ing, chamfer or other shape is frequently formed on theedge(s) of timber, stone plaster etc. These shapes arecalled ‘labours’ because their formation only involveslabour making the shape. Occasionally these laboursdo not run the full length of the item in which they areformed but are machined to form a definite ‘stop’. Thisis the proper use of the term ‘stopped end’.




Scuntion

Intersection andalternative

Quoin

Quarter batts

Halfbatt

Fig. 1.16 Half brick thick wall in stretcher or common bond.

� Only used for walls of half brick thickness,this is the only practical bond which can beused on a wall of this thickness, althoughwe can build ‘mock’ bonds of other kinds.A little of that later.

� Only shows stretchers on general face ex-cept for occasional closers and half battsused to maintain bond at quoins, scuntionsand intersections.

Walls we will consider now will vary in thick-ness from 215 mm to 327.5 mm thick – 1 to11/2 bricks thick.

We will begin with English bond and con-tinue with Flemish bond, Scotch bond andgarden wall bond. Finally we will showQuetta and Rattrap bonds, which are always327.5 and 215 mm thick respectively. Sectionalbond is shown hatched in all the figures whichfollow.

English bond (Figures 1.17 and 1.18)

� The strongest bond

� This bond maximises the strength of thewall

� It is used on walls one brick thick and up-wards

� Note how the sectional bond changes as thewall increases in thickness.

Pattern on the face of the wall shows distinc-tive courses of headers and stretchers.

Flemish bond (Figures 1.19 and 1.20)

� Not such a strong bond as English bond� It is used on walls one brick thick and up-

wards� Note how the sectional bond changes as the

wall increases in thickness.

Decorative pattern on face of wall shows al-ternate headers and stretchers in each coursewith the headers centred under and overstretchers in adjacent courses.




qh

qc

qh

qc

qh

qcqc

Note how sectional bondis offset one from the

other by quarter of a brick

Scuntion Intersection Quoin

qh -- quoin headerqc -- quoin closer

Fig. 1.17 One brick thick wall in English bond.

qh

qc

3/4 batt



qc 1/4 batt

qc

1/4

1/4 qc

Note how sectionalbond makes a right

angled turn at quoins

Scuntion Intersection Quoin

qh -- quoin headerqc -- queen closer1/4 -- quarter batt

Fig. 1.18 One and one half brick thick wall in English bond.




3/4 3/4

3/4

3/4

qc

qc



qh

qh

qh -- quoin headerqc -- queen closer

3/4 -- three-quarter batt

Fig. 1.19 One brick thick wall in Flemish bond.

3/4

qc

qh

qh

qc3/4

qh qc

3/4 3/41/4

qh -- quoin headerqc -- queen closer1/4 -- quarter batt

3/4 -- three-quarter battNote how sectional bond is offsetone from the other by three-

quarters of a brick

Fig. 1.20 One and one half brick thick wall in Flemish bond.




Quetta bond (Figure 1.21)

Note that on this drawing the hatched portionis a void, not the sectional bond. This bond wasan attempt to produce a more weather resis-tant form of wall than the one brick solid wallcommonly used in housing at the time, with-out using any more bricks or splitting the wallinto two layers joined with wall ties. The ideanever took off as the brick which protrudesfrom one vertical layer and touches the othermerely serves to draw moisture from the out-side to the inside face of the wall.

The bricks are all laid ‘on edge’, thus thewall ends up one and one third bricks thick.Building the bond with brick on edge poses afew problems:

� Bricks to be exposed on the face should haveplain beds – no frogs, cavities or perfora-tions

� If the wall is to be rendered or plasteredthen choose bricks with very shallow frogsor very small perforations

� If the wall is covered over in some otherway, then the size of frog etc. doesn’t matter

Adjacent courses

Pattern on face of wall

Note that the bricks are allbuilt on edge .The bondcan also be built in blocks --

see later figures.

v v v v

v v v v

v -- void

Fig. 1.21 Quetta bond – wall is always one and one half bricks thick.

� Coverings used could include vertical til-ing, slating or cladding with weather board-ing or other sheet material

� All of these coverings would overcome theproblem of moisture crossing the wall.

Walls with this bond have been used as thebasis for part brick/part reinforced concretewalls with reinforcing rods set in the gaps and,when the brickwork is complete and the mor-tar hardened, concrete poured into the spaces.All-in-all, not an economic solution to keep-ing out the weather when the cost of over-cladding is added, but quite suitable for non-inhabitable buildings or garden or retainingwalls, especially with the concrete fill and re-inforcement for the latter.

Scotch and garden wall bonds(Figure 1.22)

Neither of these bonds is as strong as Flemishbond but they are used in situations wherestrength is not such an issue, e.g. gardenwalls.




English garden wall bond Flemish garden wall bond

6

5

4

3

2

1

Courses 1 and 5 show the typical header pattern of English bond andthe typical header/stretcher pattern of Flemish bond.

Courses 2, 3, 4 and 6 are all built entirely of stretchers on face.

Scotch bond only differs in the number of courses of ‘all stretchers’,which is five.

Seven or more courses of stretchers are never used.

Fig. 1.22 Garden wall and Scotch bond.

Rattrap bond (Figures 1.23 and 1.24)

As in Quetta bond, the bricks are laid on edge,resulting in an interesting face pattern. Thenotes regarding Quetta bond apply equallyto Rattrap bond, with an even lesser chanceof keeping weather effects out of the build-ing. A stronger bond than Quetta, it couldbe used for industrial or agricultural build-ings and of course can be made more weath-erproof by over-cladding. The bond is usuallyone stretcher to one header but it can also be

v

v

v

v

v

v

v -- void

v

Fig. 1.23 Rattrap bond by one – wall is always onebrick thick.

built with three stretchers to one header, asshown in Figure 1.25. Note that it is always anodd number of stretchers but never more thanthree as this weakens the bond considerably.

Vertical alignment

Before leaving the patterns of bond, the ver-tical alignment of perpends must be studied.Two points in particular:

� Perpends are never built immediatelyabove each other, i.e. in adjacent courses,on the face of a wall

Fig. 1.24 Rattrap bond – orthographic view of quoin.




void

voidvoid

Fig. 1.25 Rattrap bond by three.

� Perpends can be partly built above eachother within the body or thickness of thewall

� Perpends continuous through adjacentcourses are known as risbond and the jointsare known as risbond joints.

Figure 1.26 shows the face of an English bondwall and a Flemish bond wall. Note that

English bond face pattern

Flemish bond face pattern

Note that in these face patterns thereare no perpends directly one above or

below the other

Note that in the English bond above, there are no perpends immediately above one another.

In adjacent courses perpends run at right angles to each other

Note that in the Flemish bond above, part of the perpends between the stretcher courses are above one another but only in the interior of the wall. They are shown with a heavy line.

This weakens the wall.

Fig. 1.26 Risbond joint or no risbond joint.

1/23/4 3/4

3/4 1/2 3/41/21/21/2 1/2

risbondjoints overventilator

block

In this face drawing of the wall, the ventilator block has no risbond joints above or below it.

Risbond has been avoided by using half andthree-quarter batts. These can be coursed the other

way round.

1234567

123456

Fig. 1.27 Risbond joint at two course high wall venti-lator.

neither shows any risbond, but that on thecourse plans risbond can be found within theFlemish bond wall.

A common source of risbond joint, fortu-nately of minor importance, is the building ofventilator blocks into half brick thick walls,as shown in Figure 1.27. As you now know,these walls are generally built in stretcher orcommon bond.

The reason risbond appears in the uppersketch of Figure 1.27 is the obsession the brick-layer has had with getting back to a bondwhich is simply laying each brick half overthe one below, and he achieves this in fivecourses – but it is a visual disaster. A littlethought would start this off in course 7 and getback to that in course 1 as shown in the lowersketch. And just to prove that bricklayers likethis do exist and it’s not all in the author’simagination, Figure 1.28 is a photograph of afacing brick wall with one course ventilatorsbuilt into it. They mostly all looked like the onein the photograph. The photograph was orig-inally taken in colour and the contrast acrossthe different colours of bricks accentuated therisbond as well as showing up more clearly thepoor general workmanship in this small area




Fig. 1.28 Risbond joint at one course high wall venti-lator.

of brickwork. The ventilator is built into thebottom three or four courses of a two-storeygable wall of a house and the whole of thebrickwork was to the same horrendous stan-dard.

Honeycomb brickwork

This is a method of building a half brick thickwall similar to stretcher or common bond butleaving a space between every brick in eachcourse, as shown in Figure 1.29. Thereforethe bond can be by quarters or thirds, whichdescribes the overlap of bricks in adjacentcourses. With a quarter lap the void is half abrick wide, and with a lap of a third, the voidis a third of a brick wide.

v v v

vvv

vvv

v v v

vvv

vvv

v -- void

Fig. 1.29 Honeycomb brickwork.

Vertical external corner or‘arris’

Fig. 1.30 Alternative use of term quoin.

This bond is used when building short sup-port walls under ground floors which in turnare suspended over a void. The holes in thewall allow for a free flow of air in the under-floor void.

Quoins – an alternative definition

The term quoin has so far been used to meanthe corner of a wall – the complete corner –from one side of the wall to the other. It canalso be used to mean the vertical external cor-ner or arris of the wall, as illustrated in Fig-ure 1.30.

Another term associated with quoins isquoin header. Whenever a closer is used ata quoin or scuntion the whole brick nearestthe corner is called a quoin header.

Half brick thick walls

It is generally believed that a wall showing acommon or stretcher bond pattern is alwayshalf a brick thick and no other pattern can beused in a wall half a brick thick. This is not true.Half brick walls can show English, Flemish orgarden wall bond on the face as a pattern – notas a true bond – and so deceive the unwarysurveyor. How is it done? Snap headers.

Look back at Figure 1.27 which showed ris-bond joints around a ventilator and a possiblesolution. Such ventilators are frequently builtinto half brick thick walls and an appearanceof a more decorative bond could be given by




Header faces showing

Stretcher faces showing

Alternate courses

Fig. 1.31 Snap headers.

using snap headers in its construction. Indeedsome of the cuts in the ‘solution’ would betermed snap headers. The term arises becauseany headers which show on the face of thewall are just ‘headers’ (whole bricks) snappedin two with the decorative header face show-ing and the rough cut end on the inside. Thisis illustrated in Figure 1.31.

Frog up or frog down

Cellular or single frog bricks can be laid withthe depression on the bed facing up or down.The prime consideration in deciding whichway to lay these bricks is the strength of wallrequired. Economics is the secondary consid-eration.

With the frog up, mortar for bedding thecourse above fills any frog or cells in the bricksbelow. Solid filling of the depressions gives astronger wall – provided it is done properlyand not skimped by bricklayers in a hurry.

With the frog down the mortar bed over thefirst course of bricks is nominally 10 mm thick.The next course above only beds into that mor-tar around the periphery of the frog, cell orperforation. Because there is a much smallerarea of contact between adjacent courses,there is a much higher loading at these points.So for a given load, a wall built this way isstressed locally to a higher degree than a wallbuilt frog up. The wall is weaker.

The economics

Laid frog up, bricks with large depressionswill use much more mortar than when laidfrog down. This costs money in two ways:

� The cost of the extra mortar� The cost of labour spreading that extra

mortar.

‘Tipping’

No, not slipping the bricklayer a small appre-ciation of his services but a much more invidi-ous practice by the bricklayer in a hurry.

To lay the bricks at all, a full bed of mor-tar is required. It is not possible to skimp onthis as the bricks would not sit properly in thewet mortar if it were anything less than fullyspread. However, it is possible to skimp on themortar put into the perpends. This is done bythe bricklayer who is hurrying to get enoughbricks laid to earn a bonus, or is just lazy. As abrick is laid it is tamped or pressed down intothe wet mortar which squeezes out each sideof the wall. This surplus mortar is immedi-ately cut away with the trowel. The next brickis picked up and that mortar on the trowel is‘wiped’ off onto the header/stretcher arris ofthe brick. That end of the brick is then laidagainst the previous brick and from the out-side of the wall, the perpend looks as if it wasfilled – but it is an illusion! This weakens thewall. The bricklayer should cover the headerface of the brick and press this against the pre-vious brick so that every perpend is filled solidwith mortar.

Where very thick walls are built it is com-mon to expect tipping at exposed perpends,and probably no perpends filled in the bodyof the wall at all. Many old specifications6

called for each course to be grouted. Grout-ing is the filling of voids – in this case the per-pends – with a mortar of a fairly fluid consis-tency. The lay reader will of course expect thatthe spreading of the next bed of mortar wouldfill all these perpends anyway, but this is notthe case. Building mortar is of too stiff a consis-tency to run down into a 10 mm wide gap for a

6 A specification is a document which was commonlyprepared by the architect but is now done by the quan-tity surveyor. It lays down minimum standards for ma-terials, components and workmanship to be used on aparticular contract.




depth of about 65 mm. The inevitable dryingaction caused by contact with bricks stiffensthe mortar even more and prevents it runninganywhere. Even when a bricklayer pushesmortar into the perpends with the edge of histrowel, the filling is not complete.

Common and facing brickwork

So far we have only looked at the bonding ofthe bricks in a wall, without looking at whythey are built into the wall. There are basicallytwo ways in which bricks are built:

� As common bricks, or� As facing bricks.

Common brickwork is used to describe brick-work which will generally have a ‘finish’ ap-plied to it or whose external appearance isnot of great importance. Examples of the lattermight be the insides of garages, boiler sheds,etc., whereas the interior walls of a housewould be plastered.

Despite the name and the fact that this workis rarely seen in the finished building, the lay-out and cutting of bricks to form bond mustbe done carefully and well. Just because it is‘common brickwork’ is no excuse for shoddyworkmanship.

If common brickwork is to have a finish ap-plied, then the beds and perpends have themortar raked back to provide a key for thatfinish. The finishes are either plaster or a mor-tar render, which are applied ‘wet’. Rakingback the joints allows the plasterer to pressthis wet material into the joints, thus giving agood grip or key.

Another way to finish common brickworkis to fill the beds and perpends full of mor-tar and strike this off flush with the face ofthe bricks. This is called flush jointing. Beforestriking the mortar off flush, the bricklayerwill press the mortar down hard with a trowel.This gives a dense hard joint not easily bro-ken down by weather etc. He or she may thenrub the wall face down with a piece of sack-ing. This is called bag rubbing. Alternatively

he or she could brush it with a stiffish handbrush. Both techniques give a softer textureto the face of the wall. Flush jointing and bagrubbing or brushing are often used internallyon walls as a precursor to some form of paintfinish applied direct to the brickwork.

Facing brickwork

Facing brickwork is the term applied to brick-work where special attention has been paid tothe external appearance, including the use ofdecorative or coloured bricks and decorativeor coloured mortar beds and perpends.

Facing bricks can have one or both of thefollowing features:

� A special colour either inherent in the basematerial or added to it or applied to thesurface

� Texture deliberately impressed on the sur-faces or as a result of the base material tex-ture and/or the manufacturing process.

At least one stretcher face and one headerface must have a ‘finished’ surface. Frequentlyone stretcher face and both header facesare ‘finished’. This allows any wall of onebrick thick to be built with two ‘good’ facesprovided the lengths of the bricks are reason-ably consistent and the bricklayers are skil-ful enough to stretch or compress the thick-ness of the mortar in the perpends to accom-modate any inequality in dimensions of thebricks. Finishing only one header face allowsany wall over one brick thick to be built withtwo ‘good’ faces.

Figures 1.32 and 1.33 illustrate how bricksrequire to be ‘finished’ on more than just thestretcher face to give a good appearance in fac-ing brick work. Finishing half brick thick wallswith a good face both sides is restricted thistime by the accuracy of the width of the brick,and there are no internal perpends which canbe adjusted in thickness to accommodate in-adequacies in the dimensions of the bricks.Naturally any snap headers must be built withthe cut face as a perpend so that both sides




Plan of one course of English bond in the scuntion of a one and one half brick thick wall.

The finished faces of the facing brick are shown in heavy lines -- remember only one stretcher face

and one header face are finished

Fig. 1.32 Scuntion of one brick thick wall built bothsides fair face; bricks finished one stretcher and twoheader faces.

show finished faces. In this case the cuttingis best done with a power saw. If you doubtthat, just go back and look at Figure 1.28 andthe perpends at the half bricks. The wavy edgeof a brick cut with a chisel or a brick hammeris clearly seen.

This is the quoin of a onebrick thick wall in English

bond and the header facesare shown with a heavy

line. This shows that for awall of this thickness, bothheader faces require to be

‘finished’.Only one stretcher faceneeds to be finished.

Fig. 1.33 Quoin of one brick thick wall built both sidesfair face; bricks finished one stretcher and two headerfaces.

Pointing and jointing

Naturally the finishing of the joints of facingbrickwork is important, but any brickworkwith joints exposed to the weather requiresa good finish to the joints to prevent weatherpenetration. Pointing and jointing are done toenhance the weathering characteristics of themortar in the beds and perpends since theseare the most vulnerable part of the whole wallwhich might be exposed. Should the build-ing mortar be unsuitable for exposure to theweather, then jointing is not an option; point-ing must be carried out where the mortar canwithstand any weathering effects. Apart fromthese practical considerations, aesthetics fre-quently play their part in choosing a point-ing/jointing shape. Some of these shapes areshown in Figure 1.35.

On facing brickwork, flush jointing is occa-sionally done but bag rubbing or brushing arenever done as this carries mortar over the faceof the bricks, spoiling their appearance. Lookat Figure 1.28 for mortar staining of the bricks.

It is more usual to strike the mortar off at anangle or some other shape to emphasise thejoint and also to add to its weather sheddingproperties, as shown in Figure 1.34. The figure

Mortar used for building pressed back hard and

trowelled into an angled shape

Fig. 1.34 Weather struck joint in pointing or jointing.




shows a weather struck finish, the most com-mon shape used to finish off a mortar bed orwith which to finish pointing. The slope is de-signed to shed any water striking the surface.It is a shape preferred by bricklayers as it fin-ishes off the upper edge of the course which isthe bed set level with the builder’s spirit levelor the builder’s line. Note that when the shapeis worked on the mortar used for building itis termed jointing.

There are many reasons for, and occasionswhen, the mortar used for building facingbrickwork is not suitable for the finished ap-pearance of the wall: wrong resistance toweathering, wrong colour and wrong textureare the most frequent. It is generally consid-ered too expensive to use coloured mortars forbuilding. What is done then to keep the build-ing mortar cost down and still have effectiveand expensive looking joints? Simply, buildin the cheaper mortar, rake back the joints10–15 mm and add coloured and/or texturedmortar to finish the joints. This process iscalled pointing.

It is important to understand the differencebetween jointing and pointing:

� Jointing means giving the mortar used forbuilding a finished shape.

� Pointing means raking back the buildingmortar at the joints and adding a sepa-rate mortar with a colour or texture andgiving that a finished shape. The additionof the coloured or textured mortar is al-ways done after the building mortar hashardened.

Different shapes can be worked on the mortarin both jointing and pointing operations, anda selection is shown in Figure 1.35.

Starting at the top left and working clock-wise, flush or flat jointing or pointing has al-ready been described. Despite its simple ap-pearance it is not an easy option when build-ing with facing brick as it is all too easy tosmear mortar across the face of the brick andso spoil the appearance.

Recessed jointing or pointing used to bemuch more common on the Continent than inthe UK. It is a good clean method of finishing

Flush or flat jointing or pointing

Recessed jointingor pointing

Bucket handle jointing or

pointing

Reverseweather

jointing or pointing

Flush or flat keyedjointing or pointing

Tuck pointing

This piece is done after the rest of

the building mortar has

set

Fig. 1.35 A selection of joint shapes.

a joint although it does not shed water out ofthe joint, as weather struck jointing or point-ing does.

Tuck pointing is very decorative and withthe distinctive, exposed, regularly shaped,rectangle of (coloured) mortar draws the eyeaway from any irregularity in the size or build-ing of the facing bricks. It is frequently used,therefore, for renovation work in old brick-work where the old mortar is raked out, thebeds and perpends flush pointed and a groovestruck, and after being allowed to set, the‘tuck’ is formed, perhaps in a contrasting mor-tar. A tool for tuck pointing is illustrated inFigure 1.36.

Section

Fig. 1.36 Tuck pointing tool.




Keyed pointing or jointing is shown hereworked on a flush bed or perpend. The key issimply a shallow line marked in the still plas-tic mortar and ruled straight down all the bedsand across alternate courses in the perpends.The idea is to disguise any unevenness in thebrickwork. Keying can be applied to recessedpointing or jointing.

Reverse weather jointing or pointing is afairly obvious description but its effects arequite different to the regular weather struckjoint. Being made from the lower edge of thecourse of brickwork, it tends to follow a moreirregular line and so could, if below eye level,show up any such irregularities. However, inwalls generally much of the work is above eyelevel so it has the effect of softening the line ofthe bed pointing.

Bucket handle or grooved pointing is eas-ily achieved with a simple tool made by thebricklayers themselves from a bucket handle.Not just any old handle but the galvanisedsteel one from an old fashioned galvanisedbucket. The handle is straightened out and cutinto lengths (generally two or three) which arethen shaped into a very open Z, as shown inFigure 1.37. Either end can then be used topress the mortar back into the joints, givingthe required shape to the joint and density tothe mortar.

It is important to realise that good point-ing/jointing is all for nought if the bricksthemselves are:

� Badly laid� Of poor quality� Of uneven sizes� Chipped on edges or corners.

These faults are emphasised by attempts topoint them up straight and true. The onlypointing which might help would be tuckpointing where the base mortar is tinted tomatch the brick, the groove is struck straightand even and the tuck is placed in a contrast-ing coloured mortar. It is for this reason thattuck pointing is favoured by many profession-als when carrying out re-pointing of old andworn brickwork.

Bucket handle tool

This is the handle endwith the groove up. This end is the tool with

the rounded surface down

Section across buckethandle

Bucket

Fig. 1.37 Bucket handle pointing/jointing tool.

General principles of bonding

To recapitulate all we have learned aboutbonding brickwork we list here the sevenprinciples which, if followed carefully, will en-sure the proper layout of each course of brick-work in any thickness of wall. As you readeach one you may wish to refer back to theillustrations of bonds, and note where each ofthem occurs. To start you off the figure num-bers are given for the first principle.

Bonding 1

Correct lap set out and maintained by:

� closer next to quoin header (Figure 1.17, theupper course shows queen closers next tothe quoin headers), or




� three-quarter batt starting the stretchercourse (Figure 1.19, the upper course showsthe scuntion beginning with a pair of three-quarter batts).

Bonding 2

� Perpends in alternate courses kept verti-cal (any figure showing face patterns ofwalls).

Bonding 3

� No risbond joints on faces of wall� Risbond is allowed in the thickness but

must be kept to the minimum ‘allowed’ bythe particular bond.

Bonding 4

� Closers only used next to quoin headers.� These can occur at quoins and scuntions.

Bonding 5

� Tie bricks at intersections and quoins mustbond properly, i.e. by a quarter brick.

Bonding 6

� Bricks in the interior of a wall should belaid header wise across the wall as far aspossible.

Bonding 7

� Sectional bond should be properly main-tained.

� Front and back faces of walls should linethrough (Figure 1.38 illustrates this vitalpoint).

Bricks in thickness of the wall laid header wise, across the long axis of the wall

Sectional bond lines through back tofront of wall as shown by heavy lines

Fig. 1.38 Alternate courses in a two brick thick wallin English bond showing bricks in centre of wall andlining through of sectional bond.

BLOCKS

Block materials

Modern blocks are generally made from someform of ‘concrete’ and are the subject of BS6073, Precast concrete masonry units. Blockscan also be made from fired clay, the subjectof BS 3921.

Concrete blocks

There are three basic categories of concrete7

used for block manufacturer

� Dense concrete� Lightweight concrete� Autoclaved aerated concrete (aac).

Dense concrete is usually described as con-crete which has a dry bulk density8 in excessof 750 kg/m3.

7 Concrete is discussed more fully later in this chapterand in Appendix D.

8 Bulk density is the mass of any material in unit vol-ume, voids in the material being measured as part ofthe overall volume.




Lightweight concrete is therefore concretewhich has a dry bulk density up to 750 kg/m3.

Aac is a special concrete made from a mix-ture of cement, lime and siliceous materialwhich is heated under pressure. It is aeratedbefore ‘cooking’ and has a dry bulk densityfrom 350 to 750 kg/m3.

Dense and lightweight concretes

These concretes are made from three basicingredients:

� Aggregates� Binder� Water

Additives such as colouring may be intro-duced.

� Aggregates are of two kinds, fine (sand) andcoarse (normally gravel or crushed stone orother inert material)

� The binder is normally ordinary Portlandcement (OPC)

� The water is necessary to trigger and com-plete the chemical reaction which makes themixture ‘set’.

Materials for aggregates in dense orlightweight blocks can be: natural graveland sands, crushed stone, crushed stone dust,slags, furnace ash, exfoliated vermiculite,expanded perlite, crushed brick, crushedconcrete, crushed clay blocks, wood fibreand sawdust or wood chippings. The woodmaterial is usually treated with petrifyingliquid. This involves impregnating the cellsof the wood with minerals in solution – thepetrifying fluid – so that they become almosta ‘fossilised’ replica of the original cells.

Autoclaved aerated concrete

� The aggregate is a mixture of sand andpulverised fuel ash (PFA). The proportionsvary but grey coloured aac has a higher

proportion of PFA than sand, and creamcoloured aac has a higher proportion ofsand.

� The binder is a mixture of OPC and buildinglime.

� Water is used to make the mixture set.� Aluminium powder reacts with the alkali in

solution to generate hydrogen, the bubbles‘aerating’ the wet mixture before autoclav-ing.

� The mixture of alkaline and siliceous mater-ial combines under the heat and pressureto form calcium silicate, the same materialas sand or flint lime bricks. The Victoriansinvented the name Tobermorite for calciumsilicate.

All blocks of whichever concrete come in avariety of sizes. For a complete range of facesizes refer to BS 6073. Thicknesses generallyavailable are: 50, 65, 75, 90, 100, 199, 125, 150,200, 215, 250, 300 mm.

Dimensions of standardmetric block

As in brickwork, blocks have coordinatingdimensions within which the block and onelayer of mortar are accommodated. The worksize of the standard metric block is 10 mm lesson face than the coordinating size, so 10 mmmortar beds and perpends are the norm.

Aac blocks are only ever manufactured assolid rectilinear shapes. Dense and light-weight concrete blocks are manufactured assolid, cellular and perforated shapes, asshown in Figure 1.39.

With a wide range of thicknesses available,walls built with blocks only have a single, ver-tical layer of blocks in them. In other words,the thickness of the wall is designed to suit oneof the thicknesses of block available. It followsfrom this that blockwork is built in commonbond. Blocks will overlap in adjacent courses.The overlap may be by thirds or halves. Thechoice of bonding by thirds or halves can de-pend on the setting out of the length of the wallto accommodate proper bonding at quoins




Coordinating height 225Working

height 215

Coordinating length440

Working length430

Range ofwidths

depends onmanufacturer

Cellular or perforated block.If cellular the bottoms of the two rectilinear holes are sealed off.

Either type of block may have the voids filled with plastic foam,

usually polystyrene.

Fig. 1.39 Standard metric block dimensions and typeby shape.

and intersections. The thickness of the blockalso affects this decision.

These last points are illustrated in Fig-ure 1.40. Note the cuts at the quoin.

Unlike brickwork, block cutting is generallyconfined to cutting to a particular length to

cut

cut

Fig. 1.40 100 thick block wall quoin.

cut

cut

Fig. 1.41 150 thick block wall quoin.

accommodate the bond required. Dense con-crete blocks can be cut with a bolster9 andhammer or with a power saw. Lightweight ag-gregate blocks can be cut with a bolster andhammer, a power saw or chopped with a brickhammer. Aac blocks can be cut with a bol-ster and hammer (the bolster tends to sinkdeeply into the block without making muchimpression!), a power saw (this is a waste ofpower), a tungsten-tipped hand saw (goodbut labour intensive) or an axe or zaxe10 (botha bit crude).

Figure 1.41 illustrates a quoin in a 150 mmthick wall. Note the cuts at the quoin. Al-though the 150 thickness is slightly more thanone third of the coordinating dimension of ablock length, there is no need to actually cut allthe blocks at the quoin. The bricklayer simplymakes the perpends in each course a little lessthan 10 mm between the quoins. He doesn’tactually measure this as a fraction of a mil-limetre at each perpend. His skill allows himto make the adjustment a little at a time purely‘by eye’ or ‘by feel’. Only if there is insufficientlength between the quoins to make sufficientadjustment, are blocks cut.

9 A bolster is a cold chisel with a broad cutting edge,anything from 50 to 150 mm wide. 100 mm wide is acommonly used size in these circumstances.

10 A zaxe is a tool used by slaters to cut roofing slates.It resembles a meat cleaver with a spike on the backedge.




Whys and wherefores of mortar

Mortar is what we use to build masonry.Mortar ‘glues’ bricks and blocks together, al-though the gluing effect is minimal. Mortartakes up irregularities in the shape and size ofbricks and blocks, especially if built ‘frog up’.By providing full contact between adjacentunits, mortar spreads loads evenly through-out masonry. Mortar prevents weather pen-etration between bricks and blocks, preventspenetration of walls etc. by fire and sound,and can be a decorative feature of masonry.

Mortar is composed of:

� Fine aggregate� Binder� Water.

Additives such as colourings, waterproofers,accelerators and air entrainers are frequentlyintroduced.

The most usual aggregate is washed build-ing sand. The binder may be:

� ordinary Portland cement (OPC)� building lime, or� a mixture of both, or� masonry cement.

Water is added to trigger and complete the‘setting’ process of the binder(s). Water makesthe mixture plastic, thus allowing the bricksand blocks to ‘bed’ into the layer of mortar.

The proportions of the principal ingredientsare determined by the use to which the mor-tar will finally be put. The proportions are ex-pressed as parts by volume. Where OPC orlime is used as the whole of the binder thisis expressed as one volume. Where OPC andlime are combined, the OPC is expressed asone volume and the lime in its correct propor-tion to that one volume of OPC. Fine aggregateis the final volume.

For example, a mortar mix described as 1:2:9is a mixture of one part OPC, two parts build-ing lime and nine parts fine aggregate. Thismix is termed a cement/lime mix. A cementmortar mix described as 1:3 is a mixture of one

part OPC and three parts fine aggregate. Themix is described as a cement mortar mix asopposed to a lime mortar mix.

Cement

Cement is the best known ingredient of mor-tar and one most commonly mistaken for themortar itself. Remember we build nothing ‘incement’. We put cement into certain mortarsand concretes where its properties as part ofthe binder are used to advantage. There areoccasions when the use of cement would notbe appropriate.

The most common type of cement used isordinary Portland cement (OPC) which is thesubject of BS 12. The term ‘Portland’ cameabout because the colour of the cement – agreeny grey – resembled Portland stone.

Ordinary Portland cement was the inven-tion of Joseph Aspden in 1824. To manufac-ture cement, clay and limestone are brokeninto small pieces, mixed and calcined in a longrotary kiln. The fuel can be oil, gas or pow-dered coal. A little gypsum powder is addedto the clinker, which is then ground into afine powder. The result is ordinary Portlandcement. The cement can be filled into papersacks of various weights, the most commonbeing 50 kg, with 25 kg bags finding favourwith jobbing builders and the DIY market. Itcan also be shipped out of the works in bulk,either in railway or road tankers. This materialis loaded into silos at the plant or site whereit is to be used.

The chemical composition of OPC is com-plex and varies according to the source of theminerals used in its manufacture. The harden-ing or ‘setting’ of mortar and concretes madeusing OPC is due to a chemical reaction be-tween the OPC and the water used for mixing.Air is not required for setting to take place, nordoes the mortar or concrete need to ‘dry’. Be-cause cements of this type will set under waterand without air, they are often referred to ashydraulic cements.

White cement can be manufactured but inthis instance the clay is white china clay and




the limestone used is pure natural chalk. Coalis not suitable for firing the kiln. Powderedgypsum is added after the clinker has beenfinely ground.

Portland cement can be obtained in a rapidhardening variety. The setting process isspeeded up by grinding the powder evenmore finely, thus increasing the exposed sur-face area of the cement particles. The surfacearea of the particles is given in square cen-timetres per gram of cement – the specific sur-face. The specific surface of rapid hardeningcement ranges from 7000 to 9000 cm2/g. Theincreased area in contact with water speedsup the setting process.

For Portland blast furnace cement (BS 146)30%, by mass, of blast furnace slag is addedto the clinker produced by calcining clay andlimestone before grinding. Blast furnace slagis a material which demonstrates pozzolanicproperties, i.e. a ‘setting’ action takes place inthe presence of water.

OPC is a complex cocktail of chemical com-pounds, one of which – tricalcium alumi-nate – is attacked by sulphates in solution.By reducing the tricalcium aluminate contentin the cement greater resistance to sulphateattack can be achieved – sulphate resistingcement.

Masonry cement is made by taking OPCand adding a filler and a plasticiser. The fillermight be a material such as pulverised fuelash (PFA). PFA is the ash produced in ver-tical furnaces used in power stations whichburn powdered coal. The ash is very fine, greypowder which is trapped in electrostatic pre-cipitators and mixed with water before beingdumped in lagoons. The PFA must be keptwet in order to stop it spreading out over thecountryside in even the slightest breeze. Theplasticiser is usually an air entraining agentcomplying with BS 4887.

Lime

Before the invention of OPC, lime was themost common binder used. Builder’s lime ismanufactured from carbonates of calcium and

magnesium. The most common form is madefrom calcium carbonate – chalk or limestone –which is calcined in a kiln to give calcium ox-ide or ‘quicklime’.

The equation for the process looks like this:

CaCO3heat−→ CaO + CO2

The calcium oxide is slaked by immersing inwater and forms the hydroxide, calcium hy-droxide – CaO2(OH).

The burning of the lime used to be car-ried out next to sources of limestone and alsowhere coal could be easily obtained to fire thekilns. The quicklime was extracted by handwhich was dangerous and dirty work. Quick-lime generates large quantities of heat whenwetted, and on moist skin or in eyes will pro-duce horrible burns.

Builders slaked the quicklime on site by cov-ering it with water for at least three weeks.The excess water was drained off, leavingthe wet hydroxide known as lime putty. Theputty was run through a sieve to take out anyunslaked lumps and then mixed with coarsesand to give a mortar.

Lime manufacture is now a factory processand the lime is supplied to the trade as a drypowder known as a dry hydrate – CaO2(OH).This can be mixed direct into mortars, pro-portioned by volume. The relevant standardis BS 890.

Limes based on calcium carbonates are themost commonly used for general buildingwork. They are known as ‘pure’ or ‘fat’ limes.They are also termed non-hydraulic limes,as they require air in order to set. In partic-ular they require the carbon dioxide in the airsince setting is the action of the CO2 convert-ing the hydrate back into the carbonate, re-leasing water which simply dries out.

Another lime available is a magnesium-based lime which must have at least 5% ofmagnesium oxide before slaking. In practicethe percentage is something in the order of30+%.

Hydraulic limes are also made and theseexhibit properties similar to OPC, as the lime-stone from which they are made is a grey lime-stone containing traces of clay.




BS 890 covers other limes – semi-hydraulicand eminently hydraulic. These are not of in-terest at the present time.

Sand

Modern mortars use washed building sand –not any old sand but an aggregate where thereis a definite grading of the particle sizes suchthat the proportion of binder to sand at around1:3 to 1:4 allows for complete filling of thevoids between and coating of the sand parti-cles by the binder. Sand is covered by BS 1200 –pit and river sands. Sea sand is never used.

Water

Water is the final ingredient in any mortar. It isno use specifying and procuring lime, cementand sand to the required standard if the waterused for mixing is impure. For the majorityof work it is sufficient to describe the wateras ‘potable’ and to further reinforce that byspecifying that the supply must be obtainedfrom a main supplied with water by the lo-cal water authority. Natural sources of watersuch as rivers, streams ponds, etc. can be con-taminated with excess sulphates, nitrates andother soluble chemicals which can weakenthe mortar by destroying the cement or otherbinder, or can cause staining or even odoursin the building when it is heated.

An example of the latter was found when alarge office block in the centre of Edinburghwas having its heating system commissioned,although the fault in the end was not ‘water’.A strong smell of old ammonia was detectedon one floor and was finally tracked down toone or two rooms. When questioned, the sub-contractor quizzed his employees and even-tually found out that they were in the habit oftaking plastic, soft drinks bottles of water intorooms with them. The water was occasionallydrunk but more often was used to ‘knock up’an already setting mortar. Occasionally, hav-ing drunk too much, the bottles were used forpersonal relief! One wonders if the workmenever got the contents mixed up in other ways.

Which mortar mix?

Guidance on which mortar mix to use is givenin the Building Regulations by reference toBritish Standard 5628. The editions quoted inthe Regulations have all now been supersededand mortar is now included in Part 3, Codeof Practice for the use of masonry; materials andcomponents, design and workmanship.

Choice of mortar depends on the units beingbuilt, strength required and the season of theyear with its attendant weather conditions.Further factors are:

� Location within the building� Degree of exposure to weather, particularly

driven rain.

We cannot, therefore, consider which mortarto use until we understand what we mean byexposure.

For external walls, location within the build-ing is divided into three areas:

� To a height not less than 150 mm above fin-ished ground level – from the damp proofcourse level down

� Between the level of the top of the dampproof course (DPC) and the junction of wallwith roof

� Gable haffits – the triangular shaped pieceof wall from eaves level to the ridge level.

Exposure is rated in accordance with theBritish Standard Code of Practice 8104, As-sessing exposure of walls to wind-driven rain. Itis based on a fairly complex series of data pre-sented in the Code which can be selected andapplied to any building according to its pro-posed geographical location. It is instructiveto note that in this Standard the exposure ofthe wall of a house between DPC level andeaves can vary by as much as a factor of 2 to3 and if there is a gabled wall, the top of thegable can vary by a factor of from 3 to 4.

There are two measures of exposure given inthe Code, the unit of measurement being litresof rain driven onto the wall/square metre ofvertical wall surface. Note 1 litre/m2 is theequivalent of 1 mm of rainfall.




Exposure will be calculated as either annualor spell. Annual exposure is the total amountof water hitting the wall surface in one yearand would be used to gauge the overall mois-ture content of the wall and the prospect ofmoss or lichen growth, etc.

Spell exposure is the total amount of waterhitting the wall surface in relatively shorterperiods of time with up to 4 days in the spellwithout any appreciable rain at all; it is usedto determine peaks in rainfall and so enablethe wall designers to take these into account.For example, if a wall were to have an an-nual exposure of 400 litres/m2, that would in-dicate an annual rainfall against the wall of400 mm – not a lot; but if a spell exposurerating of 400 litres/m2 were indicated over aperiod of less than one day, the wall wouldhave to be designed very differently. Such anexposure would indicate that a cavity wallwould have solid water on the inner face of theouter leaf and that weeps at the bottom of thewall might be overwhelmed in their efforts toclear the water; more of this when we look atwalls.

A number of factors are taken into accountin the calculations:

� Airfield index – Imagine that on the siteof the building there is a large open spacewithout obstructions – an airfield for exam-ple – and that in the middle of this spacethe quantity of driven rain 10 m above theground is measured at the orientation of thewall being examined.

� Geographical location – (see airfield indexabove). Maps have been prepared to a scaleof 1:1 000 000 showing the increments ascontours. Maps give the key to finding thewind roses from which the base for the ex-posure calculation is derived.

� Orientation – 12 directions are given on thewind roses (see airfield index above).

� Topography – ground slope and height.� Terrain roughness – allows for the general

terrain type up wind of the wall.� Obstruction – nearby trees, hedges, fences

and buildings, their distance and spacing.� Overhangs and surface features – refers to

eaves overhangs, canopies, string courses,etc. on the wall itself.

� Occasional features such as hills, cliffs andescarpments can also be factored into thecalculation. The maps, and the factors of ori-entation, topography and terrain roughnesswhich they introduce in the key to the windroses, generally account for ranges of hillsand mountains but cannot account for iso-lated features such as these. They must bedealt with separately.

Mortar used for building masonry is only oneof many parts of British Standard 5628, Struc-tural work in masonry. The Standard covers alltypes of masonry – brick, stone, concrete, etc.– and is a large and comprehensive document.As a result, the mortar mixes given in Tables13 and 14 of the Standard cover all these ma-sonry types. Our text here deals only withbrick and concrete block and so the choicenarrows slightly. The mixes and advice sum-marised in Table 1.2 should prove adequatewith the limited masonry types under discus-sion.

‘Fat’ mixes

Mortar mixes suitable for building masonryunits should be ‘fat’. This is a difficult term todefine, but imagine you are trying to spreadout a mixture of plain sand and water into athin layer. Such a mixture is gritty and stiffand if it is being applied to a porous surfacethe water is absorbed. To improve the spread-ability of a mortar, to reduce the grittiness andloss of water on absorbent surfaces, one can doa number of things to the mixture:

� Include lime in the binder, or� add an air entraining chemical, or� use masonry cement as the binder.

Highly absorbent surfaces on bricks orblocks – especially in hot, dry weather – cantake so much water out of the mortar that thechemical reaction between the water and thebinder cannot fully take place and a weak bedand perpend are formed.




Table 1.2 Mortars for clay bricks and concrete blocks.

Mix Below DPC level Above DPC level

A 1:1:5 or 1:1:6, or 1:5 usingmasonry cement

For all exposures and in/at anyseason

For all exposures and in allseasons

B 1:5 or 1:6 using OPC and aplasticiser

For all exposures and in/at anyseason

For all exposures and in allseasons

C 1:2:8 or 1:2:9, or 1:6 usingmasonry cement

— For sheltered and moderateexposures and in spring andsummer

D 1:8 using OPC with a plasticiser — For sheltered and moderateexposures and in spring andsummer

E 1:3 using hydraulic lime — For sheltered and moderateexposures and in spring andsummer

F 1:2 using hydraulic lime — For all exposures and in allseasons

G 1:3 using either OPC ormasonry cement

For all exposures and in anyseason but to withstand aheavy loading

For severe exposures and at allseasons but to withstand aheavy loading

Adding lime to the mix

Using a cement/lime mortar has a numberof advantages beyond providing the brick-layers with a ‘fatty’ mix. After it has set, themortar:

� Is weaker than a pure cement mortar andwill not damage the bricks or blocks

� Exhibits greater elasticity than a pure ce-ment mortar, thus taking up thermal andmoisture movement of the blocks or bricks.

If cement/lime mixes are not usually used, amasonry cement mortar or a cement mortarwith an air entrainer are the next best, in thatorder.

Adding an air entrainer

Air entrainers are based on an industrial de-tergent and during mixing introduce lots of airbubbles into the water of the mortar. Becauseof the air bubbles the mix feels ‘fatty’, thushelping the laying and bedding of the bricks orblocks. Because the water surrounding the airbubble is held there by high surface tension,

it is less likely to be taken up by an absorbentsurface.

Using masonry cement

Using masonry cement as the binder gives afatty mortar since the cement exhibits a com-bination of characteristics akin to an OPC andlime mix. The set mortar is also softer andmore elastic than a pure cement mortar.

General rules for selection of mortar

� Adequate strength to resist crushing loadsin the beds of the masonry

� Weaker than the masonry units so thatshrinkage of the mortar does not crack theunits

� Sufficient elasticity to absorb movement ofthe units

� Good ‘spreadability’ with proper water re-tention on absorbent surfaces

� Appropriate for the season of the year� Capable of withstanding the exposure to be

experienced.




Mortar additives

Additives frequently introduced are:

� colourings� waterproofers� accelerators� air entrainers� frostproofers.

Colourings

OPC, which is grey/green in colour, can betinted dark red, green, buff, brown, darkgrey, etc. by the addition of powdered tint-ing agents. To obtain pale colours silver sandand white cement must be used in the mortar.

Waterproofers

In some instances the term ‘waterproofers’ isa misnomer. Some additives are only waterrepellants.

Water repellants prevent water penetratingthe pore structure provided there is no pres-sure behind the water. Under hydrostatic orhydraulic pressure, mortar, even when treatedwith a repellant, will allow water to passthrough its pore structure. An old form of wa-ter repellant was the inclusion of linseed oil.This was dispersed as an emulsion in hot wa-ter and then added to the mixing water. Mod-ern water repellants are based on syntheticresins which coat the pore sides, preventingwater from entering unless under pressure.

To make mortar completely waterproof, onehas to fill every pore or seal off every poreinto a closed cell structure. Again, modernchemistry has given us synthetic resins whichare capable of blocking off the pore struc-ture, even under considerable pressure, butbe aware that they are not all ‘equal’. For ex-ample, there are waterproofers based on EVA(ethyl vinyl acetate), available in a liquid anda powder form. Both are simply added to themortar mix in the recommended proportions,but the liquid form is quickly washed out of

the pore structure whereas the powder formis more permanent in its effect.

Accelerators

Accelerators are chemicals added to a mortarto speed up the reaction between the binderand the water, known as ‘setting’. Calciumchloride was widely used for this purposebut too high a proportion ‘killed’ the set com-pletely and if in contact with ferrous met-als caused corrosion. If a very fast set is re-quired it is better to use quick-setting Portlandcement.

Air entrainers

Air entrainers are based on industrial de-tergent and are ‘wetting’ agents. Numeroussmall air bubbles are formed during mixing.While this can allow a reduction in the amountof water used when making concrete, with amortar the effect is to reduce the loss of wa-ter to dry or porous building units. This has anumber of effects:

� The mix appears ‘fatty’ for laying the units� Water is retained in the mortar beds and not

absorbed so readily by dry, porous units� Water is not lost so readily in dry hot

weather� There may be a lowering of strength.

Frostproofers

Frostproofers are nothing more than acceler-ators. Acceleration of the ‘set’ generates moreheat, thus ‘proofing’ the mix against imme-diate frost attack. An accelerated setting timealso means the mortar has a shorter time spanduring which it can be frozen.

Mixing in additives

Additives in liquid form are best put into themixing water. Mixing water is generally keptin a steel or plastic drum next to the mixer.




So to a known quantity of water the correctproportion of additive may be mixed in. Nomatter how much mortar is mixed, it will thencontain the correct amount of additive. Mostdry additives, i.e. in powder form, are addedin proportion to the mass of OPC or otherbinder. A proper container should be manu-factured and calibrated to ensure the correctproportion of additive is placed in the mixerwith the dry ingredients.

Mixing mortar

Mixing of the ingredients is generally donein a mechanical concrete mixer. The term ‘ce-ment mixer’ is frequently used but is quiteincorrect.

The dry ingredients are placed in a drumand mixed to a uniform colour. Water is fi-nally added and mixing continues to give auniform consistency. Never mix beyond thatpoint. Mixing for longer than is necessary togive a uniform consistency will entrain an ex-cessive amount of air into the mix, which willweaken the mortar.

Mortar should never be remixed, nor shouldold mortar be introduced into a fresh batch.Old mortar should not be ‘freshened up’ or‘knocked up’ by adding more water.

Once the water is added to the batch of in-gredients, the chemical reaction of ‘setting’commences. It takes up to about 3 hours toreach an ‘initial’ set. This is the point beyondwhich the mortar should no longer be used forbuilding. In practice mortar should be usedwithin two hours of mixing.

Schematics of drum-type concrete mixersand pan mill mixers are shown in Figure 1.42,and a photograph of a drum mixer is shownin Figure 1.43.

Black ash or old soft bricks were often usedas aggregates for mortar, instead of washedbuilding sand. These were placed in a pan millwhich had heavy rollers rather than paddles.These rollers first ground the brick or lumpsof ash to a powder, after which the binder andwater were added and the rollers squeezedthe mixture out to the sides and so mixed it toan even consistency.

Pivot

Trunnions

Rotating drum-type concrete mixer.The drum rotates on the pivot and tilts

up on the trunnions for mixing ordown for emptying

Paddles inside drum

Paddle

Pan Pivot

Pan-type mortar mixer.Either the pan rotates and

the paddles are fixed orvice versa

Dischargedoor

Fig. 1.42 Schematic of drum and pan mortar/concretemixing machines.

Mortar during this period was frequentlya lime mortar or a cement/lime mortar, thelime being bought in as ‘slaked lime’ and inlumps. Pieces of slaked lime could be grounddown in the pan mill with the ash or brick andwater, giving a ‘fatty’ mortar. Black ash is no

Fig. 1.43 Drum concrete mixer.




longer used as an aggregate. Corrosion of fer-rous and galvanised fittings has proved to be aproblem because of aggressive minerals fromthe black ash, notably sulphates which alsocause deterioration of the OPC binder. Soft,broken bricks are no longer used as an ag-gregate, the supply of washed building sandbeing cheap and readily available. Pan millmixers with rollers are no longer used but hor-izontal pan mixers and their modern equiva-lent, the turbo mixer, are used. Both types usepaddles to ‘stir’ the ingredients. The paddlesmay rotate with the pan fixed or the paddlesmay be fixed and the pan rotates.

To ensure correct and consistent propor-tions in the mix a gauging box should be used.This is a four-sided box which is placed on aclean board and filled once with binder. Thebox is lifted up, replaced on the board andfilled, say, five times with sand. The six vol-umes are shovelled into the mixer and the re-sult is a 1:5 mortar. Figure 1.44 illustrates thetechnique.

Mortar can be mixed by hand but onlyfor small jobbing work. Consistency betweenbatches is difficult to achieve with hand mix-ing. Gauging boxes should be used and theingredients mixed dry on the board with a

Only one gauging box isused

1 volume of binder

5 volumesof fine

aggregateMixing board

This system of mixing by volumes can be used when hand mixing where mixing

actually takes place on the mixing boardOR

the board is placed in front of the concrete mixer and the measured materials areshovelled into the drum or pan for mechanical

mixing, and either discharged ontothe board or into a wheelbarrow

Fig. 1.44 Use of gauging box for proportioning ingre-dients in a mortar.

shovel, turning the pile over at least threetimes. Water is added and the whole mixed toan even consistency. Loss of binder frequentlyoccurs when hand mixing and, in conse-quence, many operatives put more OPC intothe mix just in case it is lost. The loss is care-lessness and better mixing technique shouldbe employed, with no additional binder addedas the mix might well end up being too strong.

Good or bad weather

Conditions affecting the building of masonryare hot dry weather, frost and snow, and rain.

Hot, dry conditions

� Hot, dry conditions dry out bricks andblocks and built masonry.

� Absorption of water from mortar increases.� Dampen down bricks and blocks and cover

stacks. Don’t soak the bricks and blocks.� Use an air entraining agent or plasticiser in

the mortar.� Cover built masonry with wet hessian to

prevent premature drying out of the mortar.

Frost and snow

� Bricks, blocks and building sands should becovered with waterproof covers before frostand snow occur. A low heat can be appliedunder the waterproof cover.

� Materials which are frozen must not be usedfor building.

� Even if materials are not frozen, buildingmust stop when the air temperature reaches4◦C on a falling thermometer.

� Building can recommence when the tem-perature reaches 4◦C on a rising thermo-meter.

� Mortar which freezes has no free water toreact with the binder to achieve a ‘set’.

� When mild conditions return the mortarwill be found to have dried out ratherthan ‘set’ and will therefore be weak andcrumbly.




� Masonry affected has to be taken down;the bricks and blocks can be cleaned andrebuilt.

� It is important that the site has an accuratethermometer displayed outside the site of-fice during the winter.

� If a clerk of works11 has been appointedto the site he will keep a written recordof weather conditions including twice dailytemperature readings plus max./min. read-ings every 24 hours.

11 A clerk of works is the architect’s representative onsite. He is employed, usually on large or high valuesites, as a full time, on site, representative. He usuallyhas wide authority to approve or condemn workman-ship etc. and can be of inestimable assistance to all theprofessionals who only visit the site at intervals.

Rain

� Rain makes bricks and blocks unsuitablefor building, the excessive moisture con-tent leaving mortar in beds and joints sosloppy that it runs out – particularly thebinder, leaving a weakened mortar behindand trails of binder down the face of thewall.

� If frost follows rain, then the water in themortar and the units may freeze.

� Masonry just built should be protectedfrom penetration by rain. Waterproof cov-ers should be placed on top of the wall andweighted down. A good overhang beyondthe face of the wall may be necessary. A fullcovering on the windward face may be nec-essary.

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2 Substructures

Excavation generally 34Topsoil 35Subsoils 36

General categorisation of subsoils andtheir loadbearing capacities 37

Foundations 37The principal considerations 38Simple foundation calculations 39The mass of buildings 39Mass, load and bearing capacity 40Foundation width and thickness 41Reinforced concrete foundations 44Failure of wide, thin, strip foundations 44Trench fill foundations 45Critical levels and depths 46Level 46Finished ground level 47Bearing strata 48Depths and levels 48Step in foundation 49

Setting out 49The site plan 49Where do we put the building? 49

Equipment required for basicsetting out 49

Setting out procedure 50Excavation 53

Marking out the excavation 53Excavation for and placing concrete

foundations – and not wastingmoney doing it 53

Building masonry walls from foundationup to DPC level 57

Ground floor construction 59Detail drawings 59

Wall–floor interfaces generally 62Precautions 62

Solid concrete floors 62Single and double layer concrete floors

with hollow masonry wall 62Hung floors 64

Hung timber floors 64Hung timber floor alternatives 66Hung concrete floors 67

Blockwork substructure 71

Buildings can be divided into two parts: thepart generally below the ground floor andwhich extends down into the ground, and thepart above the ground floor, which is termedthe superstructure. This chapter is, therefore,quite a long one as it deals with ground con-ditions into which a building is set, right upto and including ground floor construction.

The sequence of events which will be dis-cussed in this chapter begins with an exam-ination of the ground conditions on a site,the excavations which must be done, the con-crete foundations which must be placed, thewalls built off these foundations, a variety of

ground floor constructions, and treatment ofthe ground inside the building. Besides this,the chapter describes how the builder willplace the building on the site with a gooddegree of accuracy, and details of techniquesin general for that work are described in Ap-pendix B.

Excavation generally

Generally speaking, ground conditions are di-vided between two types of soil: vegetable soilor topsoil and subsoil.

34

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Substructures 35

Over much of the world’s land surface veg-etation grows, and the layer of soil in which itgrows is what we refer to as topsoil. Althoughthe basic ‘ingredients’ of topsoil are mineralin origin, it contains a very high proportion oforganic matter as well as bacteria, insects andother creatures such as worms.

Below the layer of topsoil there is a thin layerof material which is neither topsoil nor subsoilbut is considered to be topsoil for our pur-poses in building construction. It is a transi-tional layer where the two forms of soil meet.The subsoil layer lies below and has no or-ganic constituents in its make-up, although itmight be home to living creatures that burrowbelow the topsoil.

Topsoil

Topsoil must be removed before a buildingis built. This is a requirement of all BuildingRegulations. The reasons are threefold:

(1) Topsoil has no loadbearing capacity andtherefore cannot hold a building up, norhas it sufficient cohesion, depth or massto allow the anchoring of a building in it.

(2) Topsoil contains organic matter which ifleft under the building would rot andcause a health hazard and/or give offmethane gas which could cause an ex-plosion, or hydrogen sulphide which hasan unpleasant odour as well as beinga health hazard. Rotting vegetation alsoattracts vermin, particularly insects, andthis could be a source of disease etc.

(3) Vegetable roots, bulbs, corms, seeds or tu-bers left behind could sprout under thebuilding and cause damage to any treat-ment applied over the area of the build-ing, particularly damp proof layers. Rootscan penetrate drainage and ducting sys-tems, blocking or disrupting supplies ordischarges.

The author remembers his father laying a con-crete floor for a garage. The concrete was laid

Fig. 2.1 Site with topsoil removed – concrete founda-tion in foreground.

over a deep layer of compacted ashes obtainedfrom a neighbouring farm. Some time afterthe floor had set hard it showed evidence oflocalised cracking and eventually sprouts ap-peared through the cracks. The spot was dugout and underneath were found a few pota-toes buried in the ashes. It seems incrediblethat a potato could force its way through aconcrete slab but this was during World WarII and cement was in short supply, so perhapsit was not a particularly strong concrete.

Figure 2.1 shows a building site with the top-soil removed and concrete foundations for ahouse in the foreground.

The thickness of the topsoil layer will obvi-ously affect the cost of the building. The morethere is to remove, the costlier the exercise be-comes. So how thick is that layer? Generallythe layer of topsoil is fairly uniform acrossmuch of the UK, at around 200 mm where theground has been cultivated, down to around100–150 mm for naturally occurring land withgood vegetable growth. Of course there arelarge areas where there is only heather grow-ing, large areas of heathland, and large areaswith peat and little growing on it. Also thereare sites where there has been demolition andfilling in, commercial dumping or filling in ofdepressions, holes, quarries, etc. There couldbe little or no topsoil in these areas, althoughthere may well be vegetation growing. We willnot go into foundations appropriate to theseextreme conditions but will deal only with asimple topsoil layer over a stable subsoil.

One of the problems with presenting drawninformation is that many of the drawings are

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prepared with no particular site in mind, andthe designer therefore has to assume some‘standard’ which can be followed by anyonetrying to interpret the drawings. This idea ofhaving a standard is something which willcrop up on other occasions as we progressthrough the text. The ‘standard’ thicknessadopted for topsoil is usually 150 mm, so thatis what we will adopt in any drawings in thistext.

When only one building is being erected it isusual to remove the topsoil from the immedi-ate area of the building, but with mechanicalexcavators so readily and cheaply availableit is much more usual to follow the ‘big site’practice and remove the topsoil from the areaof the site. The topsoil is most often requiredfor the garden ground around the buildingand so must be preserved in some way. Anyexcavated material is called spoil and can bedisposed of in a number of ways:

� It can be removed from the site to a placewhere it is either disposed of, or stored, orused.

� It can be immediately spread around thesite being worked on. This is accompaniedby the process of bringing the surface toa particular level and so is usually termedspreading and levelling.

� It can be put into heaps called spoil heapsand re-excavated at a later date for use onthe site being worked on. Spoil heaps of top-soil should not be higher than approx 2.50 mas storage in higher heaps can be detrimen-tal to the soil in the longer term.

Subsoils

Subsoils can be divided into three groups:

� Those capable of carrying the load of sim-ple low-rise buildings1 without the needfor special techniques or precautions

1 A low rise building is one generally not exceeding twostoreys in height plus a pitched roof.

� Those capable of carrying the loads im-posed by much larger buildings with ap-propriate foundation techniques

� Those requiring special techniques or pre-cautions for even the most simple of struc-tures.

We will concentrate on the first category.But first the subsoil. Subsoil does not start

at a line dividing it from the topsoil. There isa gradual transition from one to the other andgenerally occupies a layer 50–75 mm thick.This transitional layer is not generally suitablefor building on and so the rule is that founda-tions are placed in the subsoil, not on top of it.

So, is there a standard thickness of subsoil?No there is not, simply because in theory thereis no real end to the subsoil, but for practi-cal construction purposes it can hardly extendinto the hotter depths of the earth’s crust. Isthere a standard kind of subsoil? Again no;subsoil varies across the world, different kindsof soil being encountered at different depthsand for different thicknesses.

No matter what the thickness or kind of sub-soil, the most important consideration for any-one placing foundations in it is the loadbear-ing capacity, which can be defined as the forceacting on a unit area which will cause the foun-dation to just fail. So we need to have a marginof safety and arrive at the safe loadbearingcapacity. Where we have only our eyes andexperience to guide us, a good rule of thumbis to halve the loadbearing capacity we mightassess from visual examination and use thisas the safe loadbearing capacity.

Loadbearing capacity depends on a numberof factors, not all of them applicable to everykind of subsoil:

� Kind of subsoil� Thickness of the layer of that subsoil� Kind and thickness of layers underlying the

subsoil� Degree of compaction of the subsoil layers� Moisture content and general water level� Degree of containment of the layers� The presence or not of underground flow-

ing water.

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Substructures 37

Ever since textbooks on construction havebeen written, attempts have been made byvarious authors to define what they conceiveas standard types of subsoil. They havesucceeded by and large and modern booksgenerally categorise subsoils in the mannergiven below, together with an approximationof the loadbearing capacity given in newtonsor kilonewtons (kN)2. For ordinary workthese loadbearing capacities are quite ade-quate provided they are converted into safeloadbearing capacities as mentioned above.Only when very thin layers of subsoil areencountered or odd placement of low capac-ity layers is met, does the ordinary builderrequire expert guidance through samplingand analysis of the subsoil. The techniquesused are well beyond the scope of this textbut are generally known collectively as ‘soilmechanics’. The specialists in this field cansample subsoil on a building site and test it toprovide a safe loadbearing capacity, and caneven go on to design the foundations for aparticular building. They can also predict theprobable cause and type of any failure whichmight occur due to overloading, variableweather conditions moisture content, etc.

General categorisation of subsoilsand their loadbearing capacities

Naturally Building Regulations have a lot tosay about loadbearing capacities. This usuallytakes the form of a reference, including tableswhich give widths of strip foundations in a va-riety of soil types for a range of loads. In thetables soils are generally classed as gravel,sand, silty sand, clayey sand, silt and rock.All of these must then be assessed accord-ing to their condition – compact, stiff, firm,loose, soft and very soft. Only rock allows afoundation the same width as the wall. The

2 A newton is the force exerted by a mass of one kilogramacting with the force of gravity, g. So if we know thata mass of 1 kg is laid on the ground, the force exertedby that mass will be 1 × 9.81 m/sec2 = 9.81 newtons.The kilonewton is derived from this and is equivalentto 1000 newtons.

others require a minimum width – given inthe table – depending on the subsoil and itscondition. A range of wall loadings is given,and the various widths allowable are notedunderneath.

These tables are not too different from thetables which have been promoted in a vari-ety of textbooks over the past 40–50 years, allbased on Building Research Station Digests64 and 67. As a simple example, one tablequoted in a set of Regulations gives a widthof 400 with a wall load of 40 kN per metrein a subsoil of compact sand or gravel. Aswill be seen when we consider simple widthcalculations a little later, this means that thesafe bearing capacity of the subsoil is con-sidered to be 100 kN/m2. And this is despitethe fact that maximum loadbearing capacityfor these subsoils is quoted in many texts as>300 and >600 kN/m2 respectively! By therule of thumb quoted above, the safe bear-ing capacities of the soils would be >150 and>300 kN/m2 respectively, which would indi-cate that the figures quoted in the Regulationsfor foundation widths and safe bearing capac-ities have a much higher margin of safety.

Foundations

Now that we have had a brief look at soils wereally must consider what we mean by foun-dations. There is clearly a wide variation inthe loadbearing characteristics of the soil cat-egories in the tables mentioned above, and weneed techniques to allow us to build in the ma-jority of situations these will present.

First, it has to be said that foundations aregenerally made by pouring wet concrete intoholes in the ground. The shape involved canbe simple or complicated and everything inbetween. We are really only concerned withsimple shapes – long rectilinear pieces of con-crete cast in the ground and presenting a hor-izontal surface on which walls can be built.

Concrete is discussed in more detail in Ap-pendix D and this should be read if the dis-cussion in this chapter gets away from you.

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The better the loadbearing capacity, the sim-pler the foundations may be. If we weredealing with a stable rock formation, thenthere might be no formal foundation con-structed. Instead the rock would be levelledoff and the walls or frame built straight offthe rock surface. The majority of foundationsare of two main types:

� Strip foundations3

� Deep strip or trench fill foundations.

In strip foundations a relatively wide trench(500–750 wide – wider than the walls) is dugand a layer of concrete poured in, to a thick-ness of at least 150. Walls are built off this layerof concrete. In deep strip foundations a nar-row trench is dug (only slightly wider thanthe walls) and filled with concrete almost toground level. The walls are then built startingalmost at ground level. We will study thesetwo types of foundation in greater detail laterin this chapter. The other extreme would besoft ground to some depth overlying a firmstrata with good loadbearing characteristics.In a situation like this it might be best to drivepiles into the soft ground until they sit in thebearing strata. Piles are long columns of ma-terial which can be driven into the ground, orthey can be cast in concrete and combinationsof concrete and steel tubes sunk into holes‘drilled’ into the ground. Once in, the tops ofthese piles are linked together with beams ofconcrete and the walls then built on the beams.

This is a very simple explanation of a com-plex process which has more than one way ofbeing achieved. The piles themselves can bemade of a variety of materials – concrete bothprecast and cast in situ, steel of a variety ofsections, and even timber. On the other hand,the poor layer of soil might not be too badand provided we can build sufficient area offoundation, we can spread the building loadthinly over this poor soil. In this case we arelooking at a raft foundation. This involvesplacing a layer of concrete over the area re-quired to support the building. Note that thisarea could be much larger than the building

3 Figure 2.1 showing a building site has a house founda-tion in the foreground which is a strip foundation.

but would be at least equal to the building’sfootprint. Then we could get into the moreesoteric types of foundation: for example wecould make a large box of concrete, divided in-ternally into closed cells, and ‘float’ this on thesite of the building. The individual cells wouldhave valves which allow air to be bled off andwater or silt to enter. Using these valves thebox is sunk into the ground to the requiredlevel and the building built on top4.

Obviously some of these solutions are veryexpensive and it would depend greatly on thetotal value of the building being built, as wellas how urgent the need for that building, be-fore making a final decision on whether or notto build at all.

The principal considerations

Foundations are placed in the subsoil to:

� Provide a base on which the building can bebuilt so that it will not sink into the ground

� Prevent a building being lifted out of or offthe ground

� A combination of both of the above.

The regulations further stipulate that thefoundations must not be at risk from move-ment in the subsoil. This movement can becaused by two mechanisms:

(1) The subsoil takes up or loses moisture andas a result it swells or shrinks, causing thebuilding to be displaced. Plants take upa great deal of moisture from the groundand in transpiration this passes into the at-mosphere. Trees are major users of waterand many species such as poplars and wil-lows take up more than most. Thus theycan take moisture from the subsoil dur-ing drought conditions, which can causethe subsoil to shrink. Trees are best keptwell away from buildings – or vice versa –where shrinkable clay is encountered.

(2) Moisture in the subsoil freezes and causesthe building to be displaced.

4 The BP refinery at Grangemouth is one of the largestof its type in Europe. Many of the buildings and petro-chemical plant are built this way.

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Substructures 39

The first mechanism is a feature of shrinkableclays, which exhibit extreme swelling andshrinkage as their moisture content varies.These clays are distributed mainly in a widearea across south-east England5. Until recentyears it was considered adequate to place thebottom of any foundation 1000 mm below thefinished ground level to get below the point atwhich the moisture content will vary either bydrying out or by absorption of additional wa-ter. The sides of the foundation and any wallsbuilt below ground could also be affected andthere had to be a crushable layer of materialat the sides of the foundation and the wallbelow ground level. However, ever drier andhotter summers have placed some doubt onthe efficacy of the 1000 mm depth and manyprofessionals now argue for a greater depth.

The second mechanism is a feature of allsubsoils and the only way to avoid frostheave6 is to keep the bottom of the founda-tions below a depth to which frost can pene-trate. This is generally held to be 450 mm.Some parts of the UK regularly have frostspenetrating to this depth, while it happensvery occasionally across wide areas. Localknowledge is not the only guide in this matter;the Meteorological Office has information onweather patterns across the UK and has a web-site at www.met-office.gov.uk/construction/pastdata5.html or www.met-office.gov.uk/construction/pastdata3.html

Simple foundation calculations

In the construction forms – brick and timberframe – which we will study, there is sufficientmass inherent in the form to obviate the needfor the foundation to keep the building in theground. So, we will be principally concernedwith providing adequate support.

This does not mean that within the con-struction form we can ignore the need to tieportions of the construction together or to

5 For further information on the distribution of soil typesacross the UK, contact the British Geological Survey byvisiting their website at www.bgs.ac.uk

6 So-called because water when it freezes expands andso causes the ground to expand and lift upwards.

the foundation. To determine the amount andtype of support we need to know:

� The mass we intend to support, for whichwe need to know the construction form tobe adopted

� The bearing capacity of the subsoil on theconstruction site.

The mass of buildings

Mass of buildings depends on the construc-tion form adopted, the building type and theend use of the building. Calculation of massis not difficult and a few steps will now beoutlined.

To give the reader some idea of the range ofmasses, consider some contrasting buildings:

� Two-storey house with brick cavity walls,timber floors, timber roof with concretetile covering. This is the medium to heavyweight type of low-rise building. Wallmasses where upper floors and roof haveto be taken into account range from 2500–3000 kg/m of wall.

� Timber frame house with brick or renderedblockwork cladding, timber floors, timberroof with concrete tile covering. This is alight to medium weight type of low risebuilding. Wall masses where upper floorsand roof have to be taken into account rangefrom 1500–2500 kg/m of wall.

� Timber frame single or two-storey ‘clubhouse’ with timber or ‘plastics’ weatherboard cladding, pitched roof with bitumi-nous shingles. This is a light weight con-struction. Wall masses where upper floorsand roof have to be taken into account rangefrom 800–2000 kg/m of wall.

� Only an unlined timber garage or gardenshed would be lighter.

The mass of a building is transferred to thefoundations through the walls. The externalwalls carry the bulk of this load and we nor-mally only calculate foundation size for thisamount. The building load on the foundations

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comes about because of the following:

� Walls which in turn support floors androofs.

� Roofs bear snow loads (rain also adds to theload but not as much as snow and wind soit is ignored).

� Wind loads affect the walls and roofs ofbuildings but mainly the roofs where theloads can be positive as well as negative –positive loads pushing against the walls ordown on the roof, and negative loads tryingto suck the walls out or the roof. When thewind blows, both types of load are experi-enced by the walls and roof.

� Floors bear dead and live loads – dead loadsare those imposed by the masses of otherparts of the structure such as partitions builtoff the floor and things like furniture andfittings; live loads are those imposed bymasses of the occupants, the dog and thecat, and these occupants moving about orjumping up and down.

Those walls which support both floor(s) androof will have the heaviest loadings. Normallywe calculate only for these walls and makeall other foundations the same. Where theremight be a considerable saving in excava-tion and concrete, more individual calcula-tions might be done.

Mass, load and bearing capacity

The mass in the walls is normally calculatedon a ‘per metre’ length of wall basis, measuredin kilograms (kg). The load for a given mass ismeasured in newtons, i.e. 1 newton = 1 kg × g(the acceleration due to gravity − 9.81 m/sec2). It is more usual to employ the derived SIunit of kilonewtons (kN), i.e. 1000 newtons.

Bearing capacities are also given in kilo-newtons per square metre. Safe bearing capac-ities should be taken as half of that figure ifonly a visual examination of the subsoil hastaken place. This allows not only a factor ofsafety but also for variations in strength dueto disturbance and varying moisture content.

A simple calculation will give the foun-dation area required to support a building’swalls or, for a length of 1 metre of wall, willgive us the width of the foundation. Considerthe following.

A building has a mass at the foot of the wallsof 10 200 kg/metre length of wall. The wallswill be 265 thick. The subsoil identified visu-ally has a bearing capacity of 300 kN/m2, soa safe loadbearing capacity (SLC) would be150 kN/m2. To begin the calculation of thewidth of the foundation required we take themass of wall −10 200 kg/m – and convert thatto the imposed load per metre:

10 200 × 9.81 N/m = 100 kN/m

The width of foundation required is:

Load per metre 100 kN/mSafe bearing capacity/m2 150 kN/m2

= 0.66 m

= 660 mm

In the short discussion earlier regarding sub-soil types and loadbearing capacities, a foun-dation width of 400 was given for a wall witha load of 40 kN. Applying the above formulawe can calculate the SLC:

Load per metre 40 kN/mSLC

=400 mm or 0.40 m

So: SLC = 400.40

= 100 kN/m2

This seems somewhat at odds with the load-bearing capacities quoted for the soils in ex-cess of 300 and 600 kN/m2 and our own con-tention that we should halve these for visualassessment of safe loadbearing capacity.

The thickness of the walls was given aboveand yet played no part in the calculation. Itwas included deliberately to show that foun-dation width is not necessarily related to wallthickness. In this case the foundation willproject beyond the faces of the wall by ap-proximately 200 and this does have a bearingon the thickness of the foundation – as the nextsection will illustrate.

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Substructures 41

Foundation width and thickness

First, we make no apology for repeating someof the important points already mentioned inthe text. The apocryphal tale comes to mindof the sergeant major who was congratulatedon how well his recruits were trained and hadthis to say, ‘First Ah tells ’em wot it is Ah’mgonna tell ’em, then Ah tells ’em, then Ah tells’em wot Ah told ‘em’.

So for at least the second and not the lasttime in this chapter:

� Foundations must be placed in a bearingstratum so that the full bearing potential ofthe stratum is exploited.

� Note the zone between two strata wheremixing can occur and which is not suitablefor foundations.

Both these points are included in Figure 2.2.

� The bottom of the foundation must be deepenough to avoid frost heave. This depth canvary with local weather conditions but gen-erally 450 mm is sufficient in the UK and isillustrated in Figure 2.3.

� In shrinkable clays, the bottom must bedeep enough to avoid heave due to changesin moisture content of the clay, as shown in

Groundlevel

Mixed layer

Upper limit of mixing

Lower limit of mixing keptabove bottom of

foundation

Masonry wall

Concrete foundation

Fig. 2.2 Foundation penetrating mixed layer into sub-soil.

450

Ground level

Fig. 2.3 Minimum depth to avoid frost heave.

Figure 2.4. In all but the most severe condi-tions 1000 mm is sufficient.

� Very narrow trenches are difficult to workin while building walls and once the depthexceeds 700–800 it becomes almost impossi-ble. So the cladding of foundation sides andbackfilling with crushable material shownin Figure 2.5 is only possible when the

Ground level

1000 min.

Fig. 2.4 Minimum depth in shrinkable clay.

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Claysubsoil

Expanded polystyrenebeadboard 30--50 thick

Groundlevel

Crushable, granularfill material both sides

Fig. 2.5 Protection from heave in shrinkable clay.

foundation is of a reasonable width. (Seedeep strip foundations later in this chapter.)

� In shrinkable clay, the sides of founda-tions and the walls on them must be pro-tected from subsoil movement. Expandedpolystyrene7 is frequently used, and fill-ing round the walls is done with granular,crushable material which will in all prob-ability have to be imported onto the site.The use of this material is illustrated in Fig-ure 2.5.

� When using bearing capacities quoted intables and textbooks combined with a vi-sual inspection only, the capacities shouldbe halved as discussed earlier.

We will study two types of foundation: stripfoundations and deep strip or trench fillfoundations. The method of calculating widthis the same for both types. Thickness de-pends primarily on type but for main wall,strip foundations is never less than 150 mm.Figure 2.6 shows a typical section through astrip foundation.

7 The product used is a ‘beadboard’ made by compress-ing beads of expanded polystyrene so that they adhereinto a board or sheet material. It is the same materialwhich is frequently moulded into packaging for fragilemerchandise.

Ground floorconstruction

Wallwidth

Outside of building Inside of buillding

Ground level

Thickness

Width calculated

Footings?

Fig. 2.6 Foundation with width calculated – thick-ness?

Width is calculated as before and thicknessdepends on:

� wall width� whether the concrete is to be plain or rein-

forced� whether or not there will be footings.

Assuming that the concrete will be plain andthere will be no footings, thickness must beequal to the projection of the concrete fromthe face of the wall, as shown in Figure 2.7.

Note that walls are always centred on thefoundation and that any additional widthof concrete in a plain foundation which ex-tends beyond the point where the dotted linereaches the bottom, would, in theory, tendto break off. To be effective the extra widthwould have to involve further techniques tostrengthen the concrete.

For a given wall thickness the width re-quired for a foundation increases as:

� wall load increases, and/or� bearing capacity of soil decreases.

As the width increases so does the thickness,as illustrated in Figure 2.8. There comes a time

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Substructures 43

Centreline

Centreline

Projection

Thickness45°

Fig. 2.7 Calculation of thickness of plain foundation.

Centreline

Centreline

Projection

Thi

ckne

ss

45°

Fig. 2.8 Increasing width, increasing thickness.

when it is more economic to consider a courseof action other than further increasing thick-ness. Dictated mainly by economics, two pos-sibilities are open to the designer:

� Use footings� Reinforce the concrete.

Footings are formed by building courses ofbrickwork, each course being half a brickwider than the one above, as shown in Fig-ure 2.9. The additional width is always evenlydistributed on each side. The footings coursesare usually laid brick on edge. The net resultis the spreading of the load from the wall overa widening area of brickwork and then ontoa relatively thin, wide layer of concrete. Thethickness of the concrete equals the projectionas before.

There is an economic limit to the numberof courses of footings one would place. Thatlimit varies as the relative costs of brickworkand the alternatives change with time. Quan-tity surveyors should be capable of calculatingguide figures appropriate at the time a build-ing is being designed.

The second alternative is reinforcement ofthe foundation.

Six coursefooting

Each course projects quarter

of a brick

Projection ofconcrete

Thicknessof

concrete 45°

Fig. 2.9 Wall on footing on plain concrete foundation.

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Reinforced concrete foundations

The use of reinforcement bars is generally con-fined to the foundations for larger structures.Fabric is generally adequate for reinforcingstrip foundations to low rise buildings.

Fabric is supplied in sheets, available in sev-eral standard sizes which may be cut to suitthe width and length of the foundation be-ing laid. Individual pieces must overlap withadjacent pieces by at least one ‘cell’. Cornersand intersections should fully overlap. Lapsshould be tied together with 1.6 mm soft ironwire ties. If a large number or quantity of foun-dations is being laid, it can be more economicto have the fabric supplied in roll form, thewidth being set to suit the foundation width.

Failure of wide, thin, stripfoundations

There are two modes of failure:

� Bending as shown in Figure 2.10� Punching shear as shown in Figure 2.11.

To avoid failure by bending, fabric must beplaced near the bottom of the concrete, asshown in Figure 2.12. Concrete is not strong intension but the fabric is and so the elongationof the bottom layer is prevented or controlled.Concrete is strong in compression and gener-ally does not require reinforcement in the toplayer to prevent failure by crushing. However,the narrow wall concentrates its load over asmall area of concrete and thus the wall couldpunch its way through the strip foundation.Fabric in the bottom alone cannot resist thisand so another layer is placed in the top, asshown in Figure 2.13.

In theory, the bottom layer has to be thefull width of the strip foundation and the toplayer can be narrower. In practice, unless thereis an excessively wide foundation, both lay-ers are the same width. The financial effect isminimal. The same type of fabric should beused for both layers, thus preventing errors

As concrete bendstop layer is incompression

Bottom layerin tension

Tension increases and concrete cracks

Fig. 2.10 Bending failure of plain concrete foundation.

Wall punchesout piece offoundation

Load on wallspread through

foundation at 45°

Fig. 2.11 Shear failure of plain concrete foundation.

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Substructures 45

Reinforcement in bottom prevents bottom layer from cracking in tension

Fig. 2.12 Reinforcement of bottom layer of concretefoundation.

in placing; don’t think things like that neverhappen – they do with monotonous regularity.

Trench fill foundations

Trench fill is sometimes referred to as deepstrip but to avoid confusion this text will referonly to trench fill.

In cohesive soils (which generally have anadequate loadbearing capacity) it is possibleto require a foundation to be little wider thanthe wall to be built. Money can be saved

Top layer ofreinforcement

Theoretical limit ofwidth of top layerof reinforcement

Fig. 2.13 Top layer reinforcement of concrete foun-dation.

then by digging a very narrow trench with amechanical excavator equipped with a nar-row ‘bucket’. Trench depth must still be main-tained so as to reach into the bearing strata andto avoid frost and/or clay heave.

Because of the cohesive nature of the soil,the sides will remain stable until concrete ispoured in, provided there is the minimum ofdelay between digging and filling and no in-tervening adverse weather conditions. Con-crete is filled in to a point just below the fin-ished ground level.

The operatives do not need to work in thetrench as is the case with strip foundations.Support for the foundation is not just fromthe bottom of the trench but can also be fromthe friction of the sides of the foundationin the ground. This is illustrated in Figure 2.14.

Centre line

Ground level

Fig. 2.14 Deep strip foundation.

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Crushablelayer

Ground level

Centreline

With both sides lined,support is entirely from the

bottom of the concrete

Fig. 2.15 Deep strip foundation in shrinkable clay.

Trench fill foundations do not fail by bend-ing or punching shear. Shrinkable clay is oftensuitable for trench fill foundations. In this casethe sides of the foundations should be linedwith a ‘crushable’ material such as expandedpolystyrene sheet. The additional work is il-lustrated in Figure 2.15.

Critical levels and depths

From our study so far it should be obviousthat there are a number of critical levels andheights which depend on:

� The lie of the ground, i.e. how it slopes etc.� How it will be finished around the building� The nature of the subsoil� The need to protect the foundation from

heave.

Level

The word level can be applied in two ways:

� We can refer to a ‘level’ when we refer toan absolute height above a datum or to aheight relative to some other object.

� We can describe a surface as ‘level’ when wedescribe a horizontal surface.

The most common form of datum is anOrdnance Survey (OS) benchmark. (See alsoAppendices A and B, which deal with Mapsand Plans and Levelling.) These marks carvedinto corners of buildings etc. are related tosea level at Newlyn in Cornwall. This is thedatum for all OS levelling in the UK. Themark comprises two parts: the deeply incisedhorizontal line, the centre of which is the ac-tual level referred to on OS maps, and thebroad arrow symbol underneath the line. Thearrow is a British Government symbol usedon everything from prison clothing to armyweapons.

Figure 2.16 is a sketch of a typical bench-mark. Photographs of various benchmarks areshown in Appendix B.

Temporary benchmarks are frequently usedin building work, and two examples areshown in Figure 2.17. They may relate to OSlevels or to an arbitrary value. Levels are givenin metres to two places of decimals, e.g. 84.56.

As shown in Figure 2.17, a wooden pegdriven into the ground and set round with

This is the 'level'

Fig. 2.16 Sketch of benchmark symbol.

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Substructures 47

Wooden peg in concreteblock

Using the corner of manholecover frame

Fig. 2.17 Temporary benchmarks.

concrete can be used as a temporary bench-mark. Even a length of steel angle iron can betreated this way. The pegs are usually paintedin contrasting stripes and the top is usuallypainted red. Where there is a danger that thebenchmark might be disturbed, a low fenceor rail can be built around it. The corner ofthe manhole (usually a cast iron cover) is afavourite with many surveyors but the cornerof the frame should be marked, with paint,not the actual lid.

Finished ground level

By ‘finished ground level’ we mean the heightof the ground above a datum after all excava-tion and/or filling work and the spreading oftopsoil has been carried out around the build-ing. The ground around a building does notalways need to present a horizontal surface,and the finished surface can be higher or lowerthan the original ground level.

Original and finishedground level

450

With a bearing at 400, the foundation

need only be set at 450 below ground level to avoid heave

from frost

Fig. 2.18 Finished ground level at same level as orig-inal ground level.

When original and finished ground level arethe same, as shown in Figure 2.18, the excava-tion depth is set by:

� Need to reach a bearing� Need for cover to prevent heave.

Frost cover dictates that the bottom of thefoundation is 450 below finished groundlevel, as shown in Figure 2.19. There should beno difficulty reaching a bearing with that min-imum depth. The foundation bottom must

Original ground level


450

Excavation is much more than

the 450depth required

for cover

Fig. 2.19 Finished ground level at lower level thanoriginal ground level.

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Original ground level

FIL

L

450 min.

Fig. 2.20 Finished ground level at higher level thanoriginal ground level.

reach down to bearing strata, as shown in Fig-ure 2.20. Cover is generally no problem. Notethe large amount of filling.

Bearing strata

A bearing may not be found at a convenientdepth below finished ground level. Therewill be occasions when the foundation mustbe deeper than the minima previously illus-trated, and also site surfaces are seldom trulyhorizontal, nor do bearing strata follow thesame angle of slope.

Assuming a level site, strata may be hor-izontal, as illustrated earlier, or strata mayslope slightly, moderately or steeply, as illus-trated in Figure 2.21.

If the site surface slopes, strata may be levelor strata may slope with or against the sitesurface, as illustrated in Figure 2.22.

Depths and levels

Depth of foundation below finished groundlevel is controlled by:

Finished ground level shown as a series of thin linesat various slopes

Bottom of foundation shown as heavy horizontal or stepped line

As the groundslope increases, thefoundation will have more stepsever closer together to keep bottom of foundation in the bearing stratum. Excavation bottom is always level and the foundations are always level

Fig. 2.21 Bearing strata sloping in relation to groundlevel (1).

� The need for frost cover or to avoid clayheave

� The need for the foundation to be in thebearing stratum

� The slope of the finished ground


Bearing stratum

Bottom of foundation


Bearing stratum

Bottom of foundation

Fig. 2.22 Bearing strata sloping in relation to groundlevel (2).

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Substructures 49

Height ofstep

Thickness offoundation

Lap

Symbol for concretein section

Fig. 2.23 Step in foundation – the rules.

� The slope of the bearing stratum� The need to be economic – no excessive

excavation; without stepped foundationsthere would be too much excavation.

Step in foundation

In very steeply sloping strata, the steps maynot just be frequent but the height of the indi-vidual steps may be great. For plain concrete,the step height should be restricted to theequivalent of the foundation thickness, oth-erwise the laps, and perhaps the whole foun-dation, should be reinforced with steel fabric.The step is illustrated in Figure 2.23.

The lap must equal the thickness, twicethe height or 300, whichever is greatest. Theheight of the step should be set to a multiple ofthe masonry unit height – for standard metricbricks this means multiples of 75.

The vertical faces of the step in the founda-tion need support while the concrete sets. Thiscan be done simply by placing bricks acrossthe foundation after allowing the lower layerto set a little, or for large steps timber form-work has to be erected and fixed to pins setinto the bottom of the trench.

Setting out

The site plan

If the reader has not already done so, now isthe time to read Appendices A and B, on Mapsand plans, and Levelling.

A plan of the site usually to a scale of 1:500is drawn showing the existing features of thesite. To this plan are added the proposed

buildings, roads and services, any ancillaryworks such as car parks, retaining walls,screen walls and landscaping work; newground levels may also be shown.

Where do we put the building?

The site plan so carefully and laboriously pre-pared is just so much waste paper if the menon site cannot place the building where theplan shows it to be. Foundations must beplaced at the correct level and ground floorsat the correct height above finished groundlevel. All depend on the correct transfer of theproposals from plan to the physical site in aproper, well ordered manner.

When planning the site layout, the archi-tect or engineer is frequently constrained by‘building lines’, i.e, the front of the buildinghas to be sited on an imaginary line drawnon the site surface. This line should be shownon the site plan, with its position relative toknown fixed points clearly marked.

Other examples of lines from which refer-ence can be made when siting a building are:

� A road or pavement kerb line� Extension of the frontage line of existing

buildings.

A typical site plan is shown in Figure 2.24.There may be no such building line and the

corners of the plan of the building may bemarked by dimensions taken from at least twoknown fixed points.

Equipment required for basicsetting out

� Dumpy level, tripod and staff� 50 × 50 wooden pegs 600–750 mm long� 2 lb hammer and a claw hammer� 65 mm plain wire nails� Two steel measuring tapes 30 or 100 m long� Builder’s line and builder’s level� Measuring rods� Crosscut hand saw� 94 × 19 wrot boarding.

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BM 74.56Metalpin inwall

Telegraph pole Corner of gate pillar

Stone wall

Road

Verge12

860

22 480 16 25014 500

Building line3 4003 400

House 3FFL 78.50

House 2FFL 79.00

House 1FFL 79.35

Overall dimensions of houses obtained from house plans

Fig. 2.24 1:500 site plan.

Setting out procedure

Walk round the site and identify the fixedreference points used for the original survey.Look for nails, pins, hooks or loops, marks,etc. to or from which measurements weretaken. Confirm these points by re-measuring.

Initially the ends of building lines and cor-ners of walls of new building works aremarked by placing a wooden peg in theground and placing a nail in the top of thepeg, as shown in Figure 2.25. This may ap-pear a very primitive way to mark out a spotin the middle of a building site, but it has beenused for many decades and no better way hasbeen found.

If there is no building line, mark the cor-ners of the building(s) by placing pegs in theground. Figure 2.26 illustrates how it is done.Using a steel tape from each of the knownfixed points, measure the distance given onthe site plan to the end(s) of the base lineor building corner. Where readings coincide,drive in a peg. Measure again across the topof the peg and put in a nail.

In the case of a building line, with the po-sition of the ends fixed, stretch a builder’sline from nail to nail, and using a steel tape

measure along the line to establish buildingcorners. Mark each corner with a peg and nail,as shown in Figure 2.27.

Having found two corners of the proposedbuilding, set out and mark the others. Use

Peg driveninto ground

Nail driven intotop of peg

Fig. 2.25 Peg with nail, used to mark corner of a build-ing on site.

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Substructures 51

First fixed point--a telegraph pole

Second fixed point -- thegate post. Make sure you

use the correct one!

Steeltape

Steeltape

Nail

Peg

Fig. 2.26 Finding position of peg from two points.

the Pythagoras theorem in any combination ofright angled triangles, as shown in Figure 2.28.

Having placed pegs at all four corners,check their positions by measuring the diag-onals as shown in Figure 2.29. The lengths ofthe diagonals should be within 6 mm of eachother on a medium size house.

We now know where the corners of thebuilding’s walls will be. When we place foun-dations we excavate and place concrete inthe bottom. Both excavation and concrete arewider than the walls. The pegs marking thecorners will therefore be displaced and welose our markers. This is shown in Figure 2.30.

We need to improve on these first basic mar-kers so that we have reference points which:

End ofbuilding line

Firstcorner

Secondcorner

End ofbuilding line

Steel tape

Fig. 2.27 Pegs on the building line marking building corners.

� Are not disturbed by subsequent work� Indicate vertical alignment as well as hori-

zontal� Allow repeatability of corner marks, mea-

surements and levels.

We need profile boards, as illustrated in Fig-ure 2.31. Profile boards are set up at each cor-ner of the building. Each set ‘looks’ two waysalong adjacent walls, as shown in Figure 2.32.To ‘see’ the outline of the building’s concretefoundation and walls, builder’s lines may bestretched from profile to profile. With all thatstring around, there is no room to get in toexcavate for the foundation, but see how ex-cavations are marked out below.

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Width Width

90° 90°

Length of buildingFront line of building

Fig. 2.28 Using the Pythagoras theorem to find othercorners of building.

Peg atcorner

Outline ofproposed wall

Outline oftrench

Outline ofconcrete

Peg atcorner

Line offace of wall

Outline oftrench sides

Fig. 2.30 Losing markers for building corners when excavating.

Of course you may end up working for alarge company which can afford electronicposition and distance measurement (EPDM)equipment, and then there is no need for allthese pegs and profiles. One or two strategi-cally placed pegs with a nail in the top, well

Fig. 2.29 Checking the diagonals.

protected, are left from the first survey andsubsequently used to set out every bit of workon the site from roads and sewers to housedrains and the buildings themselves. (SeeAppendices A and B.)

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Substructures 53

Concrete foundation width

Top of profile board sethorizontal and to knownlevel (see Appendix B)

Pegs kept clear ofexcavation

Line of excavation

Outline of wall Line offace of

wall

Saw cuts in top of profile board indicate edges of concrete and both faces

of wall

Fig. 2.31 The profile board.

Excavation

Marking out the excavation

Only put up builder’s line for the outlineof excavation/concrete; people with picksand shovels or an excavator could break the

With builder's line stretching from notch to notch in opposite profile boards, the intersections of the stringsmark the corners of various parts of the substructure. The black dot marks the corner of the wall to be built and should be exactly over the original peg and nail.

Fig. 2.32 The corner of the building wall.

strings. To avoid this, sprinkle old cement,lime or sawdust on the ground, following thebuilder’s line as closely as possible. Now re-move the builder’s line.

If during work the mark on the ground iserased, string up the line and mark again. Af-ter a first rough excavation, the line can berestrung to check for alignment. This can bedone as often as is necessary.

By using the other saw cuts in the profileboard, wall outlines can be established. By us-ing the known level of the top of the profileboards, the floor levels, depth of excavationand concrete thickness can be set and main-tained.

Excavation for and placing concretefoundations – and not wastingmoney doing it

Excavation is now largely carried out by ma-chine. Generally it is done in two stages:

� Bulk excavation� Trimming to size (may be done by hand).

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SIngle, slewing, hydraulicallyoperated, articulated arm or back hoe

or back acter

Pair of hydraulicallyoperated articulated arms

Heavy lines indicate hydraulic rams to operate arms etc.

Full widthfront

bucket

Rear tyres partially filled with waterto improve running stability

Hydraulic jacks eachside

Bucket size can vary from300--750 in width

Joint

Joint

Fig. 2.33 JCB-type excavator.

Accuracy is important so as not to wastemoney:

� Too much excavation is a waste of effort andof concrete

� Too little may mean too little concrete or notenough cover against frost or clay heave.

With modern machinery it is common prac-tice to remove vegetable soil from the entiresite and store in spoil heaps for later use.Work therefore commences from subsoil level.If major adjustment to ground levels was re-quired this would be done after removing thevegetable soil and before setting up profileboards for foundation excavation.

The most common excavator used is of theJCB type – a tractor-based machine havingfront and rear buckets on hydraulically op-erated arms (see Figure 2.33).

From the preliminary site investigation, thelevel of the bearing strata will have been es-tablished, vegetable soil has been removed,ground levels have been adjusted, and pro-file boards have been erected giving a datumfor the three dimensional work about to takeplace. With the outline of the concrete markedon the ground an excavator commences work,digging out between the lines to form a trench.The digger driver must dig down into thebearing strata – not too little, not too much. He

requires some assistance and some means ofgauging how deep he has dug, so has an as-sistant called a banksman who uses a devicecalled a traveller to guide the digger driver tothe correct depth of excavation, as illustratedin Figure 2.34.

The photograph in Figure 2.35 shows twomen using a traveller between two profiles setup to gauge the excavation depth for a road.

Crossbar may beadjustable

height

Base

Banksm

an

Traveller

Fig. 2.34 Banksman and traveller.

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Substructures 55

Fig. 2.35 Banksmen and traveller between profileboards.

The man on the left is holding the traveller, theman on the right is viewing the traveller crossbar in relation to the profiles in front of andbeyond the traveller. They are checking thesubsoil level to see that enough has been ex-cavated before the first layer of hardcore goesdown as a road base. They will alter the trav-eller (shorten it) to check that the hardcore bedis the correct thickness.

Travellers are made of timber, and the criti-cal dimension is the height.The depth at whichthe concrete must be laid has already been de-termined. We know the level of the top of theprofile boards. The height of the traveller ismade slightly less than the difference betweenthese two levels. The traveller is used by plac-ing it at the bottom of the excavation andhaving the banksman sight across the pro-file boards past the cross bar on the traveller,

Ground level

Banksman

Profileboard

Traveller aboveline of sight Traveller on

line of sight Traveller belowline of sight

Profileboard

Excavation

Line of sight is horizontal since allprofile boards are set to same level

Fig. 2.36 Traveller in use in an excavation.

like the matchstick banksman in Figure 2.36. Ifthe traveller protrudes above the line of sightmore digging is required. If the traveller is be-low the line of sight too much digging hasbeen done.

The bucket on the excavator will be oneof a standard range of sizes, narrower thanthe trench being dug. On some subsoils, thebuckets used will have a toothed cutting edge.These teeth disturb the bottom of the trenchexcavation.

Setting the traveller to less than the fullheight for the first pass means that the distur-bance does not penetrate below the requiredlevel. Setting the traveller now for the fullheight allows the disturbed layer to be re-moved either by hand or with a plain edgedbucket. This is called trimming and is illus-trated in Figure 2.37.

Excavating into the ground for any pur-pose can be dangerous due to the possible col-lapse of the sides. The deeper one goes, andthe more friable the subsoil, the more dan-gerous it becomes. In certain circumstancesit is impossible to excavate at all unless oneuses timbering. This is the short-hand termfor shoring, strutting and waling – a range ofsystems to support the sides of excavations toprevent collapse and protect workmen downin the excavation. Appendix F describes thetechniques involved. Shoring, strutting andwaling is temporary work and is, by conven-tion, never shown on drawings. It will not be

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Disturbed layer

Traveller set tofull height

Line of sight across profile boards

Profileboard

Profileboard

Ground level

Fig. 2.37 Trimming bottom of excavation.

shown in the drawings included in this text,except for Appendix F.

After excavation the trench should:

� Be straight� Have a horizontal bottom� Have the bottom at the correct depth in the

bearing stratum� Have an undisturbed bottom.

Straightness is tested by restringing thebuilder’s line on the profiles and checking thesides of the excavation with a builder’s level,as shown in Figure 2.38.

Line Line

Builder's level

This side of excavation OK

This side of excavationneeds trimming

Excavation

Fig. 2.38 Trimming sides of excavation.

Laying the concrete

Once the excavation sides and bottom havebeen trimmed, the concrete should be laid, be-fore the bottom of the trench can be disturbedagain or become sodden with rain.

Special precautions are taken to deal withground water. Either the water is drained offin specially dug trenches from the low pointsin the excavation, or sumps8 are dug at thelow points of the excavation and mechanicalpumps take out the water and dispose of itoutside the excavations.

The concrete must be laid to the correctthickness:

� Too thin means a weak foundation, failingin bending and shear.

� Too thick means a waste of money on toomuch concrete.

The upper surface must be horizontal andallow full courses of masonry up to DPClevel. Once you start to pour concrete intothe trench, you lose sight of the bottom of the

8 A sump is a hole dug below the lowest point of an ex-cavation. The hole need not be excessively large – a 600sided cube might be adequate for house foundations.The end of an extract hose fitted with a strum box (acoarse perforated metal filter) is fitted on the end toprevent small stones being sucked over and damagingthe pump.

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Substructures 57

ProfilesProfiles Traveller Line of sight

Bottom of trench

All pegs set to level withtraveller

Ground level

Depth of concrete

Fig. 2.39 Using the traveller to set concrete foundation thickness.

trench and all measure of concrete thicknessso provision must be made to allow this mea-surement to be made or maintained. The pro-file boards provide us with a unique referenceline to control both thickness and the level ofthe upper surface. Reduce the height of thetraveller so that it extends from top of profileto top of concrete. Drive pegs into the bottomof the trench such that the traveller interceptsthe line of sight across the profiles, as shownin Figure 2.39. Pegs are spaced out about 1metre apart down the centre of the trench.

Building masonry walls fromfoundation up to DPC level

With the foundation concrete laid, we areready to start building the walls. Start byfinding the position of each corner using abuilder’s line on the profile boards, and abuilder’s level. The builder’s level is appliedto the adjacent strings and this will fix thewall’s corner, as shown in Figure 2.40.

Corners

At each corner, bed a brick in mortar as shownin Figure 2.41. Adjust its position in both direc-tions until it falls directly below the crossoverof the builder’s lines. This is the corner of thewall.

Building the corner out

As the position of each corner is established,additional bricks are added in that first coursefor a distance of five or six brick lengths, asshown in Figure 2.42.

Keeping the course properly aligned

Alignment of this first course of bricks is doneusing a builder’s level, with reference to the

Excavation andconcrete lines

Wall lines

Builder's line onprofile

Face ofwall

Builder's level

Fig. 2.40 Finding corner of wall on concrete founda-tion.

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Section

Plan

Builder'sline

Builder'sline

Fig. 2.41 Setting the first brick of the corner of wall onthe foundation.

builder’s line strung on the profile, as shownin Figure 2.43.

Building up the corners

With the first course started at the corners andproperly aligned, the rest of the corner is builtup keeping it ‘plumb’ (vertical) by using thebuilder’s level. The brickwork is ‘racked back’on each wing, as shown in Figure 2.44.

Filling in

With corners built at each end of a lengthof wall, a builder’s line is strung from the

First brick, aquoin header

Queen closer

Fig. 2.42 Setting up the first five to six bricks at cornerson the foundation.

Builder'slines

First brickat corner

Fig. 2.43 Keeping the first course of bricks aligned onthe foundation.

brickwork about four or five courses up, asshown in Figure 2.45. Bricks are laid betweenthe corners, filling up the four or five courses.

Going up

With the first few courses infilled, cornersare built up again and infilling continued, asshown in Figure 2.46. Brickwork is built to amaximum of 1.50 m (20 courses) high in anyone working day, irrespective of wall width.This height is referred to as a lift. Buildingany higher on unset mortar brings the risk

Racking back

First brickConcrete

foundation

Side of trench

Fig. 2.44 Racking back.

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Substructures 59

Builder's line

Concrete foundation

Racking back Rackin

g ba

ck

Side of trench Side of trench

Fig. 2.45 Two corners built and racked back ready to fill in.

of the wall settling down unevenly and evencollapsing. Left overnight, another lift can bebuilt next day.

Ground floor construction

We are now approaching the point wheredrawings in the text will in the main be con-sidered to be ‘construction details’. As such,the details will show the construction of morethan one piece of the building. For example,we cannot show how a ground floor is con-structed without relating that to the construc-tion of the foundation and the substructurewalls; we cannot show how a roof is tiled with-out showing the carpentry structure involved

First lift

Builder's line

Concrete foundation

Racking back Rackin

g ba

ck

Side of trench Side of trench

Fig. 2.46 Corners extended and ready for the next lift.

at key parts of the roof, and so on. So as welook at a variety of ground floor constructionswe must also examine how or if they affectthe way in which the rest of the substructureis constructed. So we really look at the inter-face between a floor and a wall.

Detail drawings

The most efficient way to ‘describe’ the inter-face between two or more pieces of construc-tion is to draw an appropriate detail. Whenthe detail is complete it must contain:

� annotation� illustration� dimension.

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50 Dritherm fibreglassinsulation

100 thick Thermaliteblockwork

Half brick thick common

brickwork with joints rakedout to receive a rendered finish

Fig. 2.47 Wall detail – symbols for materials in section.

Annotation

An annotation is a note describing what hasbeen illustrated, e.g. Figure 2.47, and ex-panding on what is seen, e.g. a symbol forinsulation cannot distinguish between, say,polystyrene and glass fibre; it requires anno-tation to make the distinction.

Illustration

An illustration is a drawing to scale or a sketchto an approximate scale or in proportion. Dif-ferent line widths or strengths are used to em-phasise important features. The wrong linethickness or strength sends confusing mes-sages, as can be seen in Figure 2.48. Correctdrafting symbols and conventions must beused.

Dimension

All key dimensions must be given so that no-one need scale off any dimension. Scaling di-mensions from a drawing is not done if atall possible. If one would have to scale off,the drawing is missing key dimensions andshould be returned to whoever produced itfor them to put in the key dimensions. Lookat Figure 2.49 for an example of key dimen-sions.

Note use of break line

Note use ofdifferent linethicknesses

Note that thickness ofmasonry and insulation is

in proportion

Note excessiveline thickness

Fig. 2.48 Use of correct line thickness.

There is no need to give every single dimen-sion. Some can be left to be calculated. That isquite legitimate and indeed expected.

An example of a detail

The detail for a wall–ground floor interfacemust include information, where appropriate,on the foundation, the wall, the floor, DPC,DPM (damp proof membrane), joining DPCand DPM, wall insulation, floor insulation,

50 Dritherm fibreglassinsulation

100 Thermalite blockwork in cement/lime

mortar1:2:9

Half brick thick common

brick in cement/lime mortar 1:2:9;

with joints raked out to receive

render

102.550

100

Fig. 2.49 Wall section with dimensions and annota-tion.

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Substructures 61

finishes to wall and floor, ground levels andventilation. This is all part of the annotationand includes information on material, work-manship and dimensions.

DPCs and DPMs are discussed in Ap-pendix I, together with weeps and ventilationof underfloor voids.

The following is a simple example relatingto a non-domestic situation.

A one brick thick wall is supported on a con-crete foundation in ‘normal’ soil. The floor willbe a solid concrete floor with a power floatedfinish. No insulation is required. Because bear-ing strata and finished ground level are notmentioned here, we can assume that what isrequired is a ‘standard detail’, i.e. what thingswould look like with the minimum amount ofwork required, a ‘level’ site and existing andfinished ground levels the same.

The detail is shown in Figure 2.50:

� All key dimensions are given� Dimensions are always in millimetres� The illustration includes proper symbols for

materials:� Earth� Concrete� Common brick – pairs of diagonal lines� Facing brick – pairs of diagonal lines with

an opposite single line

215

515

150

300min.

150FGL

DPC90

210

Facing brick to be Bloggs

heather mixture wire cut

brick built in cement/lime

mortar 1:2:9, weather

struck jointed

Brickwork raked out to receive rendered finish

Concrete floor to be power floatedsmooth to receive paint finish

Concrete Mix C

DPC and DPM overlap

250g VisqueenDPM

Hardcore of broken brick or stone all to pass a 100 ring, blinded with sand Backfill with selected

subsoil from the excavations

ConcreteMix A

Fig. 2.50 Simple non-domestic substructure/groundfloor interface.

� Hardcore and backfill – ‘stones’ and diag-onal pattern respectively

� DPC and DPM – both shown as a pair offine lines linked with blocks of black.

The illustration is very ‘out of scale’, e.g. theVisqueen sheet is only 250 microns thick.

Annotation is added, e.g. concrete mixesand finish, fill material, facing brick type, etc.On this detail the annotation is a bit over thetop. It would be only one detail on a large sheetshowing the ground floor plan of foundations,substructure walls and partitions, plus severalother details. So it is going to be tedious torepeat a lot of the fine detail every time weproduce a detail. Much of the common infor-mation would be consigned to a column onthe right of the whole drawing under notes,and might include one statement on each ofthe following:

� Facing brick type� Building mortar� Pointing mortar� Pointing or jointing style� Hardcore specification� Backfill specification� DPC specification� DPM specification� Concrete finishes.

The concrete mixes referenced as A, B, C,etc. already refer to a printed document, thepreambles, issued to contractors when theytender for the works. This would cut downon the information surrounding the detail andwould be quite acceptable.

For this text we will still give as full a spec-ification for each figure as we feel is neces-sary to avoid ambiguity or to make a specificpoint regarding the construction used. Whereit might confuse or clutter up the drawing wewill omit it, but please remember when draw-ings are done professionally for a contract, theinformation has to be on the drawing either asannotation or in notes on the drawing.

The detail in Figure 2.50 is of course onlyappropriate for non-domestic building with avery light floor load. There is no insulation ofwalls or floor. The walls being one brick thick

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are not of a sufficient weather resisting stan-dard for domestic construction. The floor is a‘single layer’ concrete floor and the thicknessof concrete shown is the minimum allowedunder the Building Regulations.

Wall–floor interfaces generally

When studying the variety of ways in whichground floors are constructed, it is usual toconsider how they interface with:

� Outer walls� Internal loadbearing walls� Internal non-loadbearing walls� Intermediate support.

Ground floor construction divides into:

� Solid concrete floors – laid direct on an inertfill and not supported by any type of wall

� Hung floors – constructed over a voidand supported by walls and/or beams andcolumns. Hung floors can be constructedfrom timber, concrete or steel or a mixtureof these.

Precautions

Being next to the ground, these floors and thewalls require special precautions to preventmoisture from the ground penetrating up intothe structure:

� A DPC – damp proof course – is placed inthe walls

� A DPM – damp proof membrane – is placedunder or within the floor structure.

DPCs and DPMs must be joined where wallsand solid floors meet, otherwise moisture willpass between them. The joint is a particularlyvulnerable part of that form of construction.

Voids in many forms of construction requireto be ventilated. This is to allow any build upin moisture levels to dry out quickly beforedamage can occur. Voids under hung floorsmust be ventilated for this reason. We will

look at how this requirement is met later inthis chapter.

DPCs and DPMs are discussed in Ap-pendix I, together with weeps and ventilationof underfloor voids.

Solid concrete floors

Single and double layer concretefloors with hollow masonry wall

Domestic standards require floors and wallsto have maximum values for the passage ofheat, known as U-values. We will study thatconcept in Appendix K. To meet these stan-dards it is normal to utilise materials witha high insulation value and/or incorporatenon-structural insulation into the walls andfloor.

To meet the weather resistance standards,masonry walls for domestic constructionmust generally be of cavity construction.(Walls have a chapter to themselves, Chap-ter 3, where the arguments in favour of cavitywalls for domestic construction are put for-ward.) This means building two vertical lay-ers of, say, masonry with a gap between. Thevertical layers are usually half a brick thick or100 mm of blockwork. Asymmetrical layerscan be used but should total at least 200 mm.The minimum thickness for any one layer is75.

Two layers of masonry – termed leaves orskins – must be made to act together andprovide mutual support and stability. This isachieved by using wall ties spaced at 3 or4/m2.

Figure 2.51 shows a domestic wall/floor in-terface. The same wall and foundation con-struction is shown in Figure 2.52. The solidconcrete floor in this instance is of sandwichconstruction, the layers of concrete being theminimum thickness allowed under the Build-ing Regulations.

A few points must be noted:

� The construction shown is not the onlyway to provide insulated solid or sandwichfloors or insulated walls.

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Substructures 63

252.5

100 aac blockwork12.5 plasterboard/urethene

foam laminate95 x 15 S/wskirting

Concrete Mix C

50 bead board on 250g Visqueen

DPM

90

160Hardcore of broken brickor stone to pass a 100ring, blinded with ashes

Backfill with selected inert material

from the excavations

Concrete Mix A

550

150

300 min.

150min.

DPCWeeps at1115 c/c

Facing brick to be Bloggs heather

mixture wire cut,weather struck

pointed

FGLConcrete

MixD filling cavity in

wall

Fig. 2.51 Hollow wall/single layer concrete floor suit-able for domestic construction.

� Materials specified are only one or two of awide range available.

� Figures 2.53(a), (b), (c) and (d) show a se-lection of the many kinds of wall ties avail-able to tie two masonry leaves together. Fig-ure 2.53(a) shows a couple of galvanisedsteel twisted ties which are rigid. The twistis positioned in the cavity between the ma-sonry leaves, and any moisture from theouter leaf trying to cross over the tie dropsoff the edge of the twist. Figures 2.53(b), (c)and (d) show pairs of butterfly twisted wallties, stainless steel strip chevron ties andstainless steel wire wall ties, respectively.Note that each has a drip designed into itand an arrangement at either end to gripinto the mortar beds.

252.5100 aac blockwork12.5 plasterboard/40 urethane foam

laminate

95 x 15 S/w skirting

Concrete Mix C

Concrete Mix C

40 beadboard upstand

Hardcore bed

Backfill

550

150

DPCWeeps at1115 c/s

Concrete Mix D filling

cavity

Facing brick, weatherstruck pointed

6050 beadboard75

115

150

Fig. 2.52 Hollow wall/double layer concrete floorsuitable for domestic construction.

Fig. 2.53(a) Galvanised steel twisted wall ties.

Fig. 2.53(b) Galvanised steel wire butterfly wall ties.

Fig. 2.53(c) Stainless steel chevron wall ties.

Fig. 2.53(d) Stainless steel wire wall ties.

� DPM and DPC still overlap between walland edge of concrete behind the foam plas-tic upstand. Different materials for insula-tion, DPCs and DPMs will be studied intheir own right.

Note that the annotation has been minimisedon the assumption that there will be adequatenotes appended to the whole drawing.

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75 50 polystyrene beadboard on polythene slip

layer

20 Caberfloor with glued t&g joints

20

50

90

100 min.Broken stone or brick hardcore

DPM

Concrete Mix C

95 x 15 S/w skirting on S/w

ground plugged to blockwork

12 Carlite plaster

Lightweight aggregateblockwork

Fig. 2.54 Non-loadbearing block partition built offconcrete subfloor.

Partitions

While single and double layer concrete floorsdo not have intermediate support walls, theycan interface with partitions in different ways.The interface depends on the type of partitionand the strength of the floor.

The simplest interface is a non-loadbearingpartition, shown in Figure 2.54. This is simplybuilt off the upper surface of the concrete slab.The floor can be single layer or sandwich con-struction. Note different insulation and floorsurface options.

With a loadbearing partition one must ei-ther provide a foundation or, as shown in Fig-ure 2.55, thicken and possibly reinforce thefloor slab locally. A foundation might be re-quired for a very heavily loaded wall. Usingthe floor slab can be economical if the upfillis deep, even if the whole floor slab has to bereinforced.

Hung floors

Hung timber floors

The timber hung floor is popular with oc-cupiers as it has a natural resilience which

7550 polystyrene bead-board on polythene

slip layer

20 Caberfloor withglued t&g joints

20

50

90

100 min.

Broken stone or brick hardcore

DPM

Concrete Mix C

95 x 15 S/w skirting on S/w ground plugged

to blockwork

12 Carlite plaster

Lightweight aggregate blockwork

Fig. 2.55 Loadbearing block partition built off con-crete subfloor.

contributes to the comfort of the occupier andwhich is absent in concrete floors. If there isa greater than average depth of filling to bedone under the floor, or if the floor level iswell above finished ground level, then it willbe cheaper to construct than a solid concretefloor. Otherwise, costs depend on the currentrelative material and labour values. Quantitysurveyors should be able to advise on the cur-rent cost conditions for any configuration.

The principle on which the floor is con-structed is quite simple:

� The inside of the building is filled with inertmaterial up to finished ground level and aDPM laid

� The floor is built over the filling, leaving aminimum gap of 150 mm between the DPMand the nearest piece of timber

� The floor is constructed of timber planks orboarding fixed to joists supported by theoutside walls.

The joists are timbers laid on edge and these arefixed at centres, i.e. they are evenly spaced outacross the area of the floor at precise intervals.The timber or man-made board is nailed downto the joists to provide a flat, usable surface.Figure 2.56 illustrates the principle.

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Substructures 65

Centres

Joist

Joist

Boarding

Fig. 2.56 Joists and boarding schematic.

The size of the joists used depends on:

� The live and dead load on the floor� The span of the joists� The spacing or centres of the joists� The strength of the timber used.

The type and thickness of floor boarding de-pends on:

� The live and dead load on the floor� The spacing or centres of the joists.

Concrete Mix AConcrete Mix

A

100 x 50 sawntanalised S/w joists

100 glass fibre quiltlaid between joists

20 MR flooring grade particleboard nailed to each joist with 4improved nails across width of

sheet

100 aac blockwork12 Carlite plaster in 2 cts

95 x 15 S/w skirting and ground

nailed to block

3 course corbel

DPM of 10hot bitumen

Polypropylene mesh stapled

to joists

WallplateDPC

300

100

550

150

300 min.

150 min.

Concrete Mix Dfilling cavity

30 extruded, expanded polystyrene board clipped to wall ties

Facing bricks

252.5

Broken brick/stonehardcore blinded

with ashes

Backfill with selected material from the

excavations

100 x 39 Tanalised S/w wallplate

Fig. 2.57 Hollow wall/timber hung floor interface suitable for domestic construction.

These criteria are discussed in Appendix C,Timber.

The void between the DPM and floor mustbe ventilated. Support by the outside walls forthe floor must be above the level of the DPC.The means of support varies but generally themost economical option is the provision of athree course corbel and wall plate. Groundfloor joists are never supported by buildingthem into the wall.

Where the floor spans longer distances thanthe joists will bear, intermediate support mustbe provided. This is most commonly donewith sleeper walls built in honeycomb brick-work. Other options are considered later inthis chapter.

Unlike the solid concrete floor, there is noneed to join the DPC and DPM. Any moisturerising in the wall up to DPC level is free toevaporate from both faces of the wall. Evapo-ration from the inside face is carried away bythe through ventilation of the void.

Details usually show both the outer walland intermediate support wall interfaces. Fig-ure 2.57 shows a typical set of details. Theyshow ‘standard’ assumptions on the relation-ship of finished ground level, original groundlevel and bearing stratum level.

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Weeps are gaps deliberately left betweenthe ends of bricks to allow moisture to drainthrough the wall.

Wall plates are continuous lengths of timber,generally treated with preservative, which aresupplied by the carpenter and bedded hori-zontally in mortar by the bricklayer. Joints inrunning length and at corners should be halflapped9. Wall plates fulfil a number of func-tions:

� They provide a level surface on which to fixother timbers – in this instance floor joists.

� They provide a base on to which the joistsare nailed, thus fixing the joists in positionat the required centres.

� They spread the load of the joists over thesupporting masonry.

The following materials and techniques al-ready mentioned are described in more detaillater in this chapter or elsewhere in this textas indicated:

� DPCs and DPMs (Appendix I)� Weeps (Appendix I)� Air bricks and ventilation of ground floor

voids (Appendix I)� Sizing of joists� Timber types, stress grading10 (Appen-

dix C) and sizing of joists� Floor boarding, types and fixing11 (Appen-

dix C)� Alternative methods of joist support� Alternative solum finishes.

The solum is the area of ground and the fin-ishes applied to it, inside the external wallsand any partitions built off foundations. It isnot any particular finish – only descriptive ofa finished state, the ground prepared to re-ceive a finish or any stage between. One canspeak of the solum ready to receive hard-core, the solum ready to receive concrete,the solum ready to receive hot bitumen, andso on.

9 Joints in timber are described in Appendix C10 Stress Grading is described in Appendix C11 Floor boarding is discussed in Appendix C

Hung timber floor alternatives

Before looking at the alternatives to the timberhung floor shown in Figure 2.57, it is impor-tant to note that it represents the most eco-nomical solution to the provision of a groundfloor for domestic construction as well as fullymeeting the requirements of the Scottish andEnglish Building Regulations.

The next few figures illustrate alternativeforms of:

� Joist support at main wall� Joist support by sleeper wall� Solum finish:

� which can support a sleeper wall� which cannot support a sleeper wall.

It is quite feasible to ‘mix and match’ the al-ternatives.

Figure 2.58 shows alternative means of sup-port for joists at the outer wall/floor interface.

The inner leaf of the wall below DPC levelis built one brick thick (A). Note that the 252.5cavity wall is still centred on the foundation.

Intermediate support for the sleeper joists isby a honeycomb brick wall on its own foun-dation (C).

The solum finish is 65 mm thick concretewith a DPM under (B). The DPM is usuallypolyethylene at least 250 microns thick. Theconcrete is there primarily to hold the DPMin place. Note how the DPM turned up atthe edges of the concrete. At 65 mm thick theconcrete cannot be loadbearing but it wouldgive:

� A useful surface for a crawl space12

� Protection from penetration by vermin, i.e.rats and mice.

A 10 pitch DPM would do neither of these.Any loadbearing partition wall would have

to be supported by its own foundation.Figure 2.59 shows a commonly accepted

solum in many textbooks – the 100 thick layer

12 A crawl space is any area under a floor or in a roofwhich allows a workman sufficient access – even onhands and knees – to enter and work, maintain or re-pair components in the space.

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Substructures 67

252.5100 aac blockwork

12 Carlite plaster95 x 15 S/w skirting

and groundnailed to blockwork

20 MR flooring gradeparticle board

150 x 50 sawn tanalised S/wjoists

65100 min.

Polythene DPM

300

150

C

B

A

Polypropylene mesh stapled to u/sof joistsDPC

Honeycomb brick wall

Backfill with selected

material fromthe excavations

100 x 38 tanalisedS/w wallplate

Dense concrete Mix E

550

CentrelineFacing brick,

weather struck

pointed

30 extruded, expanded

polystyrene board clipped to wall

ties

150 min.

300 min.

150

Centreline

Hardcore of brokenbrick or stone

FGLWeepVent

Concrete Mix D filling

cavity

Fig. 2.58 Alternatives A, B and C of joist support.

of dense concrete. A DPM must be provided(D). The only advantage therefore is to pro-vide a greater degree of support than 65 ofconcrete can provide. A lightly loaded sleeperwall could therefore be built directly off thislayer of concrete.

The support for the floor joists is entirely byhalf brick thick honeycomb walls supportedby the 100 thick concrete layer (E). As the reg-ulations require a DPM under the concrete, noDPC is required in the sleeper walls.

Hung concrete floors

Hung concrete floors suitable for domesticconstruction are generally formed in one oftwo ways:

� Using precast concrete beams and fillerblocks

� Using permanent steel formwork or shut-tering spanning over beams or joists with athin in-situ concrete slab poured over. The

20 MR flooring grade particleboard

100 x 50 sawn tanalised S/w joists

100

Glass fibre insulation

Polypropylenenet to support

insulation

100 min.

Polythene DPM

Honeycombbrick wall

100 x 38 Tanalised

S/w wallplate

Dense concrete Mix Ein solum finish

Hardcore of brokenbrick or stone

Honeycombbrick wall

Upfill with selected material fromthe excavation

550

150

300 min.

FGL Weep

Vent150 min.

DPC

30 extruded,expanded

polystyrene board, clipped

to wall ties

Facing brick

252.5 thick cavity wall 12 Carlite plaster in 2 cts

95 x 15 S/w skirting and groundnailed to blockwork

E E

D

Fig. 2.59 Alternatives D and E.

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Concrete Mix A

440 long block

100

125

150

252.5

Facing brick

T beam and filler block structural floor

DPCDPM of 10 hot bitumen

Hardcore of broken brickor stone

Upfill with selectedmaterial from the

excavation

Prestressed, precast,concrete T beam

550

150

300 min.

Concrete Mix D filling cavity

Weeps

Vents150 min.

30 extruded, expandedpolystyrene boardclipped to wall ties

Fig. 2.60 Precast concrete T-beam and filler block floor.

joists may be of concrete or wood and veryoccasionally of galvanised cold rolled steel.

The first of these floors uses inverted T beamsof prestressed, precast concrete, placed at cen-tres across the required floor area. The spacebetween the beams is filled with blocks. It isillustrated in Figure 2.60.

Generally the blocks are ordinary build-ing blocks and particularly lightweight or aacconcrete blocks. Some proprietary systemsuse specially cast blocks which may be shoul-dered to fit into the ‘T’ beam. They may be es-pecially strong; they may be hollow or solid.Alternatives for the filler blocks are shownin Figure 2.61. Blocks can also be made ofpolystyrene; still quite strong but giving im-proved levels of insulation to the floors.

Because the beams are of prestressed13, pre-cast concrete:

� They can be built into outer walls withoutthe worry of ends rotting away, and so save

13 Prestressing is a means of increasing the efficacy of thereinforcement in a concrete member. Basically, whenconcrete is poured into a mould the reinforcement hasbeen placed in position. In prestressed concrete that re-inforcement is tensioned before the concrete is pouredin. The rebar is of high tensile steel wire. The settingconcrete grips the wire and when the mould is struckthe wire remains in tension, literally pulling the con-crete together and better able to carry a high load.

on the need for corbelling or formation ofscarcements for joist support

� They can be cast with a modest depth butin long lengths and so can span across mosthouses without the need for sleeper walls.

Spans available generally do not exceed 8 m.If intermediate support should be required, adwarf support wall of half brick thick honey-comb brickwork should be provided. The wallmust have its own foundation and should notrest on any oversite concrete layers as the selfweight of the floor is quite considerable.

A DPC is still required in the walls, andthe beams should be bedded in mortar im-mediately above that DPC. A solum finishis required, including a DPM. Choose anyfrom those described for hung timber floors.The underfloor void must be ventilated inthe same way as the void under hung timberfloors. Weeps must be provided in the outerleaf of cavity walls.

The cavity must be filled up to ground levelwith weak concrete. Once the beams and con-crete blocks are in place, this rough floor pro-vides a useful working platform for any workwhich is to follow. This was particularly thecase in Scotland where a trestles and battensscaffold was generally built inside the build-ing rather than having an external scaffold, asituation that has changed considerably sincethe 1980s.

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Substructures 69

Polysty. block over with partlebrd floor

Plain concretebuilding block

Light steel mesh inconcrete screed

Screed on filler block floorPurpose made hollow concrete block floor

t&g joints of sheetingglued

Polytheneslip layer

under floorsheeting

t&g joints of sheetingglued

Plain concretebuilding block

Filler block of foamed plastic insulation with floatingfloor on top Floating floor on insulation on top of filler

block floor

Fig. 2.61 Alternative filler block floor construction.

Once the building is weathertight, a floorfinish can be applied over the beams andblocks. A number of options are available,from simple screeds to complex built layersof insulation and board or sheet flooring.

So the beam and filler block floors providethe structural base upon which a ‘finish’ isplaced. The finish can be:

� Timber based sheet� A fine concrete screed.

Insulation may be necessary in the floor andcan be placed below either timber or screed.Timber sheet is laid ‘floating’ with the sheetclear of wall edges to allow for moisture move-ment. The sheets are wedged together untilthe glue in the joints has set.

While it is not impossible, it is inadvisable toattempt to fasten battens (and nail floor board-ing or sheet to them) on this type of floor. Gen-erally the beams are of very dense concrete,making nailing or plugging difficult, whilepower nailing is apt to shatter the concrete.Nor should one fasten to the blocks as move-ment of the timber will displace the blocksand this will result in a noisy floor when theoccupants put their weight on a batten fixedto a loose block. Timber floors therefore must‘float’ and be allowed to move separately fromthe concrete base.

The thickness of any screed placed over thesub-floor depends on the purpose to whichthe floor will be put and the type of fin-ish placed on the screed. Insulation may beplaced over the floor under the screed, inwhich case a minimum thickness of 65 screedwould have to be laid. In addition, some thinscreeds would benefit from the inclusion of alight steel reinforcement – galvanised chickenwire would be adequate to prevent seriouscracking due to temperature and/or moisturemovement14. A floor incorporating heatingcables or pipes must have a minimum screedthickness of 65.

Formwork

Two types of formwork can be used for thein-situ concrete slab hung floor and these areillustrated in Figure 2.62:

� A solid, profiled sheet of hot zinc coatedsteel

14 It is not generally appreciated that concrete productsactually expand and contract. They do so under theinfluence of temperature changes in the same way asall other materials. The movement caused by varyingmoisture content is even greater, the concrete expand-ing as water is taken up and shrinking on drying out.

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75 min. dense concrete floor slab

Hot dipped galvanised profiledsteel sheet nailed to timber joists

65 long steel drive screws

30--50 foamed plastic insulation

65 min. fine concrete screedplain or reinforced

Screed to receive floor finish -may require insulation indomestic construction

100 min.

Temporary supports

at 750 c/s.Full span up

to 2400

20.8 high rib of solid steel sheetSteel sheet expanded between ribs

89Sheet 445

wide

Sheet lengthsof 2, 3, 4and 5metres

2540

65 galvanised drive screwsthrough sheetinto joist

Solid, profiled steel sheet as permanentformwork

Hi-Rib expanded metal lath as permanentformwork

Hi-Rib metal lath permanentshutter bedded over DPC on

inner leaf of main wall

Fig. 2.62 Permanent formwork for hung concrete, cast in-situ floors.

� A sheet of hot zinc coated steel which hasribs of solid sheet adjacent to areas of ex-panded sheet.

Either way, the formwork has two functions:

� As formwork, and for that purpose it mightappear that the solid sheet would better re-tain the concrete; however, the mesh willretain a dry mix quite well and also ‘keys’to the concrete much better.

� To act as reinforcement for the floor slab.

On no account should any separating layer beplaced over the formwork before the concreteis poured, as this would destroy the essential‘key’ between concrete and metal.

Even if the floor is a lightly loaded domesticone, the span of the steel sheet or lath is limitedby the wet weight of the concrete. Wet concretehas a mass of approximately 2.5 tonnes percubic metre, so a square metre of formworkmust support 250 kg of wet concrete at 100 mmthick. Permanent supports can be placed atintervals dictated by the span, to support thewet concrete, or they can be placed as dictatedby the finished dry slab and temporary inter-mediate supports used to hold up the form-work until a full set takes place. Temporarysupports are generally as simple as scaffold

planks on short timber or dry masonry props.In any event it is unlikely that spans fromouter wall to outer wall can be achieved onanything but the narrowest of buildings.

The profiled sheet shape shown in Fig-ure 2.62 is a continental profile and until re-cently had no equivalent in the UK. The ex-panded metal lath material with the deep ribis manufactured in the UK under the nameHi-Rib. The strands of metal in the expandedmetal lath portion of the sheet are angled togive a better key in the concrete.

The thickness of concrete poured varies ac-cording to load and span, but would normallybe a minimum of 75 mm. Additional reinforce-ment meshes and thickness of concrete can beintroduced to cope with almost any domesticor light commercial load. These add to whatis an already expensive and heavy structure.

Consolidation of the concrete slab is gen-erally carried out either by tamping with ascreeding board, or by using a poker vibrator.Tamping is the best method to use with Hi-Ribas the slabs are too thin to allow use of a pokervibrator. Using the vibrator so near to the lathwould literally push the cement and waterthrough the lath and leave the larger parti-cles of aggregate behind. The vibrator couldbe used on the solid profiled steel sheet but

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Substructures 71

care would have to be taken not to separatethe cement and sand from the aggregate.

The use of such floors is unusual in theUK as they are generally much more expen-sive than the alternative forms of construc-tion. They are promoted quite heavily in con-tinental Europe where they are used for floorswhich are subject to continuous damp, suchas in bathrooms – particularly wet rooms,shower rooms, kitchens and laundry rooms.In all these instances they are frequently con-fined to small areas with short spans to load-bearing partitions. Remaining areas of floorare constructed using cheaper alternatives.

Blockwork substructure

Finally, before leaving this chapter, we shouldshow a form of substructure which is verypopular now with many house builders.The technique uses blocks which have extraheight, laid on their faces to give a solid wallwhose thickness is the height of the blocks.

A detail is illustrated in Figure 2.63, whichshows the substructure prepared to supporta timber frame building which has a facing

Concrete Mix A

550

Broken brick/stone hard-core blinded with wetted ashes and tamped firm

Backfill with selected

material from the excavations

Vent

Weep

Facing brick150 x 50 sawn

S/w sleeper joistDwang ornogging

150 x 50 sawn S/w ring beam

100 x 50 Tanalised sawn S/w wallplate

100 dense concreteblockwork

10 hot bitumen DPMFGL

150 min.

300 min.

150

Prefabricated timber panel100 x 25 sawn S/w sole plate

Dense concrete blocks,350 l x 300 h x 100 t,laid on face to make

wall 300 wide

Centreline

Centreline

DPC

Fig. 2.63 Solid blockwork wall and brick/block cav-ity wall as a substructure for a timber frame panelconstruction.

Fig. 2.64 Photograph of substructure as detailed inFigure 2.63.

brickwork cladding. The solid wall is built offstrip foundations to just below or at finishedground level. The advantages of this solid wallare:

� Blocks, even laid on their face, are verymuch quicker to lay than bricks.

� There is no need for cavity fill, thus sav-ing on an otherwise additional operationwhich always risks knocking over recentlylaid brickwork.

� Costs of blocks can be considerably less thanbricks when bricks have to be transportedsome distance.

� Using special aac or lightweight aggregateblocks, the work is not so strenuous.

We will return to this detail when we look attimber frame wall construction in Chapter 3.

Figure 2.64 is a photograph of a fairly sim-ilar construction, the main differences being

Fig. 2.65 Sleeper walls in the above substructure.

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the solum finish (polythene DPM with 65 con-crete over) and the final wall cladding, whichwas rendered blockwork rather than facingbrickwork. The photograph clearly shows theblock on flat solid wall up to what will befinished ground level, and the start of thecavity wall with one course of blockworklaid up to what will be DPC level. Abovethat DPC will be the ground floor, ring beamand timber frame construction, while on theline of the outer face will be the blockworkcladding.

The photograph in Figure 2.65 shows thesleeper walls built inside the building peri-meter. Note that they are of blockwork andso cannot be conveniently built ‘honeycomb’.

To provide ventilation across the completesolum, large gaps are left in the sleeperwalls, and these are bridged over with con-crete beams, in this case prestressed concretebeams, 75 thick. The sleeper walls have theirown strip foundations – one can be seen in theopening of the nearest wall.

The building was being built in an area thathas an abundance of concrete blocks whichcan be produced and delivered to site muchmore cheaply than common bricks. The onlyplace bricks were being used on the whole sitewas in some work in the main sewer manholesunder the cast iron covers, and even there onlyaround 200 Class 2 engineering bricks couldhave been used on the whole site.

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3 Walls and Partitions

General 73Requirements 74

Walls – environmental control 75Heat loss and thermal capacity 75Resistance to weather – precipitation 75Air infiltration 77Noise control 79Fire 79

Dimensional stability 79

Walls of brick and blockwork 81Insulation of external walls 84Timber frame construction 88

Traditional timber frame 88Modern timber frame construction 91

Loadbearing and non-loadbearing internalpartitions 96

Expansion joints 99

General

Why do we build walls? What do walls doafter they are built? How do we build walls tofulfil these functions?

The layman thinks of building walls mainlyto provide shelter, but they fulfil a number ofother functions as well:

� To support other parts of the structure – up-per floors and roof

� To provide mutual stability with otherbuilding parts

� To give privacy to the occupants from out-side the house and between compartmentsin the house

� To modify the micro-climate inside thebuilding:� Keeping it hotter or cooler than the out-

side ambient temperature� Raising or lowering the relative humidity� Keeping the effects of wind and/or pre-

cipitation from entering the house� Modifying the sound entering or leaving

the building.

This chapter will concern itself principallywith the structural aspects of domestic walls,but cannot completely ignore the other facetsmentioned above. The chapter will also con-cern itself with two basic types of construc-tion:

(1) Masonry walls of brick and blockwork(2) Timber frame walls.

And in two situations:

(1) Main supporting and enclosing walls fora house

(2) Internal supporting and/or dividingwalls within a house – the partitions.

Walls can be categorised in different ways:

� By load:� Structural – designed to carry vertical

loads plus their own weight� Stabilising – designed to resist horizontal

or oblique forces and carry vertical loadsplus their own weight

� Non-loadbearing – designed to carry onlytheir own weight.

73

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� By construction type:� Unitised construction – built with units

such as bricks or blocks bonded to avoidvertical planar continuity of joints whichmight otherwise become the locus ofcracking and structural weakness

� Homogeneous – built using plastic mater-ials which subsequently set or dry out,such as clay, earth (adobe) or in-situ con-crete; clay and adobe can be reinforcedwith straw or reed or small branches(cattied); concrete can be reinforced withsteel rods, bars, mesh and fibres or plas-tic or vegetable fibres such as bamboocane.

� Framed – posts and beams of timber, steelor concrete are erected on site and cov-ered with boarding, a masonry skin, metalsheet, plastic sheet etc. The skin plays norole in supporting the load on the frame.The frame alone carries imposed loadscaused by other parts of the structure, oc-cupancy, wind and snow, etc. The skinmay provide a measure of racking resis-tance.

Racking resistance

The simple frame, A, shown in Figure 3.1, willtend to pivot at all 12 joints when loads are

Brace or racking

fixed to all timbers

it crosses

A

B

Fig. 3.1 Frames with and without racking.

applied – wind, workmen and occupants lean-ing on the walls. This movement is calledracking. Frame B has a brace and will notmove.

In domestic construction, timber frames areclad with a skin of plywood or structural par-ticle board – stressed skin panels. The panelsare made in a factory and glues are extensivelyused. Panels may be for a part or whole walllength but will be for complete storey heightwalls. Door, window and other openings arepreformed in the panels. In calculating theloadbearing capacity of the panels the skinor sheathing is taken into account. The skinprovides all the racking resistance.

Requirements

Requirements for walls vary according to useand situation. Some or all of the followingmight be required. All but one or two sub-divisions are covered by the statutory require-ments of the Building Regulations:� Structural stability� Environmental control� Dimensional stability� Fire resistance.

Each of the above can be further broken down.Structural stability has three distinct facets:

� Resistance to overturning� Resistance to crushing� Resistance to buckling.

Environmental control includes:� Resistance to weather� Resistance to air movement� Thermal resistance� Thermal capacity� Noise transmission� Fire resistance.

Dimensional stability:� Thermally induced movement� Movement due to changes in moisture con-

tent� Structural movement – bending, buckling,

compression, deflection� Chemical reaction.

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Walls and Partitions 75

Fire resistance

� Combustibility – how easily a material willignite and sustain ignition

� Surface spread of flame – the rate at whichflame will spread across one material to ig-nite another surface

� Stability; integrity; insulation.

Walls – environmental control

Heat loss and thermal capacity

Thermal transmittance is measured in wattsper square metre per degree Kelvin and is ameasure of the rate of heat transfer througha piece of the structure – wall, floor or roof –known as a U-value. The Building Regula-tions lay down maximum values for this rateof heat transfer. It is calculated from knownvalues for thermal resistance of individualmaterials and their thickness in the construc-tion plus the inside and outside surface resis-tances. We will look at a simple method of cal-culating a U-value for a wall in Appendix K.

Thermal capacity is the quantity of heat re-quired to raise unit volume of a piece of con-struction by unit temperature. Dense mater-ials have a higher thermal capacity whichmeans they take longer to absorb heat, buthaving absorbed heat will give out that heatfor a longer period. So heavy constructiononce heated up can act as a store of heat overa period when heat input is not available. Alightweight construction does not have thisstore effect but of course the occupied areaheats up much more quickly and the occu-pants feel comfortable more quickly.

Resistance to weather – precipitation

Principally, resistance to weather means theability to resist the effects of precipitation.There are basically two ways to stop waterpenetrating a wall: provide an impervioussurface, which can be very thin, or providea relatively thick and finely porous claddingor complete wall.

Note that water penetrating a wall does soas solid water which dries out by evaporation,i.e. as water vapour. Drying out therefore cantake a lot longer than the initial wetting.

Thin impervious claddings can be in largeor relatively small pieces. The use of largepieces means a shorter total length of joints.The use of small units such as tile or faiencegives a large total length of joints. Jointing ma-terial is usually a mortar and so is porous.It is at the joints that water penetration willtake place. Water penetrating these joints willspread rapidly in the porous masonry back-ing and will only be able to evaporate fromthe joint line – the more joints the quicker thewall will dry out. If there is a frost before thewall dries, there is a risk of the water freezing,and the thin facing material will burst off thewall face. This can happen not just with tileetc. but also with some facing bricks whichhave a dense impervious face. With the intro-duction of ever stricter U-values, the risk ofthe wall freezing becomes greater.

Porous wall finishes are not necessarily theanswer unless the materials used give a veryfine and evenly distributed pore structure.The ideal situation is to have a pore size suchthat take up of solid water is inhibited, isevenly distributed over the face and thicknessof the wall and the water is as free as possibleto evaporate from the surface. Aac blocks ex-hibit these qualities but are rarely acceptableas a finish on the outside of buildings.

Water is not retained in the wall so the riskof bursting on freezing is reduced. The fineporous structure can take up some of the ex-pansion of the ice formed on freezing withoutbeing damaged.

Although beyond the scope of our studyof domestic structures, it is as well to pointout that if one wishes to use thin impervi-ous materials, the joints should be arranged toprevent water penetration, e.g. use interlock-ing metal panels or timber or plastic ‘weatherboarding’.

Within the scope of our study, the use offinely porous materials would include the useof suitable facing bricks with a compatiblemortar mix, or the use of common brick or

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concrete block which is covered with a 20 mmlayer of rendering in the form of either a wetor dry dash finish.

Wet and dry dash finishes comprise a mini-mum of two layers of a suitable mortar spreadevenly over the wall surface. The actual mor-tar mix depends on the type of surface towhich it is applied, the suction of the surfaceand the degree of exposure to be experienced.All renders have a fine porous structure.

A dry dash finish comprises two coats ofmortar, the second applied after the first hasset. Before the second coat has set, 6 to 10 mmnatural or crushed stone gravel is thrown or‘dashed’ against the soft mortar and pressedgently in; it adheres to the surface, giving adecorative effect.

Figure 3.2 is a photograph of four renders.From the top:

� A new wet dash render� An old (weathered) wet dash render� Two new dry dash renders.

A wet dash render or ‘roughcast’ comprisesa coat of mortar applied to the wall and al-lowed to set. A second coat of mortar to which6 mm gravel has been added is applied on topof the first coat. The second coats of these wallfinishes frequently contain ‘tints’ or colouringagents, and to achieve a ‘white’ finish, a whiteOPC is used together with a pale colouredsand and white chippings of crushed quartzor marble. The first coat is always scored toprovide a mechanical key for the second coat.

Two-coat work of this type is expected to fin-ish 20 mm thick overall. This can be achievedquite easily on good flat, straight brick orblockwork. If the wall is badly off the straightwith lots of bumps and hollows, it is normalto apply three coats, the first coat more or lessfilling up the hollows and the two successivecoats being the dry or wet dash as alreadydescribed. This first coat is termed a straight-ening coat. The next coat is a rendering coatand the final coat is a dashing coat.

The efficacy of a wet or dry dash finishshould never be underestimated. Under theharsher climatic regimes experienced in west-ern, northern and in particular the coastal

Fig. 3.2 Dry and wet dash renderings.

areas of the British Isles, equivalent wall con-structions at a height of about three or fourstoreys for the outer leaf of a cavity wall wouldbe:

� one brick thick facing brick, or� half brick thick common brick with a 20 mm

two coat wet or dry dash finish (no additivesnecessary).

There are a few points to note:

� The rendered wall is cheaper to construct� The rendered wall is considerably lighter in

weight and so a lighter foundation might beused

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Table 3.1 Mortar mixes suitable for external rendering of masonry walls.

Undercoats Final coat

Background and typeof finish

Mix for severeexposures

Mix for moderate orsheltered exposures

Mix for severeexposures

Mix for moderate orsheltered exposures

Dense, strong andsmooth/moderatelystrong, porousbackgroundsWet dash H or A H or A H or A H or ADry dash H H H H

Moderately weak,porous backgroundsWet dash A A A ADry dash H or A H or A H or A H or A

� The wall as a whole will be half a brick lessin thickness so a narrower foundation canbe used

� In harsh climates with higher levels of insu-lation, the chances of facing brick spallingoff are greater than the chance of damage toa rendered wall.

So don’t despise regional preferences forwet and dry dash finishes. They have beenused all these centuries for very good reasons.

Waterproofers are sometimes added to theouter coat to improve the resistance of the ren-der to weather penetration, but where the ma-sonry behind is of adequate thickness or isthe outer leaf of a properly constructed cav-ity wall, there should be no need for such anadditive.

Waterproofers

In the 1950s the author was shown an ‘oldroughcaster’s’ method of waterproofing.About 3

4 litre of boiled linseed oil was added toa bucket of hot water and whisked to emulsifyit. This was added to the mixing water for themortar used for the dashing coat of a wet dashfinish. The quantity of OPC in the batch beingmixed was a full 112 lb bag – approximately50 kg.

More modern materials include PVA(polyvinyl acetate) and EVA (ethylene vinylacetate). PVA will wash out of the render in

a year or two while EVA is fully waterproofand should last the life of the render.

EVA can be purchased as a powder or a liq-uid. Recent research has shown that liquidEVA blocks most of the pores, thus helpingto retain water within the wall rather than let-ting the wall ‘breathe’. Powdered EVA is moresuccessful. It does not block the pores but linesthem, thus repelling solid water trying to en-ter while retaining a fully open surface forevaporation.

Silicone-based additives, like PVA, are notlong lasting, their efficacy wearing off in amatter of a few years. Silicone-based coat-ings for application after the render has setare equally short lived but of course re-application is always an option.

Old editions of Building Regulations gavelists of mortar mixes suitable for all coats ofsuch wall finishes. An up-to-date version ofone list is given in Table 3.1, with the rec-ommendations for use as a dry or wet dashwall finish. Mortar mixes suitable for render-ing were given as:

� A 1:1:5 or 6 or 1:5 using masonry cement� H 1: 1

2 :4 or 4 12 or 1:4 using masonry cement.

Air infiltration

There has been a long period when any air in-filtration through the fabric of a building wasseen as a bad thing, and great effort was put

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into reducing it to zero – or as near as onecould get. In the few instances where this wasachieved, the occupants discovered that theycould experience one or two of a series of phe-nomena:

� A build-up of lived-in type smells – mostlyunpleasant

� A lack of fresh air, a stuffiness� An increase in the relative humidity in the

house� Dampness and mould growth� Timber rot� Increased insect activity� Greater susceptibility to colds and respira-

tory infections.

By keeping air from penetrating the building,moisture laden vitiated air was allowed to re-main inside to the detriment of the building,its contents and the occupants. The draughtyconditions of the old housing stock, whereevery joint allowed a gale of wind to comethrough, were not desirable, especially whenthe cost of heating the air lost was considered.But the old buildings had a plentiful supplyof fresh air coming in. What is required is abalance.

Any resistance to the passage of air will alsoexclude damp air, thus helping to keep the in-terior of the building dry during wet weather.Masonry structures of brick or block are dif-ficult to ‘seal’ against infiltration, failure usu-ally taking place at their interfaces with othercomponents such as hung floors, roofs, doorsand windows. The structure itself if renderedand/or plastered will be quite windtight. Tim-ber frame construction being made with pan-els covered with a ply or particle board makesvery tight structures, only failing at interfaceswith floors, roof, windows and doors. In anyconstruction form sealing these interfaces canbe done, but it requires careful supervision ofthe workmen on site who rarely see the needfor all the small detailed work and will bodgeor omit it if you are not looking.

In masonry walls, openings can be made12–15 mm larger all round, the frames sus-pended on metal brackets in the opening andthen sealed all round with one of the expand-ing foams now on the market. In timber frame

construction, the openings are made 6–8 mmlarger all round and then sealed round theframes with silicone-based mastic.

Improving the seal between ground floorand walls is largely a matter of choosing asolid concrete ground floor rather than a tim-ber hung floor. There are then no underfloorvoids to ventilate and no chance of draughtsat the skirting boards. If a timber floor finish isrequired, lay the timber floor on the insulation(plastic foam sheet) on the concrete floor slab,or nail it to battens laid over the slab and placeinsulation (glass fibre or mineral wool quilts)in between the battens. Trying to fix these bat-tens to the slab results only in a creaky floor.Float the floor on insulation and fix it only tothe battens.

The joint of roof to walls is difficult, butagain choice of structure, design and insu-lation can be combined. There are two goodchoices:

� Do not use the roof space for rooms or wa-ter storage tanks with a cold roof1 as suchcoomb ceilings are impossible to seal. Ifcoomb ceilings must be built use a warmroof2 construction and seal the insulationdown to the wall head.

� A cold roof can be used over a horizontal up-per floor ceiling but the ceiling board mustseal to the walls and the insulation must sealto the wall head.

Much else can be done in detail to reduce airinfiltration but attention to these major areascan bring it down from a UK average of 7–9air changes per hour (ach) to 2 or 3 ach. Evenlower values can be obtained but mechani-cal ventilation must be introduced to get overthe problems of retaining foul air within thehouse. Heat recovery in conjunction with me-chanical ventilation then becomes a seriousconsideration, for if mechanical ventilation isinstalled for the whole house – not just an ex-tract fan in a bathroom and a kitchen – the

1 A cold roof has insulation inserted immediately behindthe ceiling finish, the space above being left ‘cold’.

2 A warm roof has insulation placed immediately underthe roof finish: thus the whole of the roof void is keptwarm by leakage through the ceiling finish.

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loss of heat with all the air can be an expen-sive exercise. Discussion of these options iswell beyond the scope of this module but tri-als on some housing association houses inOrkney about 10 years ago confirmed thatit was possible to build and achieve low airchange rates without re-educating the workforce, and that heat recovery could workreally well in a normal domestic situation butdid require a better understanding of how thesystem could be used to advantage by the oc-cupants. The workmen of course were all reg-ular employees of the contractor(s) and wereused to building tight houses in what is a verywindy environment. Some occupants took theadvantage in monetary terms by saving en-ergy – others took the advantage by enjoyingmuch higher ambient temperatures over thewhole house than they could otherwise haveafforded. One cannot say that one group wasright and the other wrong.

Noise control

Noise is a complicated topic and far beyondthis basic text. Suffice to say that the detailsshown will provide satisfactory sound levelsin adjacent compartments of a building. Thesedetails rely mostly on a high mass to achieve asuitable reduction on sound levels. The wallswe will include in our discussion will gener-ally perform within any requirements of theBuilding Regulations when built in a singledwelling.

Fire

Structures are usually designed to have sta-bility, integrity and insulation. In the eventof a fire:

� Stability postpones collapse to allow occu-pants to escape and fire fighters to tackle theblaze.

� Integrity is designed to postpone the pas-sage of flame, hot gases or fumes into ‘safe’areas where they might harm occupants es-caping and fire fighters working, and alsoprevents ignition of neighbouring areas.

� Insulation prevents conduction of heatwhich would otherwise ignite neighbour-ing areas and might even scorch escapingoccupants or fire fighters.

The walls we will discuss will generally meetthe requirements of the Building Regulationswhen used in the context of single or terracedhousing.

Dimensional stability

Thermally induced movement is a phe-nomenon which is fairly well understood bymost lay people but in terms of building struc-tures is far from obvious as structural featuresgenerally either have low or similar coeffi-cients of thermal expansion. Where thermalmovement causes problems it is generallyin conjunction with some other parts of thebuilding, such as the services where metaland plastic pipes etc. expand and contract athigher rates than the structure generally, caus-ing the pipes etc. to fail, but structural damageoccasionally takes place.

Moisture movement is quite critical in termsof building structure. Timber and masonryhave quite high expansion and contractionrates as they take up moisture and subse-quently dry out. Of course, if parts of astructure are kept in stable conditions, nomovement will take place and no damagecan occur. As details are studied, methods ofavoiding the effects of this movement will behighlighted.

A common example of damage caused bymovement due to moisture content is shownin Figure 3.3. It is the cracking of masonrydown the side of an opening spanned by aconcrete lintel. The lintel on shrinking pullsthe masonry away under the wall rest, caus-ing a crack. Two pieces of DPC under the lintelrests would allow it to move independently ofthe masonry.

Structural movement, i.e. associated withdeflection, buckling and compression, are ei-ther static or progressive. If progressive thebuilding collapses, but if static, the buildingcomes to rest and may be perfectly safe but

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Dense concrete lintel

Crack in masonry

Fig. 3.3 Cracking in lightweight concrete wall withdense concrete lintel.

will show unsightly cracking or gaps betweendeflecting members and other surfaces. Thesegaps and cracks, if on the external face of thebuilding, can allow weather to penetrate anddamage the building.

Chemical reactions are sometimes difficultto predict, especially with new materials ornew techniques, but an example might be theeffects of sulphate attack on OPC-based prod-ucts. Mortars and concretes subjected to pro-longed attack by strong solutions of sulphatesswell up. This is a case of the irresistible forceand the (almost) immovable object. For ex-ample, walls resting on foundations being at-tacked will start to lift out of the ground andthe effects in terms of cracking of walls will beseen wherever there is a weakness in the wallabove, e.g. at quoins, scuntions of openings,where there is a lack of wall ties in cavities,lack of proper bond, etc.

A good example comes from an at first baf-fling consultancy where the author was calledto look at a house which was displaying crack-ing in unusual places as well as at a corner ofthe building close to some windows. In fact,what was unusual about all the cracking wasthat it was taking place on the outer leaf ofthe cavity wall but not on the inner leaf. Therewas also a hung timber ground floor whichwas rising and falling, especially under a door

into an integral garage. As it rose it jammedthe door into the garage.

The householder was unwilling to let theteam go down into the void under the floor,which at that point was quite deep. Usinga bore scope, the cavity in the walls wasexamined. A builder was hired to dig out thefoundations on the outside and trial boreswere taken of the subsoil under the concrete.Nothing unusual was observed during thisprocess except a certain scarceness of wallties round the corner area of the wall up tofirst floor level. In one of the excavations thebuilder actually found a bundle of wall tiesready bound up as supplied by a merchant.A bore scope examination of the underfloorvoid showed nothing obviously wrong. Wallties were inserted to bring the numbers upto the required standard and tell tales werefitted across the cracks so that the movementcould be monitored.

Six months passed and still the buildingcracked. Winter was coming so the crackswere pointed up to exclude the worst of theweather. Another six months and still thebuilding cracked. By now the possibility ofthe foundations overturning or sinking hadbeen ruled out, but there was nothing seenwhich could be the cause.

Then we were allowed to go into the un-derfloor void and we found that the floor go-ing up and down was due to two things. Thefloor joists were built into the walls – but notthe one with the big crack – very tightly andit was very humid under the floor. The joistswere moving and since they were restrictedlongitudinally, they hogged – they bowed up-wards, lifting the floor. What was making ithumid under the floor was the fact that al-though there were vents built into the outerleaf of brickwork, there was no hole throughto the inside of the wall – no ventilation to al-low the space to dry out. What was surprisingwas that there was no sign of rot in any of thetimbers.

A builder came in and cut the vents through,and with great difficulty eased off the grip ofthe walls on the timbers. The floor problemwas immediately sorted out, but we were stillleft with the wall cracks. They continued to

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extend. After the builder had gone, the under-floor void was monitored and on the secondor third visit we saw that the hot pitch DPMlooked as though it was bubbling in one ortwo places. We didn’t think too much about itbut on the next visit these bubbles and morehad burst and a white crystalline effusion wasgrowing from the gaps. It was pure sulphateof something! Digging out some of the DPMrevealed that the upfill below the DPM wasspent coal waste, very rich in sulphate, andhere it was piled into a deep substructure cor-ner against a brick wall and over the inneredge of a concrete foundation.

Now we understood what was happening.The OPC-based mortar and concrete in the in-ner leaf and the foundation was expanding.This in turn lifted the inner leaf. There was adearth of wall ties in the ground floor wall,so while the inner leaf kept rising out of theground, the outer leaf wanted to stay whereit was except that above the first floor levelit was tied to the inner leaf. So, upper andlower parts parted company. The new ties in-serted were flexible so had had no effect onthis movement in the short time since theirinstallation. The only cure was to take out thecoal waste, remove the damaged brickworkand concrete and replace with sulphate resist-ing cement based products. No movement hasbeen observed since this was done but it wasan expensive underpinning exercise while itlasted.

Walls of brick and blockwork

Cavity walls have already been introducedin our study of substructures but not in anygreat detail, and certainly without some back-ground as to their current popularity.

The Industrial Revolution brought with itthe need to build cheap housing quickly toprovide for a burgeoning workforce movingfrom rural to urban areas. In many instancesthis meant building in brickwork, and thefavoured method was to build one brick thickwalls to support the roof and occasionally anupper floor. From a structural point of viewthis was quite adequate but the methodology

was inadequate on practically every otherfront.

One of the first of these inadequacies to betackled was the problem of water penetration.Rain blown against a one brick thick wall willpenetrate the full depth of the wall if it is ofsufficient volume and for a sufficiently longperiod of time. Even if penetration does nottake place during the rainfall period, moisturein the outer layers of the wall will percolate tothe inner face, making the inside of the housedamp. Together with a lack of ventilationand substructures without DPC or DPM, thisdampness caused many of the health prob-lems associated with that type of housing.

To overcome these problems, DPCs andDPMs were introduced into the substructureand the cavity wall was introduced. Instead ofa single layer of brickwork one brick thick, twolayers were built each a half brick thick andthe layers were joined together with wroughtiron wall ties placed at 3 per square yard. Thelayers are called leaves or skins. The primepurpose of the cavity is to prevent moisturereaching the inner leaf of masonry.

Wrought iron was a good choice of materialfor wall ties, for although it rusted, it was notas liable to rust to destruction in a short timeas the later invention – mild steel. With theintroduction of mild steel came the need forbetter protection from corrosion and this wasapplied by hot dip galvanising. Alas, the stan-dard of galvanising applied was found in the1960s to be insufficient and one or two build-ing failures led to an assessment of the amountof hot zinc applied to metalwork in general.An updated British Standard was promul-gated and no further failures have occurredalthough monitoring of walls with the old tieshas to continue.

The first cavity was set at 2 inches, a littleover 50 millimetres. Subsequent work on cav-ity walls has revealed that the cavity can be upto 75 wide, with ordinary wall ties at 4/m2.The wall ties are there to stabilise the sep-arate layers of brickwork laterally, one withthe other, and to spread some of the verti-cal loadings from one layer to the other. Be-cause they bridge across the cavity there isa danger that they could provide a route by

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which water could be carried across the cav-ity. For this reason their design incorporates adrip or series of drips. Water will always formdroplets on the lower edge of anything, andso it is with the drips formed on the wall ties.The ties are enclosed in the cavity and so in aspace which has no disturbance from wind orweather. The water trying to cross runs downthe drip and when sufficient has gathered willform a droplet which falls to the bottom of thecavity. The inner leaf remains dry – but onlyif the wall tie remains clean and the drips arebuilt in the correct way up. The latter possibil-ity has been obviated by designing the ties insuch a way that a proper drip is always pre-sented no matter which way the tie is builtinto the wall.

As cavity walls form the major part of alldomestic construction work in some way, it isappropriate to widen our knowledge of them,but first what are the objections to solid wallsas a whole?

Solid walls of masonry can be used for do-mestic construction. However, they must be aminimum of 250 mm thick:

� In blockwork this means using a 250 block� In brickwork this means building a wall one

and one half bricks thick – 327.5 mm usinga metric brick.

Such walls would be very ‘cold’ and would re-quire insulation to be added. Insulating wallsin general will be covered as the chapterevolves.

The reason for that minimum thickness re-quirement is solely the exclusion of weather,particularly precipitation. Walls 200 thick areadequate to bear all normal domestic struc-tural loadings, so to keep out the rain we arebuilding very heavy blockwork and a massiveamount of brickwork.

Early mass industrial housing was fre-quently built as cheaply as possible, whichmeant the provision of one brick thick walls.In very exposed areas a one brick thick wallwill allow rain to penetrate, and in other lessexposed areas dampness will be present onthe inner face of the wall. Rendering the outerface and applying strapping, lath and plasteron the inner face did nothing to prevent these

conditions arising, although these methodswere frequently employed in remedial work.

Only the adoption of a radically new form ofconstruction would be of any help, and it camein the form of the cavity wall. First built as two‘skins’ or ‘leaves’ of half brick thick brickworkwith a 2 in cavity between, the leaves weretied together by the insertion of wrought ironstraps of metal at a rate of about 3 per squareyard.

The idea of the cavity was a simple one – likeall good ideas. Water penetrating the outerlayer of masonry would simply run down theinner face until it reached the foundation. Theinner leaf never got wet. The wall ties en-sured structural integrity and mutual supportfor the leaves. The wall ties were shaped toprevent water running across the tie and wet-ting the inner leaf in. Figure 3.4 illustrates theprinciple.

Cavities have been set at a standard widthof 50 (or 2 in) since their introduction. Widercavities can have advantages in terms of be-ing able to hold more insulation, but goingbeyond 75 makes the use of ordinary wall tiesa problem structurally. A wall with a widercavity has to be specially designed, perhapshaving special wall ties and so on. In additionto the structural problems, a wider cavity al-lows the air in the cavity to start circulating.

Halfbrickthick

Halfbrickthick

Wall tie built

into brick bed

Twist or kink in shank of wall tie to shed water

Pre

cipi

tatio

n

Fig. 3.4 Schematic of cavity wall.

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Warmed air rises, cold air descends and thiscirculation adds to the heat loss through thewall. Of course, reducing the effective widthby putting in insulation will reduce this effect.

Cavity walls of masonry construction arenow built in any combination of brick or con-crete block, reconstructed stone and naturalstone, and any type of brick or block can beemployed in either leaf. Where the brick orblock in the outer leaf cannot withstand theeffects of weather it would be coated with adry or wet dash. The leaves don’t have to be ofequal thickness as long as the sum of the thick-nesses equals at least 200 mm, although a leafof less than 65 mm thick will not be very sat-isfactory from a structural point of view. Be-yond that thickness, ordinary wall ties are notreally effective and a better arrangement hasto be made to connect the two masonry leaves.Figure 3.5 shows the open end of a cavitywall built with concrete block leaves, each 100thick. A wire butterfly wall tie can just be seen.

Wall ties are no longer made of wroughtiron (which resisted corrosion well) but of gal-vanised mild steel, stainless steel or plastic.

Fig. 3.5 Block cavity wall.

A B

C D E

F G H

Fig. 3.6 Selection of wall tie types.

Some metal ties are made of wire, someof sheet or plate. Plastic ties are injectionmoulded from polypropylene. All have splitor shaped ends to build into the mortar bed,and shanks with various forms of drip. Fig-ure 3.6 shows sketches of several differenttypes of tie:

A – A tie of galvanised mild steel, the endsfishtailed and shank twisted to provide adrip. Also available in stainless steel

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B – A tie of light gauge wire formed into adouble loop and the ends twisted togetherto form a drip. Very flexible tie, used inparty wall construction where solid tieswould transmit too much noise

C – A ‘plastic’ tie. The plastic used is poly-propylene, which is very tough andstrong. The ties are very flexible and haveno corrosion problems

D – Another shape of plastic tie. The fish tailends each have a small local thickness toimprove grip in the mortar bed

E – A ‘chevron’ tie. Available in galvanisedsteel or stainless steel. Both made fromthin sheet stamped with very shallowgrooves in the chevron pattern. Gal-vanised version prone to corrosion fail-ure. Flexible tie with a poor grip in themortar bed

F – A wire tie, a heavier gauge of wire, avai-lable in galvanised mild steel or stainlesssteel

G – Another shape of wire tie in stainless steelH – An unusual tie of stainless steel strip,

twisted into a continuous helix.

One of the difficulties of bridging the cavitywith wall ties is that the ties can gather mortardroppings. The droppings can allow water torun or be drawn across the tie onto the innerleaf. If the quantity of water drawn across isexcessive, a damp patch appears on the insideface of the wall.

To prevent mortar gathering on the ties itusual to specify that a batten will be used toprevent droppings resting on the ties, and itwill be drawn up by cords through the cavityjust before the next course of ties is built in,as shown in Figure 3.7. Alternatively, a strawrope can be laid over the ties and drawn upthrough the cavity before the next course ofties is laid, although that material is ever moredifficult to source.

Insulation of external walls

While the adoption of the cavity wall and theever-growing variety of wall ties have solvedthe problems of water penetration of walls,

Batten laid on wall ties, cords to one side

One leaf built to next course for ties, mortar droppings gather on batten, cords put over leaf

just built

Other leaf built to next course for ties, batten pulled out of cavity by cords, bringing up mortar

droppings with it

Fig. 3.7 Keeping the cavity clear of mortar droppings.

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On the outside

of the wall

On the inside of the wall

Partially filling the cavity

Fully filling the cavity

Fig. 3.8 Possible placement of wall insulation.

they did nothing to improve the thermalperformance of the cavity wall – which ismuch better than a one brick solid wall.

Initial attempts centred on the use oflightweight concretes or aac to make blocksused only for the inner leaf. These are not suf-ficient in meeting today’s much higher stan-dards but can be used in conjunction with lay-ers of insulation material. We will see howthey can contribute when looking at U-valuecalculations.

Insulation material can be placed in one ormore of three positions in a masonry cavitywall, as shown in Figure 3.8:

� On the outer face� On the inner face� In the cavity

� filling the cavity� partly filling the cavity (always on the in-

ner leaf).

Insulation can be placed in the cavity as thewall is built, or it may be injected or blowninto the cavity.

Injection of insulation into the cavity willbe examined a little later, as it is generally anexercise carried out on an existing wall.

Insulation applied to the outside of a wall isgenerally done as part of a refurbishment ofan existing structure. It involves specialist fix-ings and a new weather resistant finish. Thesesystems are not examined in detail in this text.

One, for example, uses a composite layer ofinsulation: perforated building paper, a stain-less steel mesh and a two-coat render. The in-sulation is polystyrene bead board to which isstuck a layer of building papers3 with verticalslots, and through the slots is woven a stain-less steel wire mesh. This first layer is fixedto the masonry wall with a fastener not un-like a framing anchor (see Appendix G) with alarge plastic washer. Once fixed, a two-coat ce-ment/lime render is applied which effectivelyseals off the insulating layer from the weather.

This leaves the study of systems built or in-jected into the cavity, or built onto the innerface of a wall.

Materials available are:

� Glass fibre� Mineral wool� Foamed plastics:

� polystyrene beadboard or as loose beads� extruded polystyrene� urea formaldehyde foams� phenol formaldehyde foams� polyurethane foams.

Glass and mineral wool materials compriseglass or mineral wool fibres laid down into aquilt or mat, a slab or batt, each of differinggrades or densities. The fibres are coated witha resin-based adhesive; the lighter the prod-uct the less adhesive is used. Quilts are avail-able in rolls, generally 1200 wide, and thesemay be further ‘split’ inside the pack into 2 ×600 mm or 3 × 400 mm, which suit standardstud centres. Quilts can be supplied ‘plain’ or‘paper covered’ one or both sides. The ‘papers’can be plain, bitumen bonded, foil faced, etc.

Batts are supplied as flat sheets in a varietyof sizes and to a variety of densities. Higherdensity batts have better loadbearing char-acteristics but poorer thermal performance.Those to be built into a cavity are of low

3 Building paper is specially formulated to resist the pas-sage of ‘solid’ water but allow the passage of watervapour. Resistance to the passage of solid water is onlyeffective in the long term if the ‘wet’ face of the materialis left uncovered, i.e. it cannot be pressed up against thewet inner leaf.

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density, high thermal performance and havea width of 440 mm – six courses, which is gen-erally the vertical spacing between wall ties.Those batts designed to fill a cavity have the fi-bres aligned across the width to assist with thedrain down of any ‘solid’ water which pene-trates the outer leaf.

The foamed polystyrene and polyurethaneare supplied as sheet material, which can beapplied to inner wall surfaces or as a partialor full cavity insulation. A variety of densitiesis available. The extruded polystyrene is verytough and for the same thickness has a bet-ter thermal performance than beadboard ofthe same density. The extruded board is com-monly referred to as blueboard and it has agood resistance to water vapour and is water-proof. The beadboard is quite tough but ismore compressible than the extruded boardof the same density. It does have some load-bearing ability and special grades are man-ufactured and commonly used under floorscreeds and floating timber floors. It is notwaterproof and is very permeable to watervapour. It should be familiar to anyone whohas unpacked a computer, TV or white goods,its main use being in moulded packaging, al-though the grade used is much more friablethan the one used as insulation.

Polyurethane board is very friable and isusually supplied faced on one or both sideswith bituminous felts, foils and papers. It isslightly more expensive than polystyrene butwhen first made has a higher thermal resis-tance due to the foaming process leaving thepores filled with carbon dioxide rather thanair. However, after a period of time the car-bon dioxide leaches out and is replaced withair, and there is no thermal advantage. Theother plastics are more expensive and so havelargely been overtaken by polystyrene.

Filling the cavity with sheet or batt insula-tion presents one problem – how to stop watercrossing the insulation to the inner leaf. Withthe batts, the fibres are aligned vertically in thecavity; provided adjacent sheets of the mater-ial are pressed firmly together, water will notcross at the joints even at the wall ties. Thejoints in the batts must be kept clear of mortardroppings.

Extruded polystyrene and urethane foamsheets are generally waterproof and water canonly penetrate at the joints. To stop this a re-bated or tongued and grooved joint is formedon the sheets. Polystyrene beadboard is veryporous but it is thought that it is not so loose asto allow water to drain downwards, the wa-ter tending to cross the insulation to the innerleaf. For this reason, some professionals areof the opinion that it should only be used forpartial filling of a cavity.

Partial filling of the cavity is usually donewith foamed plastic sheet. The problem here isto ensure that the sheet remains firmly againstthe inner leaf. As the masonry skins are built,the sheet will be placed against the innerleaf between the courses with wall ties, theties passing through the horizontal joints ofthe sheets. Most tie manufacturers now makepatent plastic ‘clips’ which fasten to the shankof the tie and can be pressed firmly against thesheet, holding it in place. Figure 3.9 illustratessuch a device.

The urea formaldehyde and phenolformaldehyde plastics are chiefly used as‘foam in place’ insulation or sealers. Theyform a waterproof layer between the twoleaves of the wall, therefore water penetratingthe outer leaf will concentrate on the faceof the insulation. As insulation in buildingthey are used to insulate existing cavities byinjecting through holes drilled through themortar joints of the outer leaf.

Problems of dampness crossing the cavityhave been experienced and have been largelydue to mortar droppings left on wall ties.In the ‘clean’ cavity, the foam seals roundthe shank of the wall tie. If there are mortar

Walltie

Retaining disc

Insulation

Fig. 3.9 Retaining clip for partial fill insulation incavity.

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droppings the foam will not form a seal, andwater is concentrated at these weak points andpenetrates the foam and the inner leaf. Thiscan occur even if the unfilled cavity had notshown signs of being bridged prior to inject-ing the foam.

Using loose foamed polystyrene beads is aneffective way of filling a cavity with insula-tion, generally as a retrofit or upgrading exer-cise. Holes are drilled through the masonryjoints and beads are blown into the cavitythrough a fairly large nozzle. At the nozzleanother smaller pipe injects an adhesive intothe stream of beads. Once the adhesive has set,any cutting of holes through the wall in the fu-ture does not result in all the beads runningout and having to be replaced. Because the in-sulation formed has a very open texture, wa-ter penetrating the outer leaf drains down anddoes not cross the cavity. Again, wall ties mustbe clean to prevent water crossing at the ties.

Another ‘injected’ insulation technique isto blow mineral wool fibres into the cavitythrough holes drilled in the outer leaf. Theholes have to be much larger than those usedfor the injection of foam and so are more dif-ficult to camouflage on completion.

A more worrying aspect in the early devel-opment of the technique was the habit of thefibres to slump to the bottom of the cavity af-ter a period of time. This was quite a naturalprocess caused by the excess air between thefibres being replaced with the fibres above.This effect could be exaggerated if there wasvibration caused by traffic or the occupants,and was made even worse if the fibres becamewet due to rain penetrating the outer leaf. Bothwere overcome by including a waterproof ad-hesive with the wool during injection. Oncethe adhesive set, the wool maintained its loft,and its open texture allowed water to draindown rather than pass across the cavity.

Placing the insulation on the inside face ofthe wall can be done in a number of ways.Traditionally, thin timber or strapping is fixedvertically to the inside face of the wall at cen-tres. Further timbers are fixed horizontally atthe top and bottom of the wall, as well asshort lengths of strapping (noggings) in twoor three rows between the verticals. Among

this framework, a quilt or batt of glass or min-eral wool can be placed and the whole cov-ered with a suitable sheet material – generallyplasterboard. To prevent hot moist air fromthe room penetrating to the cold wall face,condensing and causing rot etc., a layer ofpolythene sheet is placed between the plaster-board and the strapping. The layer is knownas a vapour barrier, vapour check or vapourcontrol layer. Vapour barrier is too definitea term – it is impossible under constructionsite conditions to put up a vapour barrier.At best one can control or check – as in de-lay or reduce – the flow of moisture throughthe layer. Check is indefinite in meaning, sovapour control layer would be the best op-tion. Figure 3.10 illustrates the wall strappingtechnique. Vapour control layers will be thesubject of a more detailed discussion as theyoccur in various other parts of the construc-tion process.

CeilingContinuous timberalong top of wall

InsulationPlasterboardNogging

NoggingVapour controllayer

Continuous timberalong bottom of wall

Floor

Continuous timberalong top of wall

Continuous timberalong bottom of wall

Verticaltimbers

Nogging

Fig. 3.10 Wall strapping on inner face of wall withinsulation and boarding.

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More modern practice uses a laminate of aplastic foam sheet onto a finishing type board,such as plasterboard, plywood, fibre board,etc. This material is applied direct to the insideface of the wall and fixed using a gap fillingadhesive. The use of a vapour barrier underthe finishing board is not usually necessaryas the foam plastic sheet is fairly well mois-tureproof. Simple precautions with a sealingmastic can be used at joints.

If a very positive moisture seal is required, itis possible to have the laminate made up witha polythene or foil layer between the foam andthe board. The joints of the boards can be madeup with a rebate and the two layers of film orfoil can be sealed together. This is illustratedin Figure 3.11. Adhesives used are generally

30 plastic foam sheetbonded to plasterboard

Vapour control layerson adjacent sheetsoverlap. A mastic

sealer may be applied

Rebate

9.5 taper edgedplasterboard

Boardfinish

Insulation

Gap filling adhesive stripes

200 wide and at 400 centres

Fig. 3.11 Plasterboard/insulation laminate bonded towall with adhesive stripes.

based on a gypsum plaster and may be modi-fied by the addition of a bonding agent. Thesheets have the insulation bonded to the plas-terboard so that the insulation projects on twoadjacent edges and the board projects on theother two adjacent edges. The vapour controllayer covers all exposed faces in the joint areaand so can be sealed together.

Because of their friable nature, urethanefoam boards are not suitable for laminating toa sheet finish or adhesive fastening to a sub-base. For this reason polystyrene foam is moreusual.

Timber frame construction

Traditional timber frame

The definition of ‘traditional’ depends on howfar back in time one wishes to go. Early con-struction forms used the natural shape oftrunks and limbs, which might have beenhewn with an adze into a roughly rectangularcross-section – all as recently discovered onreality television!

For the purposes of our explanation, we willconsider timber frames built on site using onlysawn timbers for the frame. Within our defi-nition there are two distinct forms of timberframe:

� Balloon� Platform.

Both are built off masonry/concrete walls andfoundations.

Choice of ground floors is:

� Hung timber� Solid concrete

For the hung timber floor shown in Fig-ure 3.12:

� Both forms start with a ‘ring beam’ builton a wall plate, both beam and wall platebeing fixed to the masonry base withragbolts.

� The ring beam could be either a single tim-ber or two timbers nailed together.

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Joists at450 centres

Ring beam of doubledtimbers nailed together

Ragbolt

Ring beam 150--200 deep

and 50 or 75 thick Joists at

450 centres

Ragbolt

With timber joistedground floor-- joists at

right angles to ring beam

With timber joistedground floor-- joistsparallel to ring beam

50 thick wall plate

Threadedend

Ragged endgrouted

or built intomasonry

Ragbolts forged in mild steel, generally from 12 dia. upwards. Supplied

with large washer and square nut

Fig. 3.12 Timber joisted floor as base for traditionaltimber frame building.

For the solid concrete floor, two possible foun-dation details are shown in Figure 3.13:

� A masonry wall and a separate floor slab.� A poured concrete foundation, wall and

floor slab all in one. This is a common formof construction in North America where thepoured concrete wall can form a basementor semi-basement.

In either case there is no need for a timberring beam, and the wall plate is ragbolted tothe edge of the concrete floor slab.

On the timber ring beam construction it wasnormal to fix a sole plate made up of two tim-bers on their flat. With a wall plate on a con-crete floor edge, a single layer sole plate wasused. In either case vertical studs rose, theirheight depending on whether the construc-tion form was to be balloon or platform.

In-situ concrete slab

50 thick wall plate

Ragbolt

One piece in-situconcrete floor

slab andfoundation

Ragbolt

50 thick wall plate

Fig. 3.13 Starting a timber frame building off a con-crete floor slab.

Platform construction means having thestuds one storey high. On top of these studsthe joists for a floor or platform are set; thenanother sole plate is set down and studs foranother storey height are set up. In balloonconstruction, the stud height is set for twostoreys. It is not generally possible to get tim-bers long enough for more. The intermediatefloor is joined to the middle of the studs. Fig-ures 3.14 and 3.15 illustrate both forms of con-struction.

In both instances there is a certain amountof prefabrication but this is done on site andis usually done by taking one layer of the soleplate and fastening studs to it, all for one wallplane, capping the studs off with a single run-ner. All this is done on the ground or the floorslab. Bracing of thin wide timber is let into re-cesses cut in the sole plate, studs and runners,

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Sole plate nailed to bottom of stud and to wall plate

Stu

d le

ngth

Stu

d le

ngth

Stu

d le

ngth

Runner nailed to stud

Double runner nailed on after panel

erected

Double sole plate nailed

to joists

Runner nailed to stud

Double runner nailed on after panel erected

Platform frame construction

Balloon frame construction

Fig. 3.14 Schematic showing platform and balloonframe construction.

Stud

Stud

Sole plate or runner

Joist halvedround studRiband let

into studs

Housing joint of studinto sole plate orrunner

Fixing first floor joist to stud inballoon frame construction

Fig. 3.15 Some joints in traditional frame construc-tion.

and dwanging is added in the height of thestuds at about 750 centres. Finally, the frameis erected on the wall plate or other half ofthe sole plate and the two are nailed together;temporary bracing keeps the panel vertical. Adouble runner is added on top of the panels.

With platform frame, the double runner ontop of the studs is used as a bearing for theupper floor joists. Once these are in position,another single sole plate is nailed to the top ofthe joists, to which a further storey height setof panels is added; and so on. An additionalrunner is added to the top set of panels andit is to this double set of runners that the rooftimbers are fixed.

In this form, and in balloon construction, it isimportant that all timbers – joists, studs androof timbers – are fixed at the same centresso that loadings from roof and upper floorspass direct to studs, direct through joist ends,etc.

In balloon frame construction the floor join-ing halfway up the stud length is supportedpartly by a thin, wide timber called a ribandlet into the inner face of the stud, and partly bya halving joint of the joist to the stud and nail-ing to the stud. The top of the framed panel ismade with a single runner and if a roof has tobe refastened at that level, the runner is dou-bled up the same as in platform frame.

Typical sizes of timbers used are:

� Studs, runners, sole and wall plates,dwangs or noggings – from 150 × 50 to 200× 75

� Bracing and ribands generally 32 or 38 thickand 200 to 250 wide

� Joists from 150 × 50 to 300 × 75� Diagonal boarding 20 or 25 thick, 150 or

200 wide� Clap boarding and weather boarding 15 or

20 mm thick� Matchboarding 15 mm thick.

Once the frame is erected, the outer face willbe covered with plain edged sawn boardingabout 150–200 wide and 20–25 thick, laid di-agonally; the edges are butted tightly togetherand nailed twice through the face to every tim-ber – studs, sole plates, runners and dwangs.

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This diagonal boarding has two purpose:

� Being diagonally laid, it contributes to theracking resistance of the frame

� Nailing twice to every underlying struc-tural timber holds the frame tightly togetherand contributes significantly to its strength.

Once the diagonal boarding is fixed, a weath-ering surface is fixed in place. This can be of avariety of materials but is generally of the im-pervious layer type, where rain is shed ratherthan absorbed and allowed to evaporate. Ma-terials commonly used are:

� Timber weather boarding, shingles, claytiles and slate

� A layer of bituminous felt placed over thediagonal boarding. Very early examplesused a ‘tarred paper’.

Modern materials used in refurbishmentwork include plastics weather boarding, con-crete tiles, synthetic slate and tile, fibre boardsheathings, and profiled metal claddings.Whatever is used it is important to allow theold frame to breath. It must not be sealed offin any way.

The inner face used to be finished either withtimber match boarding and/or with timberlath and plaster. The void between the two lay-ers was frequently left empty, which means acold house. If the climate was very cold, veg-etable fibre such as straw, grass, reed, heather,moss, etc. would be packed in to improve theinsulation. If the weathering face broke downand leaked water into the construction, rapiddecay took place as none of the timber wastreated with preservative. No vapour controllayer was placed behind the internal wall fin-ish – they hadn’t been invented.

With no vapour control layer, moisture fromthe inside was able to pass through the walluntil it reached the tarred paper or bituminousfelt. Here it condensed and the now ‘solid’ wa-ter wetted the boarding, causing rapid rottingof board and timbers. Without either tarredpaper or vapour control layer, the wall could‘breathe’ and any condensation on the backof the outer boarding could evaporate away,even if only slowly.

Once the mechanisms of moisture passingthrough the wall to condense behind a coldimpervious layer are understood, it is easy towork out a solution: provide a gap betweenthe tarred paper or bituminous felt and thediagonal boarding, and allow this gap to beventilated, as shown in Figure 3.16. This canbe done by simply applying vertical battens tothe face of the diagonal boarding to which thepaper or felt is fixed, followed by the weather-ing skin. Allow this space to remain open topand bottom and it ventilates naturally.

Modern timber frame construction

Modern timber frame is built on the plat-form frame model and employs many of thesame ideas as the traditional forms but ap-plies factory prefabrication, regularised stressgraded timbers, and plywood or orientedstrand board sheathing panels instead of diag-onal boarding, so there is no need for bracings,the sheathing shares the loading supported bythe wall panels, timbers are much smaller incross-section, and cladding is generally a ma-sonry skin 75–102.5 thick of brick or block, fac-ing brick or with a roughcast finish. Weatherboarding is also used, either of natural timberor of plastics, and this would be fixed to verti-cal treated timber straps nailed over the studsin the panels. The skin of sheet boarding issuch an integral part of the prefabricated pan-els that the construction form should really betermed a stressed skin panel.

Once the panels are delivered to site, theyare erected on a sole plate or wall plate. Theyarrive covered with a layer of building pa-per. The building paper is stapled on througha heavy plastic tape which prevents the pa-per tearing at the staples. The tape is similarto that used to by suppliers and merchants tobind bundles of their material. Building pa-per can be a bitumen bound fibre paper or,more commonly now, a synthetic fibre (usu-ally polypropylene) felted membrane such asCorovin or Tyvek.

Once the panels are erected and braced up-right, the roof is put on. When weathertight –

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Match boarding orlath and plaster

Nogging

StudWeatherboard

Tarred paperor roofing felt

Clap boardor shiplaptimbers

Tarred paperor felt

Diagonal boarding

Weather boarding

Verticalbattens

Verticalsection

Horizontalsection

Ventilation space

Stud

Stud

Verticalsection The 'solution'

Fig. 3.16 Traditional timber cladding and improved methods of cladding over insulated walls.

at least the roofing felt in place and counter-battened – the panel frame is filled with a glassor mineral wool insulation, a vapour controllayer may be fitted and a finish applied, usu-ally plasterboard but also other sheet materi-als as well as timber panelling.

Figure 3.17 shows a timber frame construc-tion at substructure level, Figure 3.18 shows

11 OSB sheathing

Breather membrane

Insulation in 97 stud space

12.5 plasterboard onvapour control layer

97 x 47 S/w sole plate and doubler

147 x 47s/w joists

Polypropylenenet stapled tounderside of

joists

147 x 47wall plate

Weep

Concretecavity fill

150

300

150 min. VentDPC

147 x 47ring beam

Addl membrane on panel turned down over ring

beam

32 x 3 galv. m.s. strap nailed to studs and built

under wall, at 2000 max. centres

550

Fig. 3.17 Modern timber frame – substructure detail.

that construction at eaves level, and Fig-ure 3.19 shows the junction of an upper floorwith the timber panels. Note that a lot of theannotation has been omitted for the sake ofclarity. On properly scaled detail drawings theannotation would be complete, either on thedetail or in a notes column.

Line of eavesconstruction Line of roof

timber

97 x 47 S/w top runner and doubler

100 gauge polythene vapour

control layer

12.5 plasterboard

97 x 47 nogging

Masonry skin

Breather membrane on

11 OSB sheathing

Wall ties, 3 or 4/m2

nailed to studs with 2 x 65 galvanised

improved nails

Weep

50 x 50 S/w firestop with

DPC over face

Fig. 3.18 Modern timber frame – eaves/wallheaddetail.

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JoistRing beam

Nogging for floorboard edge

Nogging for ceilingboard edge

Weep asventilator

Weep asventilator

50 x 50firestop &

DPC

Wall tie nailedto studs

Masonry skin

Breathermembrane

on sheathingof plywood

or OSB

Timber framepanel Glassfibre

insulation instud framing

12.5 plasterboardand VPC

Sole plate or doubler

Head plateor doubler

12.5 plasterboard

12.5 plasterboardand VCL

Nogging

50 cavity

Bottom runner of panel

Top runner of panel

Note: The void in the depth of the floorimmediately behind the ring beam is a

potential cold spot and a roll of glassfibreinsulation should be placed there

Fig. 3.19 Modern timber frame – intermediate floorjunction detail.

It is important to note:

� The hold down straps built in every2000 mm or so and nailed to the studding –Figure 3.20. Note the building paper turneddown to cover the joists and ring beam.

� The weeps built in every fourth or fifthbrick, top and bottom of the outer leaf ofmasonry to ventilate the cavity and keepthe surface of the timber frame panels dry –Figure 3.21. Note how this weep projects be-yond the face of the blockwork to accommo-date the render to be applied.

On the outside, a layer of masonry is built offthe foundation to give a 50 mm cavity nextto the timber frame panel. Wall ties are nailedto the studs of the panel and built into thecourses of the masonry – three to four persquare metre with additional ties at quoins,intersections and openings. Nailing shouldreally be with two nails per tie, and an im-proved nail would be best. Unfortunately few

Fig. 3.20 Hold down strap from timber frame to topof foundation.

bricklayers are prepared to go to the lengthof putting in more than one nail and the useof improved nails is always a matter of whatthey are given!

Special ties are available which have holesfor nails at one end and a ‘grip’ for beddinginto a mortar joint. Figure 3.22 shows two sam-ples of these ties.

The masonry can be of:

� Half brick thick brickwork, either facingbrick or commons, with a roughcast

� Blockwork 75 or 100 thick and roughcast� 100 thick fair faced blockwork, natural,

artificial or reconstructed stone.

Although the weeps have been previously de-scribed as ‘ventilating’ the cavity formed, thebottom weeps also act as cavity drains. Theventilation of underfloor voids must be sealed

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Fig. 3.21 Weep as a ventilator at the wallhead.

Fig. 3.22 Just two of many types of wall tie suitablefor timber frame cladding.

off from the cavity by use of a liner to preventthe spread of fire from the underfloor void tocavity and vice versa. A fire stop of timber atleast 50 thick (measured in the direction of thecavity) must be nailed to the top of the panelsto prevent the spread of fire either to or fromthe intermediate floor or the roof void. Each‘separation’ of the cavity must be ventilatedwith weeps top and bottom.

The details you don’t see are often more im-portant than the ones you do. The wall panelswe have looked at earlier were all fire stoppedhorizontally to reduce the spread of fire up thecavity between panel and cladding. However,there is a need to fire stop vertically to reducethe spread of fire horizontally. This is done atcorners; Figure 3.23 gives a detail while Fig-ure 3.24 shows a photograph. What is not im-mediately apparent in the photograph is thatthe horizontal fire stop is there but shows onlyas a bump under the breather paper – just be-low the scaffold boards. Now why didn’t thebuilder put up the corner ones as well, thenput on the building paper? Well, the panelscome with the paper already fixed in place andso when the paper from the upper panels wasdropped, it covered the fire stop. Then theycame along and fixed the corner fire stops.

Stud

Stud

Stud

Stud

Facing brickCavity

11 OSB and breather membrane

50 x 50 S/w firestopDPC stapled to

firestop

Fig. 3.23 Vertical fire stopping at corners of a building.

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Fig. 3.24 Vertical and horizontal fire stopping.

But they got it all wrong. The breather pa-per, while repelling solid water, does so onlywhen the surface of the membrane is open,free, and unobscured. If it is pressed betweena fire stop and the masonry, it will draw inwater like a sponge after a short period. Whatmade things worse on this particular site wasthat the builder did not put any DPC on top ofthe fire stops, which were untreated timber –a recipe for rot and subsequent disaster.

But there was one mistake which was neverphotographed. The metal straps joining upperand lower panels across the upper floor ring

beam should be fixed before the breather pa-pers are fixed down so that the workmen cansee where the studs are. On this site, they werefixed afterwards so that some of the strapswere nailed only to the frame sheathing, theOSB (oriented stand board), which was only11 thick, and they were bent to come over thehorizontal fire stop. They were therefore notstraight and tight across the junction. This alsomeant that they were pressing against the in-side face of the masonry skin.

The rules, therefore, are:

(1) Fit straps across upper floor junctions be-fore fixing the membranes down or firestopping is fitted.

(2) Fix straps down into substructure beforefixing the membranes down.

(3) Breather membranes should be completedover the whole of the timber frame panelsbefore any other work is carried out.

(4) Fire stopping of treated timber should befixed over the membrane.

(5) DPC material should be fixed over the firestopping.

(6) No breather membrane between the firestop and the masonry cladding.

Figure 3.25 is a photograph of a ground floorbuilt on a block substructure and ready tohave wall panels erected on it. The wall platewith the ring beam and the sole plate dou-bler are all there round the outside. Then thereare sleeper joists with solid strutting acrossthe mid span of the joists. What is not terri-bly clear is the two sleeper walls with wallplates supporting the joists at their mid thirdpoints.

Fig. 3.25 A ring beam and joisted base ready for panelerection.

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Fig. 3.26 Panels erected on a base.

Figure 3.26 shows panels erected on such abase4. However, one can identify all the tim-bers. Notice how the vent liner has not beenproperly built in.

Following chapters will deal with openings(Chapter 5), doors (Chapter 8) and windows(Chapter 9), and more detail of the wall tech-niques will be revealed then.

Figure 3.27 shows a photograph of the up-per floor wall panel junction. The wall panelbuilt in here incorporates a window openingand we will return to this photograph whenwe look at openings in walls in the next chap-ter. For now, note the top runner right acrossthe opening and the ring beam and upperfloor joists, with the nogging for the edge ofthe floor boarding.

Loadbearing and non-loadbearinginternal partitions

Partitions can be built from a variety of mater-ials. We will concentrate on:

4 The reader will be aware by now that the photographshave been taken on different sites. This was not a goodsite; the last photograph, Figure 3.25, was a good site.Basically the difference was that good sites had the con-tractor’s own workforce, who knew what was expectedand had been trained in or inducted into the techniquesrequired. Bad sites used labour hired by the squad, andthe contractor knew little about and had no interest intheir abilities or basic knowledge.

Fig. 3.27 Intermediate floor/panel junction over awindow opening.

� Simple half brick thick brick and narrowblock partitions, 50, 75 and 100 thick – thesecan be loadbearing or non-loadbearing.

� Stud partitions clad with sheet materialboth sides – these can be loadbearing or non-loadbearing.

Whether a partition rises from the floor or sub-floor surface or an independent foundationdepends on:

� Whether it is load or non-loadbearing� The floor structure� The mass of the material from which the

partition is made.

Brick partitions in domestic constructionmust generally be built off an independentfoundation, whether load or non-loadbearing,as the self weight is high:

� Foundation minimum width 450� Minimum thickness 150.

However, with a concrete sub-floor (not oversite concrete) it would be possible, depending

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Floor structurezone in

hatched area

DPC level in

outer wall

FGL

150 min.

450 min.

150 min. 100 min.

100 min.

Two layers of fabricreinforcement

Lightly loaded partition

Highly loaded partition

Only suitable for single layer concrete floors with a slab thickness of at least 100 mm. NOT suitable for use with minimal sandwich floors

Suitable for all hung

floor construction types and for solid or

sandwich floors of concrete

Fig. 3.28 Partition foundations (1).

on loadings, to build off the sub-floor – but seeFigure 3.28 for the options available.

Dense concrete block partitions must betreated like brick partitions. They have sim-ilar or greater self-weight. Medium and lightweight block partitions would generally needan independent foundation if they werehighly loadbearing, or could be built off a sub-floor if lightly loaded, as in the brick partitiondetail in Figure 3.28.

Non-loadbearing block partitions are builtoff the floor surface if it is of concrete, solidor hung, or a timber runner if it is of timber.A timber runner is also fixed to the ceilingand the blocks are built between the two run-ners. These are shown in Figure 3.29. Note thatthe floor structure must include either an ad-ditional joist or noggings or, preferably, solidstrutting between two joists under the runner.

Non-loadbearingpartition on solid floorLoad bearing partitionsrequire the floor to be

thickened locally

Non-loadbearingpartition on sandwich

floor

Floor joistsSolid strutting

Structuralceiling

Top runner 38--50 thickand same width as the

block

Bottom runner or soleplateFloor boarding

or sheeting

Block partition running parallel to the joistssupported by solid strutting or a pair of joistsnailed together. If running across the joists,

a simple nogging would be adequate toprevent the boarding from sagging.

Fig. 3.29 Partition foundations (2).

A lightly loaded block partition built off asandwich floor must be built off the lowerlayer of concrete, which in turn should bethickened locally as well as reinforced lo-cally – see Figure 3.30.

Stud partitions are made from sawn or reg-ularised timber starting with a runner or soleplate fixed to a floor surface; a runner is fixedto the ceiling surface and there is an infill of

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150 min.

Fig. 3.30 Partition foundation (3).

vertical timbers or ‘studs’. Generally, dwangsare fitted between the studs at a maximum of800 intervals vertically to prevent buckling ofthe studs. This is particularly important if thepartition is loadbearing. See Figure 3.31 for therange of options for junctions etc.

The frame is generally clad with plaster-board 9.8 or 12.5 thick, the 12.5 thickness be-ing used to give fire protection on the side‘at risk’ from fire damage. The plasterboardcannot contribute to the loadbearing charac-teristics of the partition. Timber sizes used aregenerally the same in any one partition and,in domestic construction, can range from 50 ×38 to 100 × 50 depending on the requirementsplaced on the finished partition.

Brick and block partitions, because of theirmass, have good sound insulation quali-ties, whereas stud partitions tend to act like‘drums’ and transmit sound easily; indeedthey seem to magnify certain frequencies. At-tenuation is possible by filling between thestuds with fairly dense rockwool or fibre glassquilts, and even doubling up thicker layers ofplasterboard – at least 12.5 thick – while fixingadjacent layers broken jointed.

A further technique shown in Figure 3.32is the provision of a separate set of studs tosupport the boarding on each side of the par-tition, and the fibrous quilt is wound betweenthe studs. Again heavy plasterboard – 12.5 or19 thick – is used to give mass.

Stud partitions can be loadbearing, partic-ularly in lightweight domestic construction.

Vertical section

Alternativecorner details

Alternative corners ofboarding

Alternativejunction details

Horizontal section

400 cskCeiling structure

800 csk

Dwangs ornoggings

Packingpiece

Timber corner reinforcementPlain edge board

Taper edge board

Metalreinforced

cornertape

Packing piece

Stud nailed to runners

and dwangs

Toprunner

Bottomrunner

Floor structure

Fig. 3.31 General partition details.

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50 x 38 min. studs

50–75 thick glass fibre or

mineral wool quilt

Single or double layerof plasterboard

Single or double layer

of plasterboard

Fig. 3.32 Acoustic partition.

They are always supported by the floor struc-ture which might require bolstering under theline of the partition. This is perfectly feasible,as the details show. A structural partition willrequire fire protection on both sides even if itis within a building of ‘single occupancy’, i.e.a single house.

In the mass local authority housing of the1960s and 1970s, it was common practice tobuild 2400 high loadbearing partitions with50 × 38 studs at 400 centres, 50 × 38 soleplates and two rows of 50 × 38 dwangs. Thepartition was clad with 12.5 plasterboard bothsides. Such partitions were set on the groundfloor of hung timber at right angles to thesleeper joists and within a quarter joist span ofa dwarf or sleeper wall. They were designedto carry a share of the load from the first floorand sometimes a further partition above – butnot carrying any of the roof load. Both parti-tions had to be directly above one another.

Racking resistance is generally not requiredas ends are usually fixed to outer walls, andtop and bottom runners are fixed to ceilingand floor structure respectively.

Non-loadbearing stud partitions can bebuilt off the upper surface of any concretefloor. Loadbearing stud partitions would nor-mally be lightly loaded and so could be builtoff the top of a single layer floor without thick-ening it. With a sandwich floor, it would bebetter to build off the lower layer, as shown inFigure 3.33, purely because the upper layer isrelatively much thinner and cannot be thick-ened locally without losing the insulationlayer.

The sequence of construction of such a par-tition is as follows:

60

40

75

Bottom runner nailed to packing pieces

Packing piecesdirectly

under studs

Sole plateplugged toconcrete

floor

Fig. 3.33 Partition base in sandwich floor construc-tion.

� Sole plate is plugged to lower layer of con-crete, and short packing pieces or a contin-uous piece nailed to it. The top of the pack-ing piece is set to finish at the same level asthe upper layer of concrete. Short packingpieces must coincide with the ends of studs.

� Lay underfloor insulation.� Lay upper layer of concrete.� Nail bottom runner of partition to packing

pieces and complete partition in usual way.

Expansion joints

External walls can be very long when appliedto terraced housing, and when combined withmaterials with a high rate of moisture or ther-mal expansion and contraction there can beproblems with walls cracking etc. in awk-ward places. Apart from being unsightly, ran-dom cracking can cause structural instability,largely due to the fact that the two parts ofthe wall are not tied together in any way andcan move independently of each other underother influences – thrust from floors or roof,wind loadings, etc.

The effect of this moisture and thermalmovement is most keenly felt in concreteblock masonry. The simple solution is to break

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long stretches of wall into short lengths and ateach length provide an expansion joint. Thereare two ways of doing this:

(1) Physically stop building the wall, leave agap filled with some flexible material andthen begin building the wall again, onlyjoining the two separate walls with someflexible jointing material.

(2) Build in flexible jointing material in alength of the wall and then cut a slot inthe wall with a power saw to about halfits thickness.

The first is a conventional expansion joint, thesecond more of a stress relief joint or inducedcrack joint.

First of all, how far apart must these jointsbe placed? There is a lot of conflicting advicebut it does appear to have some correlationwith just how much the material in the wallis expected to move under different moisturecontent conditions. A two-storey terrace of

six flats not too far from the author has anexpansion joint in the centre of each gable andothers back and front on the party wall linebetween the vertical pairs of flats – about 6 mapart. So in approximately 50 m of wall thereare six expansion joints. Another building,a supermarket, has many fewer joints, oneof which is shown in Figure 3.36 and whichdraws some adverse comment for the way ithas been done – and yet there is no sign ofany adverse effects of differential movementon the building.

So the best we can do here is look at howexpansion joints and induced crack joints arebuilt. Figure 3.34 illustrates both styles of jointand Figures 3.35 and 3.36 are photographs oftwo different joints.

In Figure 3.34, detail A is an expansion jointin a masonry cavity wall. The points to noteare:

� The mastic pointing over the joint on theexternal leaf.

Mastic pointing 12 Flexcell board

Wall ties

Internal wall finish

Galvanised orSS masonry

reinforcementstrip


reinforcementstrip

Expansion joint inmasonry cavity wall

Timber frame panel


reinforcementstrip

12 Flexcell boardMastic pointing

Wall ties

Plastic extrusionPerforated edgeFlexiblecentre

Flexcell


reinforcementstrip

RenderRender

225 aac block

75 lightweight blockRWP

Wall ties

Cut


reinforcementstrip

50 Dritherm

A

B

C

D

Fig. 3.34 Expansion joints – a selection of details.

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Fig. 3.35 A proprietary joint of plastic in a renderedfinish block wall.

Fig. 3.36 A mastic sealed joint in a complex wall ofreconstructed stone and rendered blockwork.

� The two parts of both leaves are separatedby a compressible bitumen impregnated fi-bre board – Flexcell.

� Both parts of each leaf are joined across theFlexcell with a strip of expanded metal lathsuch as is used for reinforcing mansonry.This is flexible but its position prevents thefour parts of the wall from drifting apart ina horizontal plane and yet accommodatesany movement apart or together.

� The internal wall finish simply passes overthe joint, although many details will showa timber cover plate fixed to one side onlyof the point where the finish might crack.

� Most importantly are the additional wallties built in here. These are over and abovethe 3–4/m2 in the rest of the wall. They keepthe four parts of the wall from drifting apartas well.

In the same figure, detail B is an expansionjoint in the masonry skin of a timber framehouse. All the things from the previous detailapply here, but only to the cladding of ma-sonry. There is no allowance for movement ofthe timber frame.

Top right in the figure, detail C, is an en-larged view of the detail of the joint from Fig-ure 3.36 showing the rendered finish on block-work and the proprietary expansion jointcover of plastic bedded over the joint into therender on each side. For this purpose the plas-tic joint has perforated wings which key intothe render.

Finally, detail D is an induced crack joint.The inner leaf of the wall was of 225 aac block(blocks built on their flat), and as this was be-ing built the brick reinforcement strips werebuilt in opposite the partition, as well as otherstrips being left sticking out to tie into the par-tition blockwork, which was also aac blockbut only 125 thick. Additional wall ties werealso left protruding either side of where theblock was intended to crack. Finally, the cutwas made with a power saw. The outer leafof lightweight blockwork and the insulationwere added later. This blockwork was only 75thick, and if the wall as a whole had moved,it would have cracked down a similar line tothe main wall. To disguise any possible crack

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Fig. 3.37 Faulty joint being built in the block claddingof a timber frame house.

in the outer leaf, a RWP (rainwater pipe) waserected over that line. That detail was exe-cuted twice on each of two 14 metre walllsapproximately 18 years ago and there is stillno movement to tell if the detail has been suc-cessful or not. The engineer who suggested itis confident it will perform as predicted whenits time comes.

Figure 3.35 is the joint which featured at Cin Figure 3.34. The building is relatively newand shows no sign of movement.

Figure 3.36 is a joint on a supermarket. Thewall is about 6 m high with a lot of artificialstone facings, string courses, etc. with infill

Fig. 3.38 Proprietary expansion joint cover.

panels of plain blockwork which have beenrendered with a wet dash. There is no signof movement on the building but there is alot of unsightly staining where there has beenrainwater run-off concentrated at the masticfilling of the expansion joints. This is also arelatively new building.

Finally, Figure 3.37 is a photograph of a re-cently discovered joint being built in a timberframe housing development. The Flexcell isthere against the wall, which is complete, butthere is no expanded metal lath built across be-tween the different parts of the complete wall.What the photograph doesn’t show is the lackof additional wall ties. The whole wall is onlyone storey and therefore at most 3 m high. Itis only a cladding and not structural but thatdoes not make it correct.

Figure 3.38 is the expansion joint cover em-ployed in Figure 3.35. It is made from twopieces of render stop material – the galvan-ished strip with the expanded metal edges –and a central PVC extrusion. Each side of theextrusion has a groove to take the edge of therender stop and these edges are hard plas-tic. They are joined by an inverted V-shapedpiece of PVC which is much more flexibleand allows the joint to open when shrinkageoccurs.

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4 Timber Upper Floors

Upper floor joists 103Linear and point loadings on upper floors 112Openings in upper floors 113

For pipes 113For flues 114For stairs 116

Alternative materials for joisting 118Sound proofing 120

Modern sound and fire proofing 121Support of masonry walls 123Floor finishes 124Ceiling finishes 124

By the term ‘upper floor’ we mean any floornot accessed at ground level or below groundlevel. Such floors have been developed usinga variety of construction methods, materialsand techniques. In this text we will confine ourdiscussion to the most common in domesticconstruction – the timber joisted upper floor.

The functions performed by an upper floormay be summarised as:

� Providing a safe platform for the occupantsof the house

� Providing support for partitions and equip-ment at that level

� Providing a space to accommodate serviceswithin the floor, serving the space aboveand below the floor level

� In buildings of more than one occupancy thefloor may have to provide a barrier to fire,sound and even weather where part of it isbuilt over a pedestrian or vehicular access.

Timber upper floors comprise:

� Joists which can span the narrower dimen-sion of the building

� Floor boarding or sheet material� A ceiling finish on the underside.

Plus as occasion demands, additional materialand techniques to:

� Attenuate the transmission of soundthrough the floor

� Provide fire protection between upper andlower storeys

� Modify the effects of weather in pedestrianor vehicular accesses.

Upper floor joists

Upper floor joists made from a variety of ma-terials and alternatives to the most commonmaterial – natural wood – will be discussedlater in this chapter.

With regard to wood joists, the timber isused either as sawn or may be regularised,and considering the small additional cost in-volved should be treated with preservativeat the saw mill against insect and fungal at-tack. The Building Regulations require that,as this is structural timber, it should be stressgraded, the grade depending on the span re-quired to be bridged and the expected loads ofpeople and furniture; also the self weight in-cluding any sound/fire proofing on the flooras a whole or on individual joists.

103

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Bearing surface

Depthof

joist

Widthof joist

Span measuredfrom here

Fig. 4.1 Alignment of joists, span and wall rest.

It may seem obvious, especially after look-ing at ground floor construction in timber, butit has to be stated that all joists are built inwith the long dimension of the cross-section inthe vertical plane, as shown in Figure 4.1. Thereader can test the effect of using a supportingtimber on ‘edge’ or on ‘flat’ by placing a ruleracross the gap between two books and press-ing down at the middle of the span. The ruleris much stiffer when it is on edge but is proneto buckle sideways. This is due to the verysmall width/span ratio. Joists of course arerestrained at each end and in short spans donot buckle because the ratio is much higher. Inlong spans we can employ techniques such asstrutting to prevent buckling. The various al-ternative forms of strutting are discussed laterin this chapter.

The ends of upper floor joists, unlike groundfloor joists, are generally supported by build-ing them into the wall, the use of cavity wallconstruction protecting the joist ends fromdampness and decay. Before the general adop-tion of preservative treatment of structuraltimbers, protection was provided by one ofthe following:

� Brushing or dipping the ends in creosote� Wrapping with DPC� Building the joist end into a terracotta shoe.

Brushing or dipping in creosote was done tothe last 300–450 mm of the joists. The process

gave only a surface film of preservative andin order to allow the creosote to penetrate,many old specifications required the timberto be dipped and left for a number of hours oreven days. Provided the timber is dry, the cutend will absorb the preservative but generalpenetration is poor in comparison to moderntreatment methods.

The DPC used was invariably a hessian-based bituminous DPC, which if used in con-junction with creosoted timber could be soft-ened by the solvents in the creosote, resultingin failure. Even if failure occurred, the likeli-hood of damage to the timber was slight.

Building the joist into a terracotta shoe wasmore commonly used where the supportingwall was shared with an adjacent property,the shoe being required to give fire protectionto the end of the joist. It is very effective butthe shoes are now difficult to find.

Alternatively, a timber runner can be fixedto the face of the wall and the joist ends‘housed’ into the runner. Fixing of the runnercan be done by masonry bolts or by build-ing steel brackets into the wall and resting thebearer on these. (Note that steel brackets arenot suitable for a shared wall. The bracketscould become hot if there was a fire in the ad-jacent property and could set fire to the prop-erty being served by the supports.) Either waythe centres of the fixings would be from two tothree joist spacings, depending on the thick-ness of the bearer, the floor loading and thestrength of the fixing.

Figure 4.2 shows methods of supporting up-per floor joists other than building into the in-ner leaf of a brick or blockwork cavity wall.From top left, anti-clockwise:

� A timber runner Rawlbolted to the masonryand joists fixed to it using pressed steeljoist hangers. These hangers are of thin gal-vanised steel and require multiple nailingto runner and joist to make a secure fixing.

� A timber runner against the wall, this timesupported by a mild steel bracket and thejoists housed into the runner and cheeknailed.

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Timber Upper Floors 105

Inner leaf50 runner Rawlbolted

to brickwork

Rawlbolt

Outerleaf

Housed end ofjoist to runner

1 depth, max.

Cranked, mild steel bracket built into wall

Outer leafInner leaf

Plaster97 x 15 skirting10 S/w ground

Floor boarding

JoistDwang to support sheet

ceiling finishJoist end notched where

sitting on angle iron100 x 100 x 12 angle iron

Rawlbolt

PlasterCavityinsulation

Terracotta shoe

Inner leafof cavity wall

Galv. mild steel joist hanger built into bed of

brick coursing

Joist

Joist end notchedover hanger

Note: Notching the joist overthe angle or hanger allows the

ceiling finish to run through flush

3

Fig. 4.2 Alternative methods of joist support.

� A mild steel angle iron is Rawlbolted to themasonry and the joists are set on the flangeof the angle iron with a shallow notch toallow the ceiling finish to pass under theangle iron. It is not normal to fix joist andangle iron together, but the angle could bedrilled and a nail or screw driven into thebottom of the joist.

� Using a galvanised steel joist bracket builtinto the masonry, one bracket per joist end.These must be of 3–4.5 mm thick galvanisedsteel, folded and welded; thin pressed steelfolded hangers are not suitable.

� Finally, a terracotta shoe is built into the ma-sonry and the joist ends are set into the shoe.Not a general duty method but can be usedto allow building in of joists in cavity wallsshared between two properties where fireregulations will not allow joists to be sim-ply built into the wall lest fire spread acrossthe cavity.

Figure 4.3 shows a joisted construction usingheavy gauge (3–4.5 thick steel) hangers builtinto the wall to support both a trimmer joist(the very thick joist with bolt) and commonjoists. The picture shows dwangs/noggings tosupport the edge of a sheet flooring material.Note the trimmer joist built up from a num-ber of layers of common joist bolted togetherwith 12 mm coach bolts, nuts and washers.The wall is a shared wall, commonly called aparty wall, a mutual wall or, where it rises tothe top of a pitched roof, a mutual gable.

In modern timber frame construction, theupper floor joists are supported by the wall

Fig. 4.3 Heavy gauge joist hangers supporting upperfloor joists.

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panels, and a ‘ring beam’ is included for thefull depth of the joists. The ring beam serves:

� As a support and fixing point for the joistends

� As a ‘stop’ to prevent the spread of fire fromthe cavity into the floor void.

Figure 4.4 shows upper floor joists in a timberframe construction and is a repeat of Figure3.19, but the annotation and dimensions havebeen stripped off to make the actual junctionof the joists with the timber frame more clear.

Figure 4.5 is the closest it was possible to getto Figure 4.4 for comparative purposes. In the

JoistNails

Ring beam

Nogging for floorboard edge

Nogging for ceilingboard edge

Fig. 4.4 Upper floor joists in timber frame construc-tion (1).

Fig. 4.5 Upper floor joists in timber frame construc-tion (2).

photograph the joists are seen resting on top ofthe timber frame panels; the ring beam can beclearly seen, as can the dwangs or noggingsfor the edge of the sheet flooring material.The timber frame panel shows a double toprunner at the left-hand side but uses a steelchannel section to provide support over anopening under the remaining joists. See Chap-ter 5, Openings in Masonry Walls, where theuse of metal sections in timber frame construc-tion is discussed in more detail.

The size of the floor joists is determined by:� The loads and span to be carried and the

need to limit deflection to 1380 th of the span

� The spacing or ‘centres’ of the joists� The grade of timber selected� And, not so important in domestic struc-

tures, the vertical space available or re-quired for other purposes such as hiddenservices – ducts and large pipes etc.

While the sizing of upper floor joists followsthe same rules as for ground floor joists, notethat spans will generally be greater. Provisionof heavier joists round large openings is cov-ered later in the chapter.

Domestic loads are not high in comparisonwith other structures but spans can range upto 7 or 8 m on the shortest distance acrossa building. Such spans could well requirejoist depths in excess of 250 mm. A depth of200 mm marks an important economic bound-ary for timber, so spanning such distanceswith single lengths of timber can be quite un-economical. It is not uncommon to span from3.50 to 4.00 m from support to support withsingle joists.

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Elevation of structural partitionsupporting upper floor joists

Sleeper joists

Solid strutting under door openingsupports bottom runner(s) and

floor board through opening

Bottom runnerand sole plate

Dwangs/noggings

Dwangs/noggings

Header andtop runner

Upper floor joistsA

A

B

B Single solidstrutting under

partition

Double solidstrutting underpartition and

doorway

Door lintel

JoistsJoists

Joists Joists

Section A-A Section B-B

Portion in photograph

StudsCripple

stud

Fig. 4.6 Upper floor joists supported by a partition (1).

When spans start to exceed this range itis time to think of alternative ways to sup-port the joists – in other words to break thespan into two or more manageable propor-tions. The best options are to have a load-bearing partition under the joists or provide abeam of some kind. This can be done by mak-ing use of partitions on the lower storey asloadbearing partitions, as shown in Figure 4.7.The partition has a door opening spanned by atimber lintel supported on cripple studs withdoubled top runners supporting the first floorjoists. Note the joists do not fall directly abovethe studs as these timbers are at different cen-tres, but the top runner of the partition is adouble layer of timber.

Figure 4.6 shows a detail of upper floor joistssupported by a partition, while Figure 4.7 isa photograph of the same situation. In Fig-ure 4.7 the partition is shown on the right-hand two thirds of the picture, and a dooropening with a heavy timber lintel, not abeam, is shown on the left-hand side. Theprominent vertical timber in the foregroundis a temporary strut.

Breaking the span of joists can also beachieved by:

� The use of beam(s) spanning from wall towall at right angles to the joists

� A combination of the above with a partition.

Floors constructed over beams are commonlyreferred to as double floors. When beams onlyare used, it is common to have the beam spanthe shorter distance across the building, thusdividing the longer distance into two even

Fig. 4.7 Upper floor joists supported by a partition (2).

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4000

8020

100 x 50 S/w sawn joists@ 400 centres

Flitched beam

Flitched beam

Equal

Equal

Equal

Overall dimensions arehard brick sizes

Upper floor plan

Beam detailJoistJoist

10 Steel flitch

100 x 50 bearer nailed to timbers of beam

12 coach bolts @ 400 centres, top and bottom

alternately

Fig. 4.8 Flitched beam from the 1970s.

shorter spans for the joists. Two or more beamscan of course be used.

An example from a house built in the late1970s is shown in Figure 4.8. The only differ-ence if this floor was being constructed now isthe need to have all the timber stress graded;C3 would be perfectly adequate. It is inter-esting to note that when the calculations forthe beam were carried out, the two 100 × 50bearers for the joists were not taken into ac-count except for the self weight of the beam,and yet the design was perfectly adequate inall respects.

Any supporting partitions can be built us-ing brickwork, blockwork or timber stud. Re-fer to Chapter 3, Walls, for details of partitionconstruction.

Beams can be of timber, steel or a combi-nation of timber and steel. Figure 4.9 showssome alternative beam constructions and Fig-ure 4.10 shows a flitched beam elevation, il-lustrating the extent of the steel plate. Note

Joists housed into top of solid timber beam. Beam is of wrot or dressed timber with a bead worked on the exposed

corners

Laminated timber beam.Timbers are cut and reversed

end for end before being glued back together to form a beam. The glue is usually a resorcinol-based glue which

is water and boil proof

Beam made from two 50 min. thicktimbers with a steel plate -- the flitch--bolted between. 12 coach bolts alongcentre of the beam and 450 apart aregenerally adequate. Flitch thickness

varies with load carried, from 6upwards. Plate gives strength, timber

gives stiffness to resist buckling

Flange on RSJ is tapered androunded.

Flange on UB is parallel andsquare

Webs are parallel and thinnerthan flanges

RSJ (Rolled Steel Joist) UB (Universal Beam)

Fig. 4.9 Alternative beam constructions.

that the steel plate was not the same thick-ness end to end. The middle part was calcu-lated to require 10 thick plate, and the two endeighths 6 thick plate. The whole was weldedtogether and packers used to ensure that thebeam ended up a uniform thickness for its en-tire length.

Supporting joists at flitched beams have al-ready been shown – a bearer nailed to eachside (Figure 4.8). Alternatively, joists could behoused into the timbers, pressed steel hang-ers used or the joists passed over the beam.The large amount of notching for the housingswould have to be considered when calculatingthe size of the timbers to be used. In practice,the next larger timber size available would beused. Similar means of support could be usedwith laminated and solid timber beams.

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Mild steel hot rolled strip

12 coach boltsat 400--600

centres

50 thick sawn or regularised white wood or stress

graded timberDepth to suit load applied

Minimum thickness of 50 mm required to providelateral stiffness to beam

Weld Weld

Elevation of a beam with steel saving.The hatched shape shows three pieces of steel welded together and the clear area represents packing of plywood or timber strip. The precise extent of the steel can be easily calculated.

6 plate and packers 10 plate 6 plate and packers

Fig. 4.10 Flitched beam detail with minimal steel content.

Occasionally reinforced concrete is used butits high self weight makes it expensive to putinto place. With steel beams, it is not possi-ble to notch into the beam or nail hangers toit, so joists can only be set over the top ofthe beams or into the bosom of the beam.Figure 4.11 shows typical details of a steelbeam supporting joists.

Combining beams and partitions can be suc-cessfully done using brick or block partitionswith steel or flitched beams, although mod-ern examples of timber frame construction areincreasingly showing steel beams resting oncripple studs. Where timber stud partitionsare built, timber or flitched beams can be used.To keep beam sizes down, columns can be pro-vided in the lower floor. These are generallyof masonry, one brick square.

The spacing of joists is dictated to some de-gree by the loadings involved and there arethree standard centres which can be used –400, 450 and 600. There is nothing, frankly, tostop the joists being placed at other centres,the only snag being that the standard loadspan tables don’t cater for centres other thanthe standard ones already mentioned. How-ever, spacing out the joists is dictated more by

Metal or timber restraining strap

Joist Joist

RSJ

Metal or timber restraining strap

JoistJoist

RSJ

Continuoustimber bearer

Fig. 4.11 Rolled steel joists, beams or columns as floorsupport beams.

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External wall

2nd and subsequent joists at 400centres measured starting at wall.

First joist put in 50 clear of wallUpper floor finish

Lower floor ceiling finish

External wall

Spacing of last pair ofjoists less than 400

50 clearance at wall

50 clearance at wallmaintained despitecloser spacing oflast pair of joists

Maintaining joist clearance of 50 at each end allows thermal or acoustic insulation to be placedefficiently. Narrower spaces are difficult to deal with properly

Fig. 4.12 Centres of joists in relation to wall and ‘last space’ and last wall.

the need to support a sheet material for thelower floor ceiling finish and occasionally bythe floor finish as well.

Modern construction more often than notuses plasterboard, having a sheet size of1200 × 2400. Joist centres therefore must be at400 or 600 mm centres. Joist layout followsthe same principles as for ground floors; thefirst joist is set 50 mm clear of the wall andthe remainder fixed at centres, finishing, ir-respective of the gap left, with a joist 50 mmclear of the opposite wall. Figure 4.12 shows asection across floor joists to illustrate the reg-ular centres of fixing and the odd space at theend.

Joist spacing uneven or greater than required for sheet finish

PackPack

Pack PackPlasterboardceiling finish50 x 38 branders

at 400 centres

Fig. 4.13 Brandering.

Should economy or other requirement dic-tate that the joist centres be some otherdimension, then light timbers can be fixed at400 centres and at right angles to the under-side of the joists, and the sheet finish nailedto these. These timbers are known as bran-ders and the process as brandering. It can bea useful technique in refurbishment work forbringing an old uneven ceiling down to a bet-ter ‘level’, as well as sorting out joists at oddor uneven centres. Figure 4.13 illustrates howthis can be done.

Figure 4.14 illustrates an alternative tobrandering using cheek pieces to support aceiling. This can only be used if the centre of

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Joists or ceiling ties

Cheek pieces

Branders

Plasterboard ceiling finish

Fig. 4.14 Cheek pieces.

these timbers is not critical – unless brandersare nailed to the cheek pieces.

Having the joists or branders at 400 or 600centres gives support to only two edges ofa standard plasterboard sheet. The edges atright angles cannot remain completely unsup-ported so additional timbers must be fittedto provide that support. These timbers arelighter in section than the joists (38 × 38 to50 × 50) or the same section as the branders.These timbers, dwangs or noggings, are fixedat a maximum of 800 centres but most com-monly at 600.

The spans of upper floor joists are inevitablylonger than ground floor joists, and, with the

A

A

A-A

Galvanised m.s or SSherringbone strut

Shape atnear end

Shape atfar end

Non-strutted joists buckling under load

Wall

Solid strutting samethickness as joists but not

so deep

Herringbonestrutting

Nailing

NailingFolding wedges between

wall and last joist

Line of ceiling board

Line of floor board

Struts do NOT touch here andare not nailed together

Fig. 4.15 Solid strutting and steel and timber herringbone strutting.

greater depth of joist, twisting or buckling un-der load is a possibility. To prevent this, oneof two forms of strutting is built in. This mustnot be confused with the dwangs or noggingsalready mentioned. Indeed, because this strut-ting is never flush with the underside of thejoists, it cannot be used for edge support. Itis done once at half the span or twice at athird span, depending on the span. The ob-ject should be to reduce the free length of joistto a maximum of around 2.5 m.

Figure 4.15 is a detail of solid and herring-bone strutting in steel and timber, and Fig-ure 4.16 is a photograph of a mock-up of thegalvanised steel strutting. Note that the joists

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Fig. 4.16 Galvanised steel herringbone strutting.

are 200 deep and the strutting is only 150 deepand fixed flush with the upper surface of thejoist. This leaves 50 mm immediately abovethe ceiling board for running services such aswater pipes and electricity cables.

Figure 4.17 shows timber solid strutting andFigure 4.18 shows a photograph of a mock-upof timber herringbone strutting.

Linear and point loadingson upper floors

The loads referred to here are loads causedby partitions built across the floors, or itemssuch as hot water storage cylinders, cold watertanks, baths, etc. The use of load span tableswill not allow these items to be taken into ac-count and their effect must be calculated sep-arately. Generally, partitions running parallelto the run of the joists are supported by placinga joist immediately under the partition. Thejoist would be at least double the thicknessof the ordinary joists; indeed, it may be twoordinary joists laid close together and joinedby a pattern of nails.

Fig. 4.17 Solid timber strutting.

Fig. 4.18 Timber herringbone strutting.

Point loads can be caused by partitionswhich run at an angle to the run of the joists,or by bath support legs or cradles or by waterstorage systems. The increase in joist width orthe introduction of additional joists are bothsolutions, but they usually require individ-ual calculation. An illustration of doubling upordinary joists – even triple and quadruplethicknesses – is shown in Figure 4.19 and thejoists are identified in the key in Figure 4.20:

� The joists numbered 1 are common joists50 mm thick.

� The joists numbered 2 comprise two 50 thickjoist timbers nailed together. The increasedthickness is required to carry a partitionload as well as the share of floor load.

� The joist numbered 3 is a special joist – atrimmer or trimming joist; it comprises two50 thick joist timbers nailed together and isrequired to support all the previous joists.

� The joist numbered 4 is another specialjoist – a trimmer joist; it supports one end

Fig. 4.19 Common joists, trimmers and trimmingjoists.

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1

4

3

4

2

2

2 1

1

11

1

1

1

1

Solid strutting

Wall line

Fig. 4.20 Key to Figure 4.19.

of the trimmer or trimming joist as well asa partition load and share of the floor load.It comprises four 50 mm thick joist timbersbolted together with 12 mm coach bolts,nuts and washers.

Note the solid strutting in the centre fore-ground.

Openings in upper floors

Openings in domestic upper floors occur fora number of reasons:

� Passage of services – water pipes, disposalsystems, small ductwork, cables, etc.

� Flues for boilers, fireplaces, etc.� Stairs – the opening shown in Figure 4.19 is

for a stair.

For pipes

In the case of small pipes for hot and coldwater, or even disposal piping up to around50 mm bore, passage through the floor is sim-ply done by piercing the floor board and ceil-ing finish and making good round the pipe.

Where the hole is exposed to view, a pol-ished metal, plastic or wooden cover plate, of-ten termed a ‘floor plate’, is sometimes fittedto mask the edges of the hole, especially where

Cover plate

Floor

Pipe

Cover plate

Clip

Pipe

Hinge

Fig. 4.21 Floor plates.

the pipe might damage the underlying finishdue to expansion/contraction, or to cover aroughly cut hole. Figure 4.21 shows a sketchof this.

Holes need not be vertical to the floor planefor small bore pipes and conduits. They canbe in the joists themselves and it is here thatspecial rules apply, and the Regulations havesomething to say on the subject. Holes such asthese can be formed in two ways:

(1) Drilling a circular section hole completelyin the joist

(2) Cutting into the edge of the joist forminga notch.

The hole should only ever be drilled throughthe neutral axis of the joist. The neutralaxis of a rectilinear joist is always on the

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centre line of the vertical face of the joist. Holesshould only be drilled from a quarter length ofjoist to 0.4 length of the joist from either end,and individual hole diameters should not ex-ceed a quarter of the depth nor be closer thanthree diameters.

Notching should be avoided if at all possi-ble but is often the preferred method whenaccommodating services in old floor struc-tures and floor boards have to be lifted to gainaccess to the void. Confine notching to the firstand last fifths of the length of the joist and keepthe start of the notching not less than 0.7 of thespan from the end of the joist. Overall depthof the notching should be no greater than oneeighth of the depth of the joist.

Any variation from the above cannot be al-lowed unless the loadbearing capacity of thejoist has been recalculated to take account ofthe hole(s) or notch(es). Any pipes or cablesset into notches should be protected wherethey pass over the joist by fixing a metal strapacross the notch.

For larger piping, such as soil and vent pip-ing of 80–150 mm diameter, it is usual to‘frame’ up the joisting. It may seem strangeto require framing of the joisting to accom-modate a pipe of some 100 mm bore, but thealignment of these pipes is critical. Only twobends are allowed from the junction with theuppermost appliance to the open vent at thetop, and the pipe must generally be fixed backto a wall or into a corner. In the either case, thejoist could be in the way.

Figure 4.22 shows how bridling of joistsround a large bore pipe might be carried out.In the case of the opening shown, the floorloading only falls on two sides of the open-ing and only one joist is shortened by the bri-dle. Because of this, the bridle would be madefrom the same size of timbers as the generaljoisting.

If more than two sides were supported ormore than one joist cut, the bridle and thejoist supporting the end(s) of the bridle wouldhave to be thicker. The increase would be inthe order of 25–50%, so in a floor joisted with200 × 50 joists, the bridle and the supportingjoist would be 200 × 63 or 75 mm thick, de-pending on availability of the timber size.

Roof line

Ceiling line

Soil, waste andvent pipe

Joists atcentres

Bridle

First joist

Wallline

First floor

Ground floor

Maximum oftwo bends

Drain connection

Fig. 4.22 Bridling joists around large pipes.

For flues

Flues are built into buildings to carry awaythe gaseous and fine particulate products ofcombustion from open fires and stoves of var-ious types. Fuels range over gas – natural andLPG – various grades of oil, wood, peat, coaland derived fuels, straw and other ‘waste’products. A variety of flue types is available toaccommodate the widely differing conditionsof temperature and products in the flue gas.

Extremes of temperature can be met withdifferent fuels burned in quite different appli-ances. For example, a light fuel oil in a cen-tral heating boiler can produce flue tempera-tures of around 500◦C, requiring either a lined

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masonry flue or a double metal wall with ce-ramic insulating lining. Solid fuel appliancescan produce even higher flue temperatures.

A fully condensing, gas fired, sealed boilerwith a flue temperature of 65◦C or less canbe flued with a plastic pipe, usually of ABS(acrylonitryl-butadiene styrene). We will con-centrate on flues with higher temperatures.

Flues carrying high temperature gases andpassing through floors introduce the possibil-ity of setting fire to the joists, floor and ceilingboarding, etc. by:

� Hot gases and/or flames escaping� Combustible material touching the hot flue� The flue radiating heat towards combust-

ible material.

Precautions are necessary to prevent this anddepend on:

� Integrity of the flue structure itself� Isolation of the flue from any combustible

materials using an air gap� Insulation of the flue from the combustible

material.

Integrity of the flue material

Provided the flue does not allow flame or hotgas to escape, it fulfils its function in terms ofpreventing fire. Flues can be of a variety ofmaterials but basically domestic constructionwill use:

� Masonry flues – built with bricks or blockswith a clay liner. A clay liner is mandatory,the masonry on its own being considered in-sufficient to stop flame or hot gases reachingthe rest of the structure

� Metal flues – a pipe of either single or mul-tiple wall construction with sealed joints.These are proprietary systems. The days ofsticking any old piece of cast iron or asbestoscement drainpipe in are far behind us now.

Isolation and insulation of the flue

Isolation of the floor structure can be carriedout by keeping the flue completely clear ofany combustible material, with an air gap

50–75 across and all round. The flue ma-terial itself may afford a degree of insula-tion, e.g. brickwork, blockwork – especiallylightweight or aac blocks, or in-situ concrete –and this in many domestic situations is per-fectly adequate.

Insulation of the structure from the hot flueis generally carried out using mineral wool,which has a high melting point. Glass fibreshould not be used as it could simply meltand set fire to some other part of the struc-ture. Other materials which can be used arevermiculite or mica, both best used as the ag-gregate in a refractory concrete rather than asa loose fill, when retaining the granules can beproblematical.

It is difficult to separate the ideas of isolationand insulation as the solutions for both are al-most identical. Depending on flue tempera-ture, a simple gap may be enough to preventradiant heat igniting the combustible compo-nents of the structure, as well as separatingflue and materials. However, isolating sleevesor a combination of gap and sleeve may be re-quired when flue temperatures are higher, es-pecially where the space for a very large gapis not available.

Figure 4.23 gives the solutions for a com-bustible floor, in diagrammatic form. Full de-tails are given of how to deal with two typesof flue for very specific fuel types in specificappliances:

� Solid fuel open fire burning either coal, coalderivatives or wood

� Gas fired closed appliance with an all metalflue.

There is a large range of proprietary fluesavailable for most types of appliance and fuelcombination. Please refer to manufacturers’literature.

The masonry flue for an open fireplace willgenerally start off at the lower floor levels asa mass of masonry approximately 1000 wideand 450–600 deep on plan. This is known asa chimney breast. This mass of masonry ac-commodates the open fireplace and is built tovery specific minimum dimensions. At ceilinglevel the dimensions of the chimney breast re-duce to accommodate the flue, its liner and the

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Flue FlueFlue

Air gapall round

InsulationInsulation

Insulation

Air gapall round

Keep flue clear of structureSuitable for low surface

temperature flues

Insulation between flueand floor

Suitable for low/mediumsurface temperature flues

Gap and insulationBest for high surface

temperature flues

Fig. 4.23 Schematic of flues passing through floor.

surrounding layer of masonry. In brick-builtflues this would give an overall plan dimen-sion of 440 × 440.

Masonry flues must now include a linerwhich is generally made of fired clay; a typi-cal example is shown in Figure 4.24. The lineris made in lengths of around 450 mm and thejoints are joggled. Note that the joggle can bebuilt upside down, which would allow anytars from the fuel being burned which con-dense on the cooler wall of the liner to runout of the joint, thus staining the surroundingsurfaces. Figure 4.25 shows a flue through afloor and top of chimney breast, with the jointin the liner laid correctly.

Fig. 4.24 Clay flue liner.

Where the flue passes through a floor whichis entirely within one dwelling, i.e. the upperfloor of a two-storey house, there is no need forthe void between the flue and the joists to befire-stopped. Only support is required. If theupper floor separates two different dwellings,i.e. flats, then there must not be a gap aroundthe flue, so only a mineral wool or a concretefill can be used for isolation and insulation offlue and structure, as it must also perform asa separation between the dwellings.

If the upper floor separates two differentdwellings i.e. flats, then the plates above andbelow the floor must give support and fire re-sistance. Proprietary flue systems have bothtypes of ‘plate’ as standard fittings. This is il-lustrated in Figure 4.26 which shows a fluethrough the floor of a two-storey house, sup-port only.

For stairs

Larger openings generally occur to allow ac-cess by stairs. Depending on the run of thejoists in relation to the long dimension of theopening, this can mean cutting a large num-ber of joists, supporting the ends with a heavytimber(s) whose ends are in turn supportedon the remaining joists. These supporting

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Clay flueliner

Bridles

100 min.

Mortarfilling

Joists atcentres

Joists atcentres

Floorfinish

Ceilingfinish

Joists

12–20 gap

Enlargeddetail of

joint

Inside faceof liner

Plan of flue

Vertical section of flue

Fig. 4.25 Flue passing through floor at top of chimneybreast.

timbers must be larger than the general joist-ing, and joints must be made which provideproper support. Readers should refer back toFigure 4.19 which shows a stairwell openingin a timber joisted floor.

When framing up openings in floors withlarger joists, the trimmer and trimmer joistjoint is usually a tusk tenoned joint (seeAppendix C). Housed joints are used forjoists to trimmer junctions. Modern domestic

Flue

Bridle samesize as joist

Ceilingplate

Jointbelowceiling

Floorboarding

Floor andsupport

plate

Gap -- refer tomanufacturer's

literature

Proprietaryflue system

Joist

No joint here

Ceilingfinish

Fig. 4.26 Proprietary flue system support and sealingplates.

construction frequently uses pressed steelhangers for all joints. Figure 4.27 shows joistsfixed to a trimmer with pressed steel hangers.Note that these hangers are of thin galvanisedsteel sheet; that many small galvanised nailsare used to fix them; and that the top of thehanger is bent flat over the top of the joist andnailed down. The latter can be seen on someof the earlier photographs.

Figure 4.28 shows a heavy gauge hanger forfixing to masonry and Figure 4.29 shows alightweight pressed steel hanger suitable forjoining joists etc.

Chapter 10 on Stairs shows how the edgesof the opening are finished off.

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Fig. 4.27 Floor joists fixed to trimmer with pressedsteel hangers.

Fig. 4.28 Heavy gauge hanger for fixing joists tomasonry.

Fig. 4.29 Light pressed steel hanger for joining joists.

Alternative materials for joisting

From an economic point of view there is nomaterial better than solid timber for joists ofaverage length. However, when solid timbercannot provide carrying capacity over a par-ticular span, or where services of large cross-sectional area require to be contained withina floor structure, then alternatives must besought. As already described, long spans canbe broken up by using beams or partitionwalls under the joists, but these may not bedesirable or may not exist.

Proprietary joists are available in a varietyof material combinations and configurationsand a few typical alternatives are describedhere. This is by no means an exhaustive listbut serves to illustrate the types available:

� Combinations of timber and steel� Combinations of natural timber and ‘recon-

stituted’ timber

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� Combinations of natural timber and ply-wood or fibre board.

The first, a steel/timber combination, is thePosijoist by Mitek Industries. The companysupply only the pressed galvanised steel lat-tices in pairs, each pair having three sets ofteeth similar to gang nailing plates. Manufac-turers of Posijoists then supply stress gradedtimber for the upper and lower chords andpress the pairs of lattice sets into the chordswith a hydraulic press. The system can obvi-ously be designed to meet a variety of loadsby altering the cross-section of the timbersused. The spacing of the chords is fixed, be-ing a function of the size of the lattices. Theauthor is grateful for permission from MitekIndustries to publish the photograph in Fig-ure 4.30 which shows a floor structure madeusing Posijoists. It amply illustrates the versa-tility of a lattice system joist structure.

For more information try the websitehttp://www.mii.com/unitedkingdom/

Another lattice style joist is made by a com-pany called Metsec. The strength is from theall steel construction, the top and bottomchords being made from pressed steel chan-nels and the lattice from a continuous lengthof steel tube bent and welded between thechannels. To make the joist ‘joiner friendly’,timber inserts are crimped into the steel chan-nels, providing a fixing for both floor boardingand ceiling boarding.

Fig. 4.30 Posijoists.

Web

Web

Upperand lower

chordsof

solidtimber

Fig. 4.31 All timber, man-made joists.

For more information try the websitehttp://www.metsec.co.uk/metsec/en

‘All timber’ manufactured joists are made,and employ stress graded timber chords eachwith a groove machined into them. A webof man-made board is then glued into thegrooves in the timber chords. Their con-struction is illustrated in Figure 4.31 and thestraight web style is also shown in Figure 4.32,supporting an upper floor. Note in the photo-graph how trimmers and trimmer joists areformed by doubling (or more) the numberof joists fixed together (the large timber run-ning from left up to right across the wholepicture). Note also the use of light pressedsteel hangers to fix the joists to that doubled

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Fig. 4.32 Upper floor supported with Finnjoists.

joist. The straight webbed joist can use a heav-ier material for the web and it is frequently atype known as OSB – oriented strand board –in thicknesses from 11 upwards. There is aproprietary system marketed under the nameFinnjoist which uses OSB for the web, but in-stead of solid timber for the flanges, it useslaminated timber.

For more information try the websitehttp://www.finforest.co.uk

Figure 4.33 shows the surface of a sheet ofOSB with a metric rule to give some idea ofthe size of the particles or ‘strands’.

Because the wavy webbed joist requires amore flexible web material, plywood or evenhardboard is used. If you doubt the strength ofhardboard you really must revise your think-ing. As part of any stressed panel construc-tion even 3 thick hardboard can be incredibly

Fig. 4.33 OSB sheet.

strong – and of course it is available in other,thicker sections.

Sound proofing

Traditional methods of sound proofing reliedentirely on increasing the mass of the floorstructure – placing a layer of clinker or ashesin the joist void and sealing between joists andjoists and walls, by covering the ashes with alayer of lime putty1. Figure 4.34 shows thistraditional sound proofing. The detail shownin Figure 4.34 could only be carried out us-ing large section joists; 250 × 50 was a min-imum and this was frequently increased to300 × 75. These sizes were necessary to carrythe large dead load of the ash layer, the sup-porting boards and the lime putty.

The heavy timbers also gave great rigidity tothe floor, and so even with the floor boardingnailed direct to the joists, attenuation of soundcaused by impact was relatively effective. Thethick flooring boards themselves, spanningonly a clear 400 mm, were also very rigid andthis reduced ‘drumming’ of that ‘skin’.

Surprisingly, the construction as detailedproved to be a good fire barrier due to thethick layer of lime plaster on the ceiling andthe lime putty over the ash layer, which notonly gave protection against flame but alsogave a seal to resist the passage of hot gases,and had the layer of ash to provide insulation.

The author had experience of a fire in alate nineteenth century house which had beenconverted into offices; it illustrated how wellthese old floors could resist fire. The floorstructure was 20 mm oak parquetry on 32 mmsoftwood tongued and groove flooring on300 × 75 joists, with ash deafening and limeputty on 25 boards and a ceiling finish to theroom below of 20 mm lime plaster on splitsoftwood lath. The fire was started by a radi-ant electric fire, plugged in and switched on tohelp dry the parquetry floor which had been

1 Slaked lime, which can be mixed to a variety of con-sistencies before being used. Each can be referred toas lime putty. In the instance quoted above the consis-tency should be ‘creamy’ to allow it to be poured overthe beaten ash base.

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Lime putty poured over ashes

Void used to run small pipes and

cables 28 or 32 t&g floor boarding

JoistLayer of

ashes min.100 thick

450 centres

Split lath20 lime

plaster in three coats

50 x 25 S/w fillets to support boarding

20 plain edged, sawn

boarding to support

ashes

Fig. 4.34 Traditional sound proofing.

mopped during the evening. The fire had beenpropped up in an unstable way. It was forgot-ten about and during the night fell over, facedown, onto the parquetry. During the night itslowly burned its way through the floor andthe ceiling, showering the room below withhot embers. The room below was an officewith desks littered with paper, which caughtfire, and this room was gutted.

After the fire brigade had been called andthe fire extinguished, the damage to the ceil-ing was found to be minimal. Some had comedown and joists and support boards for theash layer were charred but were still struc-turally sound. The only damage to the roomwhere the electric fire had been was due to hotsmoke coming up through the hole burned bythe electric fire. Papers on desks were charredand fragile but nothing had actually ignited.Smoke had discoloured everything and waseven ‘burned’ into the surface of a glass tum-bler sitting on one of the desks. The electricfire was still plugged into the wall socket andwas dangling through the hole suspended byits flexible cord. Of course the insulation to thecord was totally destroyed.

Modern sound and fire proofing

Modern construction has been forced to adoptsmaller sections of timber etc. due to ever

escalating costs. Thus we cannot rely entirelyon mass with the need for protection of largejoist sections nor on thick and rigid layersof boarding. Mass does still play a part butresilient materials are introduced to reducenoise caused by impact and vibration.

A detail common in flatted dwellings of the1960s used smaller joists, mineral wool ‘pug-ging’ to give mass (up to 100 kg/m2) and aglass fibre mat as a resilient layer between rel-atively thin flooring boards and the joists. Thepugging was formed by making the mineralwool into small pellets which could be pouredinto the void between the joists; 12.7 (1/2 in),20 gauge galvanised chicken wire was nailedto the underside of the joists to provide sup-port for the pugging in the event of the plas-terboard ceiling finish failing in a fire. Twolayers of 12.7 mm plasterboard on the ceil-ing gave the requisite fire protection and ad-ditional mass. Plasterboard sheet joints werestaggered in adjacent layers to prevent thepassage of smoke and hot gases.

The floor boarding was isolated from thejoists by a layer of fibreglass quilt draped overthe joists. The floorboards were nailed to alight batten running between the joists. Thebatten was not fastened to, nor did it touch,any part of the structure. A roll of fibreglassquilt was placed between the first and lastjoists and the wall, and the quilt draped overthe joists was turned up against the wall at theedges of the floor-boarded area.

Figure 4.35 shows a detail from the 1960s –a floor with sound and fire proofing as used

Joists at 450 centres

PuggingChicken

wire

Foldedfibreglass

quilt

Fibreglass turned up

between edge of floor boards and masonry

wall nailed to50 x 50

S/w batten

19 t&g S/w floor boards

50 glass fibre quilt draped over joists

Two layers of 12.7 plasterboard

nailed to joists with broken

joints

Fig. 4.35 Sound and fireproofing 1960s style.

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between low rise flatted housing. Note the fol-lowing:

� The floorboards are not nailed to the joists,only to the free batten between them.

� There must be a triple thickness of fibreglassquilt stuffed between the first/last joists andthe walls.

� There must be a single layer of fibreglassbetween the edge of the floor boards andthe wall.

� The pugging is a layer of mineral wool pel-lets laid at a rate of 90 kg/m2.

To keep the joist size down and still maintaingood rigidity, it was common to break the spanof the joists with partitions in the lower floor,or use an RSJ. Both partitions and RSJ hadto be protected from the effects of fire. Plas-tered partitions of half brick thick brickworkor 75 blockwork were used as these were in-herently fireproof and with their mass atten-uated the noise transmitted. The most com-mon type was stud partitioning of 50 × 38sawn S/w studs, runners and a minimum oftwo rows of noggings/dwanging. Claddingwas 12.7 plasterboard both sides to protect thewooden framing from fire. This gave a protec-tion period of half an hour.

RSJs were frequently used in associationwith one or more one brick square columns,which were plastered. The columns broke upthe span of the RSJs. Steel distorts badly in afire so the RSJs had to be protected on each ver-tical face and on the bottom. This was done byframing round the RSJ with timber and apply-ing two layers of 12.7 plasterboard as shownin Figure 4.36.

Fire proofing an RSJ spine beam is detailedin Figure 4.36. Points to note are:

� There are four layers of plasterboard be-tween the bottom of the RSJ and the lowercompartment – this provides insulation.

� The two layers immediately below the RSJare temporarily taped into position and fi-nally secured by the chicken wire layer.

� Plasterboard round the RSJ has joints ‘bro-ken’ to give a better ‘gas’ seal.

50 glass fibre quilt over joists

Floor boardsRSJ

32 x 3 m.s. restraining plate checked into top

of joists and nailed with 65 wire nails

Mineral wool pugging

200 x 50 sawn S/w joists

2 layers 12.7 plasterboard both sides

between joists nailed to timber chocks

2 layers 12.7 plasterboard

taped to underside of RSJ

50 S/w chocks wedged into bosom of RSJ and nailed to sides of joists

Corner joints of

plasterboard staggered

2 layers 12.7 plasterboard

nailed to joists with broken joints

20 gauge galvanised chicken wire

Fig. 4.36 Fireproofing an RSJ spine beam.

� The metal restraining strap is fixed acrossthe top of the joists where it is least vulner-able to fire. Many authorities insisted that itwas screwed down rather than nailed.

� The fibreglass layer is continuous over theRSJ.

A more modern form of fire and soundproofing is shown in Figure 4.37. It should benoted that the pugging can be of a variety of

19 MR flooring grade particle board nailed or screwed to battens

and all t&g joints glued

Min. 44 x 44 sawnS/w battensResilient strip197 x 44 stress

graded regularised S/w joists

Pugging -- see text

Ceiling finish-- see text

400 centres

Polythene liner

Fig. 4.37 Up-to-date sound and fireproofing of anupper floor.

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materials laid to a density of 80 kg/m2. Ful-filling these requirements would be:

� Ash, 75 thick� Dry sand, 50 thick� 2–10 limestone chippings, 60 thick� 2–10 whinstone chippings, 60 thick.

The polythene lining keeps any dust frompenetrating the ceiling, especially while thepugging is being poured. For the resilient stripon top of the joists 25 mineral wool, density80–140 kg/m3 is recommended. The ceilingfinish can be:

� 6 plywood nailed to the joists, then two lay-ers of plasterboard with broken joints togive a total thickness of 25 minimum, or

� Metal lath with 19 of dense plaster.

Support of masonry walls

All thin or slender vertical structures areprone to buckling when loaded excessively.Masonry walls are no exception, and to breakdown the height of the wall into smaller un-supported heights it is a requirement of theBuilding Regulations that walls of more than3.00 m in length are tied to the intermedi-ate floors. Where the joists are built into thewalls with a bearing of a minimum of 90 mm,nothing further need be done, but on adjacentwalls there is no connection between wall andfloor so metal restraining straps must be builtin. The straps are of galvanised steel and musthave a minimum cross-section of 30 × 3 mm.They must extend across three joists withdwangs between the joists, and packing orfolding wedges between the first joist andthe wall. Solid strutting was frequently usedinstead of dwangs/noggings when this reg-ulation was first promulgated. Clarificationin more recent editions of the regulations re-quired the dwang/nogging to be at least halfthe depth of the joists.

How the work is done is illustrated in Fig-ures 4.38–4.40. Figure 4.38 is the detail, Fig-ure 4.39 is a photograph of the metal strap-ping and Figure 4.40 shows the metal strap

Joists at centres

Noggings, half the joist depth

minimumFolding wedges

Metal strap spanning at

least three joists

Brick cavity wall

Note that the thickness of the strap has been exaggerated

and that much of the construction detail annotation etc. has been omitted for clarity

Fig. 4.38 Detail of support from floor of masonrywalls (1).

extending into the wall and nailed over notjust three joists but four. Figure 4.41 shows theview from the underside of that strap, and onecan see that the square section noggings underthe strap would appear to be undersized if itwas meant to comply with the Building Regu-lations. The end of the strap can just be seen inthe top right hand corner of the photograph.

The joists in this particular building are reg-ularised out of 250 sawn timbers so the dwangshould be at least 125 deep. It shows around50 × 50. What is not shown in the photographis the packing between the wall and the firstjoist. This was a plain square section timber.Wedging would have been better. This was

Fig. 4.39 Restraining straps.

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not a ‘good’ site. The support straps shouldbe placed no further than 2.00 m from eachcorner and a maximum of 2.00 m apart. Thesite where these particular photographs weretaken showed a lack of understanding of theneed for these straps; not nearly enough ofthem were used.

Floor finishes

As with the hung timber ground floor con-struction, floor finishes on joisted upper floorsuse either solid timber boards or manufac-tured sheeting. All that is said in Appendix Cabout this applies equally to upper floors.

Ceiling finishes

Ceiling finishes have been mentioned in sev-eral of the earlier sections of this chapter, with-out a great deal of explanation, because they:

� Influence decisions regarding the spacingor centres of joists or the need to usebrandering


t&g and V jointed boarding

t&g and beaded boarding

t&g and rebated boarding

Fig. 4.42 Match board sections.

� Contribute to fire and sound insulation ofthe completed floor.

In every instance the finish mentioned hasbeen plasterboard. Plasterboard is not justavailable in one form, but in a variety ofsizes, thicknesses, configurations, composi-tions, finishes and a host of sensible combi-nations of these. Much of that information issummarised in Appendix H, Gypsum wallboard, or can be readily obtained from thewebsites quoted there.

Fig. 4.43 Match board fixing clip.

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As a reminder, here is an extract from Ap-pendix H:

At its most basic, this sheet material is asandwich of two layers of special paperwith a layer of gypsum (calcium sulphate)between which can be nailed or screwed toa framed timber or light steel section back-ground. Thus it can be used to cover theunderside of upper floor joists to provide aceiling, or applied to walls and partitions toprovide a smooth wall surface.

Other materials can of course be used to finisha ceiling, and solid timber is probably the mostcommon after plasterboard. The section usedis usually a decorative one but incorporatinga t&g joint on the long edge, and end jointsof board are always made at a joist. Like floorboarding, the benefit here is that, unlike sheetmaterial, the centres of the joists are less im-portant in reducing waste. Boarding like thisusually finishes around 12–15 thick and is ma-chined out of no more than 75 wide sawn stock

or less, so the cover width at a maximum isaround 60–65. All the boards in one batch arethe same width. Boarding like this is knownas match boarding.

Figure 4.42 illustrates some of those sec-tions. The top and middle sections in the fig-ure have been used for panelling of walls fornigh on a century and a half. They can bothbe secret nailed in the same manner as floorboards (see Appendix C). The lower sectionin the figure came into vogue in the early1950s and while some of it was nailed tothe background framing, that tended to leavenail heads exposed, which had to be puncheddown and filled over. If the timber was var-nished, the stopping showed up as lightercoloured spots. The result was the inventionof the match board clip shown in Figure 4.43.The clips are nailed to the background tim-bers and the match board pushed against thespiked end of the clip. For reference the nailsshown are 20 long.

This style of boarding can also be used as analternative wall covering.

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5 Openings in Masonry Walls

For small pipes and cables 126For larger pipes and ventilators 127Large openings in masonry walls 127Alternative sill arrangements 136

Threshold arrangements 137Partitions of masonry 139Openings in timber frame walls 141

As we did with floors, we can classify open-ings in walls roughly by the size of openingrequired.

So we will look at:

� Openings for small pipes and cables<50 mm diameter

� Openings for pipes and ventilators >50 mmdiameter but <225 mm

� Openings in excess of 225 mm – large open-ings.

For small pipes and cables

Openings for small pipes and cables can nowbe drilled with a power tool fitted with an ap-propriate drill bit – solid for holes up to about50 mm and a core bit for larger holes up toabout 150 mm diameter. There are even somespecialised forms of bit which can cut largediameter holes but they generally require wa-ter cooling. All that is required is a power-ful enough tool, core bits requiring the greaterpower.

Because of the extensive use of cavities,sleeves must be built in across the full widthof the wall to prevent the leakage of pipe con-tents into the cavity and to prevent vermingetting into the cavity. Typical examples ofdrill bits and a sleeve in a masonry cavity wallare shown in Figure 5.1.

Sleeves should be built in with a slope downand towards the outer face of the wall to pre-vent water running through the sleeve. Thisdoes not need to be excessive – the sketchesexaggerate the slope.

Sleeves can be made of any solid drawnpipe material with a bore which will allow aneasy fit for the service pipe or cable passingthrough. Typically, copper piping or plasticspiping can be used. Ferrous material shouldbe avoided, as should aluminium which cor-rodes in contact with a strong alkali. Plasticspipe does seem to be the favoured choice onbuilding sites one visits, but it is uncertainwhether or not compatibility with cable in-sulation etc. has been considered in the choiceof pipe material.

Joints of the service pipes and cables are notallowed in the sleeve. With soft-walled pipesand cables, the ends of the sleeve should berounded or be fitted with some form of cush-ioning to prevent wear on the pipe or cable.Chapter 4, Timber Upper Floors, gives a de-scription of ‘cover plates’ where small pipesetc. pass through a floor. These can be usedwith holes in walls.

If the space between the sleeve and the pipeor cable is to be sealed, this should be donewith a non-setting mastic or sealer. Take careto ensure that it is compatible with the mater-ial of both sleeve and service pipe or cable.The space should be left large enough to make

126

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Openings in Masonry Walls 127

Masonry drill upto 50 diameter

Tungsten carbide(TC) tip on drill

Hole to allow dust toescape while drilling

Core drill -- 50 to

150 in diameter

Sleeve in masonry wall

Plastics ormetal sleeve

Service pipeor cable

TC teethset in

edge ofsteel ‘cup’

Steel cup withteeth cuts outa concentric

groove, leavingbehind a corewhich can bebroken off to

allow a further cut to be made until the hole

breaks through the masonry

NO joints in supplypipe or cable withinthe sleeve

Fig. 5.1 Drills for drilling through walls.

it easy to inject the mastic. The whole spacefrom end to end is never sealed, only about25 mm at each end.

For larger pipes and ventilators

The only large pipes to pass through domesticwalls are generally soil pipes or flue pipes –occasionally air extract ducts. In both casesthe norm is to cut a hole in existing walls orbuild in the pipe or flue with mortar, fittingthe masonry units around them. The cuttingis done with a hammer and chisel or a coredrill in a power tool. The latter causes muchless damage to the new masonry.

In the case of a flue, the heat generated maybe sufficient to damage cavity insulation. Insuch a case an approved sleeve (metallic orasbestos cement pipe) with proper clearancesmust be built into the wall. Manufacturers’literature should give the necessary adviceon the type of material to use for a sleeveand the clearances required, together with the

expected temperatures with various fuels forwhich the flue is suitable.

For pipes, ducts and flues up to 150 mmdiameter, holes would be formed or drilledto coincide with a junction of two bricks orblocks so that these units can be saved overrather than use a separate lintel, as shownin Figure 5.2. Also shown is a ventilator(not an airbrick) which generally tends to berectangular in section and made in brick coor-dinating sizes. For ventilators up to one brickwide therefore, there is no need to do anythingother than ensure that two bricks in every halfbrick thickness are evenly distributed overthe ventilator opening. Saving over relies onthe self arching effect of masonry.

Large openings in masonry walls

Large openings are made for doors, windows,hatches and the like. What differentiates themfrom previous openings is the need to pro-vide extra support over the opening as the self

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Ventilator

Sheet metal orplastics duct

These three brickssave over

Flue liner in here

Flue pipe inhere

These three brickssave over the opening

Lineroutline

Void if a flue is used

LinerCover plate on flueliner over void

Fig. 5.2 Saving over in brickwork at ducts, pipes and ventilators.

arching effect can only be partially successful.Support is provided by lintels.

Lintels can be made from a wide variety ofmaterials and a variety of shapes. Materials in-clude stone, reinforced precast concrete, pre-stressed concrete, cold rolled mild or stainlesssteel sections, and hot rolled mild steel sec-tions. Old buildings may have timber lintels or‘safes’ but these are no longer used and shouldalways be replaced during any refurbishmentwork. Shape will be discussed as we look ateach application.

There is some terminology to learn. Anylarge opening has:

� A bottom – the sill for windows or thresholdfor doors

� A top – the head which has a soffit or soffite� Two sides – the jambs which have reveals.

Once a frame is put into the opening, the soffitis divided into an inner and outer soffit, the re-veals are divided into inner and outer ingoes,and the sill into inner and outer sills. Theseare all illustrated in Figure 5.3.

Cavity walls have the cavity closed at thehead, jambs and sill or threshold. A fur-ther feature is the positioning of the door or

window frame within the opening. Wherethere is a risk of wind-driven rain, the frameis positioned back in the wall thickness, thusgiving shelter to the joint between frame andwall. This situation arises generally in Scot-land and in northern and western and south-western England, Wales and Northern Ire-land. In more sheltered areas, such as theHome Counties, Norfolk and Lincolnshire,the frame was traditionally placed flush withthe outside face of the wall, and the majority oftextbooks showed frames in this position. Thesituation has changed somewhat and the text-books now tend to show the frames set backbehind the face of the wall. Setting framesflush with the outer face of the wall had im-plications for making a weatherproof joint be-tween frame and wall which did not rely onsome form of mastic. Like flattery, mastic is thelast resort of the rogue builder in situationslike that. Mastic is advocated in the detailswhich follow but it is not relied on entirely;the detail itself has to be correct first.

Another area for concern was the closingof the cavity at head, jambs and sill. With theframe so far forward of the cavity, closure washaphazard in practice and the positioning of

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OU

TS

IDE

OUTSIDE

Frame

Outer ingo

Inner ingo

Inner sill

Inner soffitOutersoffit

Outersill

Soffit

Reveal

Sill

Jamb

Threshold

JambsSill

Jamb

Head Head

Fig. 5.3 Terminology associated with openings inwalls.

the DPC at jambs and head was never quitesatisfactory.

This chapter will deal with frames set backfrom the face of the wall – indeed the outerface of the frame should align with the innerface of the outer leaf of masonry in a cavityor timber frame wall construction. The detailsshown all work, and work well. They functionin the most extreme weather conditions in theUK.

Closure of cavities was traditionally, pre1970, done by:

� Making inner lintels wide enough at thehead with a DPC at the closure

� Making sills or thresholds wide enough,with a DPC at the closure

12--15 plaster

PCC innerlintel

Arris on plasterreinforced

15 quadrant bead

Window headTimber inner sill

Timber grounds

Bedmould

DPC behindmasonry sill

20 roughcast ordry dash finish

PCC outer sill with handling steel

Stooled end on PCC sill

Metal weather bar inmastic

frame bedding

Timber sill ofwindow frame

No mastic pointing here -- left clear to

allow water in cavity to drain out

Bellcast

20 roughcast or dry dash finish

PCC outer lintel

DPC dressed over inner lintel face

Fig. 5.4 Pre-1970s sill and head details, rendered ex-ternal finish.

� Returning the inner leaf of masonry againsta DPC layer on the outer leaf at the jambs.

These details were sound and have stood thetest of time since the early 1900s with onlyminor adjustment. The principles should notbe abandoned for the sake of fashion or whim.They are illustrated in Figures 5.4 and 5.5 in acavity wall which has a roughcast finish.

150 DPC

PCC sill

Window ingo smooth rendered

Stooled endof sill shown

dotted

20 roughcast or dry dashfinish

Outer leaf of wall

Inner leaf of wallMasonrycavityclosure

Mastic pointingTimber sill

outline

15 quadrant

Timber inner sill

Plaster arris reinforced

Horn on timber inner sill

Fig. 5.5 Pre-1970s jamb detail, rendered externalfinish.

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A few important points to note about the de-tails:

� PCC (pre-cast concrete) lintels of that pe-riod were frequently only reinforced at thebottom. Top and bottom reinforcement withlinks is now much more common.

� The roughcast finish stops short of the headand jambs of the timber frame and underthe PCC sill. At both head and sill this al-lows water to drain out. This should not befilled with mastic. At the jambs, the spaceis used to seal frame to wall to DPC withmastic.

� Joints of plaster to timber frame are coveredwith a timber quarter round beading to hidethe joint which would inevitably show acrack as the timber and plaster shrank awayfrom each other.

� Good joinery practice would have the bedmould tongued into a groove in the tim-ber inner sill. Economics now dictates oth-erwise.

� Had the wall been finished with facingbrick, many architects would have left the

Projection of outer leaf and components in itbeyond the inner leaf

While the facing brickwork is on a slightly differentalignment, the omission of the render finish to the outerface of the wall exposes the concrete lintel. This requires

a fair face on all exposed surfaces

Mastic pointingat jambs

No mastic pointing at head

Mastic bedding andpointing between sills

and roundweather bar

Fig. 5.6 Pre-1970s sill, jamb and head details, facing brick external finish.

brickwork in exactly the same position asthe common brick shown, relying in thatcase on the mastic alone to give a weatherseal instead of having the roughcast providea check or rebate. It would be better to havethis small rebate built into the wall as shownin the masonry only outlines of Figure 5.6.

Figure 5.7 is a photograph of the sill and partof the jamb of an opening in a blockwork wall.The details shown are the 200 thick outer leafof masonry; the sill with the upstand withweather bar groove; the weathered surface ofthe sill; the stool on the end of the sill; the lackof protection to the finished surfaces of the silland how dirty they are – not a good site.

Figure 5.8 is a photograph of the same win-dow opening which shows both sill and linteland masonry mullion. The mullion is beddedbetween sill and lintel but is also restrainedin position by metal dowels both top andbottom. Figure 5.9 shows how this would bedone. Note how the sill has a sloping surfacegenerally to make water run to the front edge,and how the underside of the projection has a

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Fig. 5.7 Photograph of 1990s sill and jamb.

groove – the throating – to prevent water run-ning back onto the face of the wall. Becausethe upper surface slopes, the mullion sits ona stool. This device is also used at the endsof sills which are built into walls, to allow themasonry to be bedded on a horizontal surface.

Fig. 5.8 Photograph of 1990s head, sill and mullion inartificial stone.

Mullion

Metal dowel

Groove forweather bar

Hole or morticefor dowel

Stool formullion

Weathered surface of sill

Upstand on whichwindow is bedded

Throatingon sill

Handlingsteel

Fig. 5.9 Detail of mullion–sill junction.

Figure 5.10 is a photograph of the insideof another window on the same site, whichshows a galvanised steel lintel holding up theinner masonry leaf over the opening. Steel lin-tels are sketched a little later in the text.

It is important to note that frames for win-dows or doors are only fastened to the wallsthrough the jambs and that in the traditionalmethods this fastening went into the masonry,closing the cavity.

Figure 5.11 is a photograph of a cavity clo-sure in blockwork. Note how a cut block closes

Fig. 5.10 Steel lintel at inner leaf of opening.

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Fig. 5.11 Detail of checked jamb.

the cavity and bonds into the inner leaf in al-ternate courses, and that a cut of blockwork isused for the intermediate courses, all beddedup solid in mortar and with a vertical stripof DPC between closure and the outer leaf.Compare these photographs with Figure 5.4,the general sections.

For nailing to plugs, proprietary framing an-chors or cramps are the most common meth-ods, although some window frames are stillonly wedged into place. More of this whenwe examine windows in Chapter 9.

Note that in Scotland, traditional practice isto form the opening and then fit the frame intothe opening, whereas the practice in the restof the UK is almost entirely to set the frame upon the sill, rack or brace it vertical, and thenbuild up the rest of the wall round it.

The window sections shown in all thedrawings so far have been based on standardsingle glazed casement windows. A range ofsections will be studied in Chapter 9, Win-dows.

The practice of building in frames as the wallis built up means that fixings are generally

galvanised or stainless steel ‘cramps’ fixed tothe back of the frame and bedded into the mor-tar joints. Screws should be used with timberframes, and machine or self tapping screwswith metal or plastic frames. A typical fixingis shown in Figure 5.12.

Since 1970 there have been several editionsof the Building Regulations, and the stricterrequirements regarding U-values of walls andthe need to avoid cold bridging have meantthat the traditional details have had to be up-dated. This is shown in Figures 5.13 and 5.14.

Back of frame

Screw fixings

Fig. 5.12 Window frame cramp.

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Cavity insulation

PCC inner lintel

12--15 plaster

Extruded polystyrene

cavity closure

Self foaming plastic

Windowhead

25 thick polystyrene

strip behind DPC.Blockwork reduced

to 75 thick

20 roughcast or dry dash

finish

PCC sill

Stooled end ofPCC sill

Sill of windowframe

Bellcast

20 roughcast or dry dash

finish

PCC outer lintel

DPC dressed over inner lintel and

cavity closure

No pointing here -- left clear to allow water

to drain out

Fig. 5.13 Eliminating cold bridging at sill and head.

Variations on finishing

The figures which follow show variationson the finishing of various parts of open-ings incorporating different materials and

Timber inner sillExtruded

polystyrene ascavity closure

Inner leaf

A larger gap can be leftbetween masonry and

frame which can filled withan injected self foaming

plastic

15 quadrant

Outer leaf

20 roughcast or drydash finish

Stooled endof sill

Window ingoessmooth rendered

PCC sill

Masticpointing

DPC

Window cramp built into inner leaf, screwed

to back of window frame

Galv. metal strap screwed to back of window frame and frame anchors into

blockwork

Fig. 5.14 Alternative jamb arrangements and cavityclosure to eliminate cold bridging.

components but always including wall insu-lation and avoiding cold bridging in terms ofthe current Building Regulations. The goodpractice in relation to the positioning of DPCsand frames, and mastic pointing in appropri-ate places, have all been maintained from thepre-1970 details.

Textbooks will show weeps formed or builtinto perpends of the outer leaf of brickworkover window and door openings. This cannotbe done when lintelling with PCC and/or aroughcast finish. How do you build a weepthrough a solid concrete lintel? There weretwo possible routes for water in the cavity touse to escape. The first was between DPC andouter lintel, a space which was never pointedwith mastic, as has been noted on the previ-ous figures. The second was off the ends of theDPC into the cavity itself, but at the back of theouter leaf. This was made easier by the sim-ple expedient of having the DPC project about100 mm beyond the end of each lintel. With noinsulation filling the cavity, water running offthe low end of the DPC did so against the in-ner face of the outer leaf and was in no dangerof crossing back across the cavity.

In recent years in the construction press,much has been made of cavity trays. The DPCover inner lintels is such a tray and in the badold days they formed up from hessian-basedbituminous DPC where appropriate. There isa widely held opinion that the ends of all cav-ity trays should be ‘boxed’ to prevent waterrunning off the end into the cavity. In manyinstances this is correct but is not really neces-sary at ordinary PCC lintels unless the cavityis filled with insulation. There are three solu-tions to the problem: only part fill the cavityand let the water run off the ends of the DPC inthe usual manner, or put the insulation some-where else, completely inside or outside thewall, or redesign the lintelling when we havea facing brick finish.

Alternative lintelling arrangements areshown in Figures 5.15, 5.16 and 5.17. Fig-ure 5.15 shows a steel lintel made upfrom three pressed sections spot welded to-gether, zinc plated, passivated and paintedin the factory. A separate DPC is shown but

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Weep

DPC

Mastic pointing

Self foamingplastic fill

Factory filledwith insulation

Back of lintelperforated

Metal lath to takeplaster finish

Sawn, treatedbatten clipped

to back oflintel

Fig. 5.15 Pressed steel inner lintel (1).

some manufacturers do not recommend one.Figure 5.16 shows an IG lintel, available in gal-vanised steel and stainless steel. Sealing thewindow frame and foam filling etc. is all thesame as the earlier lintels. Figure 5.17 shows avery old form of lintelling when using a facingbrick outer leaf: the mild steel angle iron. Gal-vanised angles are more readily available andare to be preferred to plain steel. Note that the

Cavity insulation

DPC

Factory filled

with insulation

Weep

Fig. 5.16 Pressed steel lintel (2).

Cavityinsulation

Weep

100 x 100 x 12galvanised

m.s. angle iron

PCCinnerlintel

DPC

Fig. 5.17 Angle iron lintel.

inner lintel here is rectangular and that a DPCis dressed over it and down over the angle toproject beyond the angle iron. DPC projectionis necessary in all the details where it is shownto provide a good drip and prevent water be-ing drawn under the lintel and so coming incontact with the frame. Mastic pointing is usedat the head of the frame.

The illustrations so far show an outer leaf offacing brick. All of these lintel alternatives canbe used with a roughcast or dry dash finishon common brick or blockwork. The generaldetail does not alter significantly. The weepscan be blocked when the roughcast is appliedand, if possible, the use of weeps with a re-movable strip or a strict regime of cleaning upafter rendering has to be adopted. Figure 5.18shows a possible solution using the IG linteland without using weeps, instead using thenatural porosity of blockwork to allow waterto escape. The detail would be the same forany steel lintel.

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Bellcast

Mastic pointing

Roughcast cutshort of edge

of lintel

Fig. 5.18 Pressed steel lintel (3).

Before leaving lintels the term ‘bellcast’ re-quires expansion. It is obvious how this pro-jection of a roughcast or rendered finish gotits name. It is not formed directly by applyingthe mortar in that shape; that would not bepossible. Instead, early craftsmen fixed a tem-porary batten to the masonry background andworked to the thickness of that batten. Mod-ern practice uses either a galvanised steel beador a plastic bead, which is fixed to the wall andthe render brought to that. Figure 5.19 shows atypical section and its application, while Fig-ure 5.20 is a photograph of three galvanisedbeads, from the left:

� The bellcast bead.� A render stop bead used to finish off plaster

at an open edge of a wall or where separat-ing plaster or render finishes either side ofan expansion joist etc.

� Angle bead for protecting arrises in plaster-work. It features in most of the sketches inthis book on openings in masonry walls.

Note that all the beads feature a mesh-likefinish to the edges which are bedded intothe plaster or render. This mesh is expanded

Blo

ck b

ackg

roun

dFa

cing

bric

k ba

se

Roughcastor

render

Bellcastbead

Expanded metal edge onbead, nailed or stuck with

mortar to blockwork

Fig. 5.19 Bellcast bead.

Fig. 5.20 Photograph of bellcast bead and corner bead.

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Mastic behind DPC

If ends of DPC boxed,then weeps in here

DPC

Thermabate

Head

Jamb

Thermabate

Mastic

Fig. 5.21 Proprietary cavity closure at head and jambs.

metal and is made by slitting the metal sheetand then pulling it apart.

The only alternative to the use of masonryor extruded polystyrene strip to close cavitiesat jambs is the use of a proprietary closure.The component is an extruded uPVC sectionfilled with expanded plastic foam. The outsidehas grooves or ridges to provide a fixing pointfor fittings and to direct water flow. Becausethe PVC outer is waterproof, there is gener-ally held to be no need to place a separateDPC against the outer leaf. This is not neces-sarily the case when the frame is set back inthe opening, as is common in much contempo-rary construction. Figures 5.21 and 5.22 showhow this can be dealt with.

Various

Flange

PVC casing Foam filled

Flanges and tees totake fittings and

stop water

Fig. 5.22 Schematic of cavity closure.

Alternative sill arrangements

Figure 5.23 shows a PCC sill with a propri-etary closure immediately under the sill ofthe window frame. The flange of the clo-sure should be over the PCC sill, bedded inmastic and trimmed back if necessary. Theprojection of the timber window sill may haveto be larger than usual. Note the use of and po-sition of the DPC. Provided the under sill DPCstarts on the inside face of the closure, thereis no need to place DPC between the closureand the outer leaf at the jambs detail.

An alternative shape for a PCC sill is shownin Figure 5.24. This shape can be cast onecourse high, although it makes a very slendersill if there is a wide window opening.

Tiles, both plain roofing of clay or concreteand quarry floor tiles, make good sills as theyare made to prevent the entry of water. Thedetail is illustrated in Figure 5.25. They must,however, be laid in a minimum of two courseswith the bond broken and in a strong mor-tar, mix 1:5 or even 1:4. Any DPC must bebrought down at least one, preferably two,brick courses below.

Window sill bedded onThermabate with mastic

Stooledend of

sill

DPC

Fig. 5.23 Proprietary cavity closure at sill.

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Outerleaf

Innerleaf

Cavity

Drip

Stooled end

Grooves for weather barfilled with mastic

Insulation to preventcold bridge

Fig. 5.24 Alternative sill in PCC.

Bullnose bricks can be laid on edge to pro-vide a sill. The plain header end is cut to abevel against the inner leaf and the bricksare canted up at the inside end to form aweathering. The sharp arris at the bullnoseend forms a satisfactory drip. A strong mortaris required to provide a reasonably weatherresistant joint. The detail is illustrated in

Cav

ity in

sula

tion

Cold bridgeinsulation

Mastic pointingand bedding

Plain roofingtiles as sill

DPC

Upper coursewith bullnose

Quarry tiles as sill

Fig. 5.25 Tile sills.

Insulation to preventcold bridging

Masticpointing

Bullnose brickbevelled one end

laid BOE

End of brickforms drip

DPC

Outerleaf

InnerleafCavity

Fig. 5.26 Bullnose brick sill.

Figure 5.26. DPCs are best taken one fullcourse below the bullnose sill.

Note that it is only on the PCC sill detailsthat stooled ends are shown because only PCC(or stone) sills are built into the walls at thejambs. Tile and brick sills are built only be-tween the jambs. PCC (and stone) sills are alsomade without stooled ends, to be built into theopening after the walls are complete and ei-ther just before or after the window frames arefixed. Such sills are termed ‘slip sills’.

Threshold arrangements

What a sill is to a window, the threshold isto a door. The same requirements apply – theneed to make this opening joint wind and wa-tertight. Many variations are seen, but withframes recessed into openings, the ideal situ-ation is to provide a step up to the floor levelwith the face of the step in the same plane asthe face of the door. Figure 5.27 shows this de-tail and points out one possible weakness – thefixing of the weather bar. Figure 5.28 shows asolution to that problem which does not jeop-ardise the integrity of the detail but providesa secure anchorage for the weather bar. The

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Platt

DPC

Dotted line showsoutline of slip step

at jamb

Cavity &closure

Outer leaf

ArchitraveSkirting

PlasterPlinthblock

Waist of slip step

Door

Line ofinsideface ofinnerleaf

Doorframe

Stop/facing

FGL

Platt

Laid to fall away from door

Slate or tile as permanentshutter

Slip step

Threshold bar pluggedand screwed to step

Plinth block

Architrave

Inner leaf with plaster

Cavityclosure

DPCOuterleaf

Stop/facing

DPC

FFL

Door frame

Doo

r

3 course corbelto support slip step

Horizontal section above slip stepVertical section through platt, slip step and bottom of door

Inner leaf

Fig. 5.27 Threshold (1).

detail is shown in Figure 5.29 and a photo-graph in Figure 5.30.

Note that the first detail, Figure 5.27, showsthe traditional full width frame in the checkedreveal while the second detail, Figure 5.29,shows a more modern small section frame

with the reveal finished in the same way asthe wall. Note that the second arrangement re-duces the amount of timber used for the doorframe and the stop facing; there are no plinthblocks to make and fix and no architrave; thereare 1

4 round beads; and plaster and skirting

This face in same plane as door

This face projects fro

m face of d

oor

Plugging and screwing the threshold bar to step is in a wider part of the step, just

where the plug expands, so less chance of concrete spalling

Stooled end

Stooled end

Plugging and screwing threshold bar to step is very close to and could spall

the concrete off

Dotted lines indicate position ofthresholds and ends of door frames

Width between stooled endsis door width plus clearance

Indicates a dowel, usually from offcuts of 15copper water pipe, let into bottom of door frameand grouted into hole in step. Dowels stop the

bottom of the door frame from twisting

Slip step shown in Figure 5.27

Slip step shown in Figure 5.29

Fig. 5.28 Slip steps.

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DPC Cavity closure

Outerleaf

Slip step

Stop/facing

Inner leafwith plaster

Door frame

Doo

r

Skirting

1/4 round bead

Threshold plugged andscrewed to step

FFL

Note how stop/facing isscribed to angled face

on slip step and so covers the joint of the cavity

closure to the top of the step -- a better finish than

the previous detail

Platt

FGL

Cavity closedwtih slate/tile

DPC

Platt

DPCStop/facing

Door frame

Face of slip step

Waist of slip step

Door

Cavityclosure

1/4 round beadCorner bead reinforcing arris

Skirting

Vertical section through platt, slip step and bottom of door Horizontal section above slip step

Plaster

Fig. 5.29 Threshold (2).

have to return into the reveal from the wallface. This detail is more economical than thefirst arrangement.

Note that in both details the type of floorhas not been shown. This is irrelevant tothe threshold construction, although for some

Fig. 5.30 Photograph of threshold (2).

floors the DPC under the slip step should beturned up between step and floor – very im-portant if the floor is of hung timber or con-crete construction. For solid concrete floors,the DPC should be joined to the DPM.

It is usual to keep the top of the slip stepslightly higher than the finished floor level ofthe building. This allows for any thickness offloor finish put down by the occupiers such ascarpet, vinyl sheet, etc.

Threshold bars come in a bewildering va-riety of types, sizes, materials and finishes.Some have moving parts (to be avoided,as they inevitably break down); others havefixed sections. Some have only one sectionfixed to the slip step; others have a numberof sections fixed to both slip step and door.

Partitions of masonry

The fitting of door frames into the openings inpartitions can be done using full width or partwidth door frames. The effect is similar to thatseen with the external doors shown earlier.

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Figure 5.31 shows lintelling arrangements.Figure 5.32 shows two typical examples – apart width frame with a PCC lintel and a fullwidth frame with a pressed steel lintel. Thechoice is deliberate as it is difficult to finishthe soffit at a part width frame where there isa pressed steel lintel of this type – how is thefinish attached to the corrugated shape of thelintel? It might be possible to introduce a stripof metal lath, but how can one convenientlyfasten that to the lintel?

Frames for doors in partitions will normallyinclude a timber threshold plate (not to be

2 or 3 brickcourse

multiples

one brickcourse

one brickcourse

Precast concretelintels

Galvanised pressed steel lintels

Depth andreinforcement

for these lintelswill depend onwhether or notthe partition is

load bearing andon the span

Pressed steellintels are made

in a variety ofprofiles for

partition work;those shownhere are only

suitable for non-loadbearing

partitions

Fig. 5.31 Partition lintels.

Plaster

Pla

ster

Plaster Pressed steellintel

Pla

ster

Plaster

Half brickwide

Stop

Doorframe

Architrave Architrave

Skirting lineArchitrave

Architrave

Architrave

Architrave

Door frame

Door frame

Door frame and fixing

Stop

Skirting line

Door

Door

Doo

r

Door

Stop

Stop

round bead

round bead

Corner bead toplaster arris

Corner bead toplaster arris

PCC lintel

Skirting line

Skirting line

41

41

Fig. 5.32 Partition door openings.

confused with a threshold bar), a plain tim-ber section, usually hardwood. This may beonly the width of the door (irrespective of theactual frame width) or the width of the frame.From a purely practical point of view, a fullwidth plate is best as any floor covering fittedto the threshold plate does not incur excessivewaste. Indeed, the occupant may be forced topurchase the next width up in floor coveringbecause of the door ‘recess’.

For fastening frames, see Appendix G, par-ticularly nailing to dooks and proprietaryframing anchors. Note in the partial widthframe situation how close the frame fasten-ing is to the edge of the frame, or if it is set inthe middle how close it would be to the edgeof the wall.

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In all the door frames fastened to masonry,3 or 4 frame anchors or plugs should be usedin internal walls and 4 or 5 for external doors,depending on the weight of the door leaf.

Openings in timber frame walls

Openings in timber frame walls share onlythree things in common with masonry walls:

� The need to provide support for the struc-ture built over the opening

� The need to provide a finish to the structurebelow the opening

� The need to close cavities and maintaina waterproof interface with whatever theopenings will accommodate.

Additional features include:

� The need to include fire stopping roundopenings

� Proper weather proofing where the outerskin is of the rain screen variety (this willbe explained as we go along)

� Provision for differential movement atopenings in structures over three storeyshigh between the frame and the facing ma-terial, particularly a facing of masonry. Thiswill not be included in this text.

We begin by looking at suitable details forone- and two-storey buildings with domes-tic or light commercial loadings, in each casetaking different cladding systems/techniquesinto account. As we have done before, we canclassify the openings according to size:

� For small pipes and cables < 50 mm diam-eter

� For pipes and ventilators >50 mm diameterbut <225 mm

� In excess of 225 mm – large openings.

Openings for pipes and cables up to 50 mmdiameter can be treated in the same way, nomatter what the storey height.

When cutting through a timber frame wallfor a pipe or cable, the elements shown inFigure 5.33 will all be pierced and the cav-ity bridged; if not treated properly this will be

Externalcladding

Cavity

Breather membrane

on sheathing

Sheathing

Insulation in voids of timber frame

Vapour control layer

Plasterboard orother finish

Fig. 5.33 Timber frame wall schematic.

a source of potential trouble as the buildingages.

Starting at the inner face, the hole in theplasterboard can be disguised by using a floorplate. Immediately behind is the vapour con-trol layer (VCL) which if badly torn will al-low water vapour to collect in the insulation –a possible source of condensation and rot onthe back of the sheathing. Cutting through thesheathing causes no problems but piercing thebreather membrane could allow solid water topass to the sheathing.

The breather membrane should be cut withan upside down T-shaped cut which forcesany solid water to run round the cut and downthe face of the membrane. This is illustrated inFigure 5.34. Bridging the cavity must be done

Top of slitpoints

upwards

Fig. 5.34 Cutting waterproof fabric for pipe passingthrough.

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Metal pipe or sleeve 50 diameter

Facing brickcladding

Cavity fire stopped atall other openings, top

and bottom and atcorners

102.5 50 110 102.5 50 110Metal pipe or sleevepassing through wall.No need to firestop

around it but must be atight fit in thesheathing.

Loose fit in the facingbrick allows for

differential expansion.

Cable or plastics pipein sleeve. Sleeve

bushed each end toprevent damage from

chafing.Gas pipes need to besealed both ends withan approved mastic

Timber framepanel with 11

OSB sheathingand breatherpaper on 97framing and

VCL and 12.5plasterboard,

decorateddirect

Fig. 5.35 Pipes and ducts through timber frame wall (1).

with any material sloping down towards thecladding, thus preventing solid water reach-ing the breather membrane. Cutting throughthe outer cladding must be done with care,not only from the point of view of appearancebut also to keep the hole to the minimum re-quired, so preventing vermin and larger in-sects getting into the cavity. Larger holes willalso allow weather, particularly rain andwind-driven snow to penetrate. This is moreimportant where the external cladding is ofthe rain screen variety.

Commencing with a masonry layer, Fig-ures 5.35 and 5.36 show some typical exam-ples; the slope to the outside is, of course,exaggerated. In Figure 5.36 a duct is illus-trated which could be circular or rectangular,plastic or metal. Whichever of these combi-nations it is, there is every need for fire stop-ping around it. Sealing of all the ducts, sleevesand pipes to the masonry cladding could bedone, but rather than using the ubiquitous sil-icone sealer, one based on polysulphide mightprove to be more long lasting. There is nothingwrong with silicone sealers but frequently theincorrect grade is used and often in the wrongplaces for a silicone-based product.

The examples shown have only illustrateda cladding of masonry which to some degreeor other will absorb rain driven onto its face.Penetration to the cavity is not a problem asmeasures are built into the general detailingto cope with this, particularly the provisionof a DPC layer between fire stopping andcladding.

Rain screen cladding

Mention was made earlier of rain screencladding. These types of cladding do not ab-sorb water to any great extent and some notat all:� Slightly absorbent claddings include slate

and tile, both clay and concrete, timber suchas shiplap or weather boarding, and boardsmade from timber, asbestos cement or silicafibre cement compounds.

� Non-absorbent claddings include metalsand glass.

Metals and glass are seldom used in domes-tic structures in the UK but are common inNorth America, as are boards of timber fibreand asbestos cement. This leaves tile or slate

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Plastic sleeve < 50 diameter and light metal or plastic duct > 50

Fire stopping

DPC

Firestopping

DPCPlastic sleeve cannot beused for gas pipe

Plastics sleevepassing through

cavity must be firestopped. Can be a

tight fit in facingbrick cladding as

flexibility allows fordifferentialexpansion Light metal or

plastics duct


cladding and timber weather board or shiplapcladding. All are quite common.

Vertical slating and tile hanging is usuallydone on horizontal battens nailed to verticalbattens which in turn are fixed through thebreather paper and sheathing to the studdingof the timber frame. The vertical battens allowa space to be formed from top to bottom of thewall cladding, and this should be finished offtop and bottom to allow a free air flow, thusallowing any water getting past the tiles ontothe breather membrane to dry out.

Timber boarding is normally fixed horizon-tally to vertical battens which in turn are fixedthrough the breather paper and sheathing tothe studding of the timber frame. Shiplapboarding is always fixed horizontally butweatherboarding can be fixed diagonally, al-though this is only done for decorative effect.Any attempt to have timber boarding fixedvertically should be avoided as it defeats therain screening properties of the boards.

Figures 5.37 and 5.38 show the same pipe,sleeve and duct situations already examinedfor a masonry cladding. Here the cladding haschanged to tile, weatherboarding and shiplapboarding.

Permutations of various finishes showingsmall pipe, sleeves or ducts through all thevarious combinations of cladding for timberframe construction would be excessivelyrepetitive here. The details already cov-ered can be extrapolated to cover mostsituations.

Lintels

Large openings are those which require sup-port provided by lintelling over doors andwindows, and here techniques have similari-ties to masonry construction:

� The opening in the timber frame is spannedby a lintel – in this case made from timberor a timber/steel combination.

� The jambs provide support for the lintel bythe insertion of additional studs called crip-ple studs.

� There can be multiple cripple studs in onejamb, some supporting the lintelling ar-rangements and others supporting a sill.

� The bottom of the opening is finished offwith a sill or a threshold

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38 x 25 vertical batten

38 x 25 tile batten

Lead slate piece nailedto tile batten at 50

centres

Code 5 leadslate piece

sized to suit tileor slate withshort spigotburned on.

Pipe passesthrough spigot.Tile cut short tolet pipe pass.Flexibility of

lead, slate ortile allowsdifferentialmovement

Metal pipe

Metal orplastics sleeve

bushed toprevent damageto cable or pipe

Metal pipe 50 diameter Metal sleeve 50 diameter


� Sills require support from cripple studs.� Masonry claddings require support over the

opening and this can be done with con-ventional lintels of concrete or steel or by

special steel lintels restrained against thetimber frame

� Jambs are treated conventionally externallyand internally but the structure within the

Shiplap boarding Weatherboarding

38 x

25

vert

ical

bat

tens

38 x

25

vert

ical

bat

tens

Metal or plastics, pipe orsleeve, through boardingand timber frame wall.

Tight fit in sheathing withcover plates inside and

outside -- see below

Cover plate Sprung lugs

Pipe

Boarding


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\

Full studCripple stud tosupport lintel

Self foaming plastic sealbetween window frame

and cavity closure

Masticpointing

Stooled end ofPCC sill

DPC

50 x 50cavity

closurewrapped in

buildingpaper

12 sheathingplywood

withbuildingpaper

on face

Glass fibrequilt

2/200 x 50 timbersas lintel

DPC

50 x 50 cavityclosure wrapped

in buildingpaper

Inner timber sill

Bed mould

75 x 50 filler piece between studs

150 x 50 sill support on a second cripple stud nailed inside thecripple studs supporting the lintel

150 x 50 studs at 400 centres

12 sheathing plywood with building paper on the face

PCC sill

Window sill beddedin mastic with galv.

weather bar

Self foamingplastic seal

between windowframe and cavity

closure

PCC lintel

20 roughcastor dry dash

Commonbrickwork

12.5 plasterboard andVCL

Fig. 5.39 Window opening in early timber frame constructioin.

jamb is made up of multiple studs and crip-ple studs in the timber frame.

� Masonry sills can be treated convention-ally in all the accepted ways but the ac-tual sill butts back against the timber framepanel with the sill DPC between. The in-ner sill is treated in a conventional mannerwith packers rather than grounds for fixingdown.

Figure 5.39 shows a version of a windowopening in an early timber frame wall. Notethat all the structural timbers were sawn andnot stress graded. These details are fairly typ-ical of early timber frame construction, whichtended to mimic the masonry cavity wall inthe order in which it was built – first the frameswere erected, then the masonry skin was builtand then the openings were filled with win-dows or doors. Now the trend is to erect theframe, fix the cavity closure to the windowor door frames, fix these back to the timberframes and then build the skin against all thetimber work. This does not leave a gap be-tween frame and timber panel to be filled witha sealant.

Figure 5.40 shows the inside of the timberframed panel once it is erected on a wall plate,the doubled top runner put in place and theupper floor joists in place. Note the use of crip-pled studs – studs cut short to provide supportat the ends of horizontal members.

Figure 5.41 shows a more modern approachto timber frame panels. This is a photograph of

Wall plate

Sole plate to frame

Noggings,two in height

Studs at 400 centres

Top runner totimber frame

Double runnerunder joists

Upper floor joistsover studs

Doubled timbersas lintel

Cripple studsupporting

lintel

Filler pieceat sill

Sill support

Cripple studunder sillsupport

Fig. 5.40 Elevation of early timber panel at windowopening.

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Fig. 5.41 Modern timber panel at window opening.

the inside of a timber frame panel but note theOSB skin, the regularised and stress gradedtimbers, the timber lintel supported on dou-ble cripple studs and the narrow sill supporton cripple studs. Now compare with Figure5.40. The studs, runners and dwangs are reg-ularised and 97 × 47 but the studs are put in

Fig. 5.42 Rolled steel channel as lintel in timber panel.

Ring beamJoist

Top runner doubler

Top runner of panel

Timber chock to holdchannel in position

203 x 89 mild steelchannel

Timber under channelgives nailing facility all

round

OSB panelcladding

2 x cripple studs andfull stud of 97 x 47

stress graded timber

Fig. 5.43 Detail of rolled steel channel as lintel in tim-ber panel.

at 600 centres and so the joists are put in at600 centres and have to be much deeper. Afurther complication is the need to keep ceil-ing heights down. If wooden lintelling is usedfor wide openings, the top of the glass in thewindow would be much too low. This can be

Fig. 5.44 Flitched beam over large opening.

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Fig. 5.45 Flitched portal frame round opening in tim-ber panel.

solved in one of two ways depending on theloadings involved.

The first way is shown in Figure 5.42 anduses a combination of a mild steel channelsandwiched between two layers of the 97 × 47timbers, with blocks at each end set into thechannel and nailed to the studs. These keepthe channel in position. Note that there aredoubled cripple studs supporting each endof the channel/timber lintel. Channel ironsare made in a range of sizes but any in therange 76 × 38 to 305 × 89 would be suitable.Figure 5.43 shows a detail at the head of theframe.

The second way is to ‘flitch’ the frame mem-bers – both studs and lintels–to make a ‘portal’round a large opening such as a patio door.Figure 5.44 is a photograph of a flitched beamspanning a wide opening and is followedby a photograph of a flitched portal frame inFigure 5.45. The steel strip in the portal framewas welded at the corners before sandwichingit and bolting up between the timbers. Notethat the steel does not extend to the bottom ofthe jambs but stops short. A plywood packeris put between the timbers.

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6 Roof Structure

Roof classifications 148Prefabrication 149Trussed roofs 150

The trussed rafter 153Verges meet eaves 159Roof bracing 160

Flat roofs in timber 162Insulation, vapour control layers

and voids and ventilation 164Traditional roofs 167Roof insulation 169

Before looking at structure, coverings, etc. forroofs it is important to understand the generalterminology of roofs. The sketch in Figure 6.1covers much of what we will need to knowfor this chapter and for Chapter 7, Roof Cov-erings. The remainder we will pick up as wego along.

Roof classifications

Roofs can be classified in a number of waysand these are a few of them:

� By roof pitch. We talk about pitched roofsand flat roofs. Pitch is the angle that the gen-eral waterproof layer makes with the hori-zontal. Up to and including an angle of 10◦

the roof is a flat roof. With an angle over 10◦,the roof is pitched.

� Flat roofs are never described as pitched butas laid to fall or, if there is more than oneslope, laid to falls, and the fall is given not asan angle but as the fall in millimetres overa horizontal distance in millimetres.

� Pitched roofs may be monopitch, symmet-rical or asymmetrical.

The next classification we will make is the roofform:

� Flat roof

� Lean-to roof� Monopitch roof� Symmetrical pitched roof� Asymmetrical pitched roof� Gabled roof� Hipped roof� Mansard roof

Not an exhaustive list but enough for our pur-poses here. Figure 6.2 shows examples of thesein outline form. As the reader becomes morefamiliar with construction, other forms willbecome apparent; many are permutations ofthe above list.

Pitched roofs can be further sub-divided byconstruction type or form into stick built orprefabricated.

Stick built roofs are not often seen now, forthe term means that timber is brought ontosite in lengths to suit the roof, but not exactlengths. Then a squad of carpenters set upa jig on the site and cut all the timbers tolength, forming all the notchings and halvingsin them before assembling the roof on the topof the walls, stick by stick. The term was evenextended to roofs with trusses or purlins etc.as the trusses were built on the ground in ajig and then hoisted into position. But we aregetting ahead of ourselves.

Each stick in a roof has a unique name, sobefore going into prefabrication we need to

148

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Roof Structure 149

Gabled end

Hipped

end

Ridge

Verge

Verg

e

Eaves

Lean-to

Half dormerwindow

Dormerwindow

Eaves

Eaves

Hip

Hip

Monopitch

Abut

men

t

Eaves Verg

e

Ridge

Verge

Verge

Eaves

Ridge

Val

ley

VergeVerge Eaves

Fig. 6.1 Roof terminology.

know what these names are. Figure 6.3 iden-tifies a few of them.

Prefabrication

Prefabricated roofs are very common now butearly attempts were seen by the author in the1950s. The roofs of the no-fines houses he sawwere supported on trusses, and these and therafters, purlins and ceiling binders were allassembled on the ground and fixed to the wallplates. Some additional bracing was built into ensure that the whole frame stayed plumband straight, and then it was lifted onto thetop of the walls by a large crane, where it wasset down on a layer of wet bedding mortar.The technique worked extremely well but wasconfined to the few very large sites where acovered work area could be built and the jigs

Angle 10°or less

Gabled roof

Asymmetrical roof

Monopitch roof

Flat roof

Hipped roof

Mansard roof

Symmetrical roof

Lean-to roof

Fig. 6.2 Roof form outlines.

for the trusses and the whole roofs could beset up inside. Also, small sites could not sup-port the large squad of carpenters required,nor employ their joinery skills later when itcame to finishings etc. Power tools for cutting

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RafterCollar

Han

ger

Han

gerRafte

r

Ridge board

Ceiling tieEaves

projection

Fig. 6.3 Stick built roof timbers.

the timber were installed, and as soon as thesite was operational roofs were being prefab-ricated and stockpiled until the concrete wallshad been poured and the shuttering stripped.The same cranes used for all that lifted the roofstructure into place.

Prefabrication now has come about with theintroduction of the trussed rafter roof – not acomplete roof but a set of units which are laidalong the line of the roof and give the pitchedform required. There are no purlins and noridge; binders are sometimes used. The eavesand verges can all be prefabricated and easilylifted into place. The main point of this typeof roof is that the components are a maximumtwo-man lift for all but a very few roofs – noneed for a crane.

Trussed roofs

Before the trussed rafter, timbers (1) for roofshad to be of sufficient strength to span quiteconsiderable distances using nails and sim-ple jointing techniques. If the timber couldn’tmake it on its own, then strong beams(2) could be used to break up the spans ofthese timbers, and if that was not enough ex-tra strong frames (3) were made up and put into break the span of the beams. Thus we had:

(1) These timbers would be the rafters, ties,collars and hangers of an ordinary roof.

(2) The beams used would be purlins to sup-port the centre of the rafters, and bindersto support the ties, generally at their thirdpoints.

(3) The frames are called trusses and couldrange from king and queen post and otherwonderful trusses from medieval times, tothe TRADA trusses of the latter half of thetwentieth century.

Figure 6.4 shows some layouts of these tim-bers in (1) in the above list for various formsof roof. For the same timber sizes, roof C isa larger span version of A, while B, the col-lared roof, is common on old properties andfarm buildings but is not as strong as roof A.The provision of a tie in roofs A and C pre-vents the rafter ends at the wall head pushingoutwards and pushing the wall over. While acollar can also do this, the lower half of therafters are put into bending as well as com-pression and so much stronger timbers have

Rafters are fixed in pairseither side of the ridge board

Tie

Rafter

Rafter

Ridge board

A - Rafters with tie

B - Rafters with collar

C - Rafters, collar, ties and hangers

D - Rafters, collar, ties and hangers in an attic roof

Bellcast orsprocketpieces

This part of rafter is ‘bending’and so whole rafter

needs to be stronger

In this roof, the load on therafters pushes them downagainst the top of the wall.

The tie prevents thembeing pushed outwards,

thus the rafters are incompression and the ties

in tension

In this case the collar is in compression

Hangers are in tension

Fig. 6.4 Pitched roof shapes and types.

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Roof Structure 151

to be employed for the same span of roof. Dis the form of C with a steeper pitch and thepossibility of having attic rooms. With thesteeper pitch, lighter section rafters, collarsand hangers can be used but the ties now dou-ble up as floor joists and so must be heavierand possibly even supported at mid span bya beam or partition. Roof D also shows a de-vice from the early roofs, the sprocket or bell-cast piece. These were separate short lengthsof timber attached at the eaves and formingpart of the eaves projection. The pitch angleon them was usually less than the main roof,which slowed the run-off of water from theslate or tile into the gutter.

Figure 6.5 shows, in step-by-step build upformat, a roof using purlins and binders span-ning from gable wall to gable wall and sup-porting rafters and ties.

Finally, Figures 6.6 and 6.7 show how atrussed roof would be built up step by step.The truss shape is a king post truss andit would be made from heavy natural tim-bers each in one piece, with the centre strut,the king post, shaped to take the struts and

A – The bare walls

B – Wall plate, purlins and ridge

in position

C -- Rafters put in both sides in pairs

opposite one another on the ridge board

Short distancebetween gable

walls-- 5 to 6metres maximum

– but see text

Ridge

Purlin

Wall plate

Fig. 6.5 Purlined roof.

Bare walls

Too far for purlins orbinders to span

Break down into lengths suitable for small sectionpurlins and binders

Fig. 6.6 Trusses and purlins.

the truss rafters. Note that the rafters whichactually support the roofing material arecalled common rafters and are laid out acrosstheir supports, the purlins, without taking anynotice of the trusses. The trusses are only thereto support the purlins.

It is possible to do without trusses if thelength between the gables is not too long, byusing trussed purlins or I beam purlins orbox beam purlins. Figure 6.8 shows sectionsthrough these, together with the position be-tween the gables that they would take. Notethat there will still be a wall plate on top of thewall for all the rafter feet, and a ridge wherethe pairs of rafters join. A plywood I beamwith a depth of approximately 900 and withfour 100 × 50 SC3 timbers as flanges, was ca-pable of supporting a tiled roof over a spanof 4000. These beams can all be made on site,the fastening being either bolts and connectors

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Truss made to this shape with slot in top for ridge board and position of

purlins and binders shown

Purlins and ridge board in position. Both can be joined at the bearing point on the truss with scarfed joints

The common rafters are fixed in position startingwith a rafter near one gable and at regular centres to the other end of the roof, ignoring the truss positions

Fig. 6.7 Trusses within the roof outline.

or pattern nailing. Figure 6.8 shows what ismeant by pattern nailing.

Other truss shapes have been made fromsolid timbers and Figures 6.9 and 6.10 showa selection of these. The queen post truss willspan much further than the king post trussand the hammer beam truss further still. Vari-ations on the latter have been used for manypublic and religious buildings and that isprobably the best place to see them. From itsshape, it is obvious that the queen post trussis very suitable for containing attic rooms somany of these fine trusses are hidden fromview, as is the mansard truss. The mansardtruss is a combination of a modified queenpost truss with a fairly low pitch king post

Trussed purlin made up from individual timbers, layered together at the nodes and bolted with

Bulldog connectors between the timbers

I and Box beams–I beams have a plywood web and four solid timber flanges. Box beams have two plywood faces and

a solid timber chord top and bottom. In addition there are solid timbers across the ply to prevent buckling of the ply layer(s)

Pattern nailing. Note how the pattern changes to fewer nails in unit area as stresses at joints are avoided. Natural timber will have to be pre-drilled where nails are concentrated in an area

near the end of a timber

Fig. 6.8 Trussed, I beam and box beam purlins.

truss on top. In every case each truss hasrafters and the purlins they support in turnsupport common rafters. The Belfast trussis, by the standards of the others, relativelymodern – late nineteenth, early twentieth

Truss timberCommon rafter linePurlin position

Queen post truss

Mansard truss

Fig. 6.9 Queen post and mansard trusses.

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Roof Structure 153

Common raftersPurlin position

Hammer beam truss

Belfast truss. Chords of heavy timber with plain or t&g boarding nailed each side but in different directions

Fig. 6.10 Hammer beam and Belfast trusses.

century – when it was widely used for indus-trial buildings.

Figure 6.8 brought up the possibility of lay-ering timbers at the nodes of the trussedpurlin. Indeed, this was the tactic adopted bythe Timber Research and Development Asso-ciation (TRADA) in their series of trusses fordomestic and light commercial roofs. Figure6.11 illustrates this type of truss. AppendicesC and G give details of the timber connectorsused.

The outline shows the general arrangementof the timbers. The pair of rafters are eachmade up of two pieces of timber, as is the tie.There is extensive use of packing timbers orspacers which are used to connect the pairs oftimbers together. The purlin struts are broadand have a check in the top to support thepurlin. The long struts are bolted to the ridgesupport at the top and, although slightly outof alignment, are bolted in between the tie

timbers. Most of the node1 details are given inFigure 6.11. Purlins are generally from 150 ×50 to 200 × 65 as the trusses are set at 1800centres with common rafters between them at450 centres, so there are three common raftersthen a truss rafter, three more common rafters,a truss rafter, and so on right down the roof.These trusses have rafters which take a shareof the roof covering load along with the com-mon rafters. Using trusses this way in the late1940s and early 1950s was the first time that ithad been done.

The author built one set of trusses in 1964,following the TRADA designs, which haddouble timbers for the ties and triple timbersfor the rafters. The purlin struts were doublebut the long struts were single. The timber wasDouglas Fir – stress grading was unknownat the time. The pitch was 22◦ and the roofwas covered with aluminium on felt on woodwool slabs. The roof is still there with nothingother than routine maintenance. About a yearafter the roof was completed and the housewas occupied, the author had occasion to gointo the roof space and while there decided tohave a closer look at the joints of the trusses.To his dismay he found that every one of thenuts on the bolted connections was only fin-gertight at best and in many instances was ac-tually loose. This was not caused by vibrationbut was solely due to shrinkage on the multi-ple layers of timber. With packers and spliceplates some of the joints were some five lay-ers thick, which represented something like250 of timber. The nuts were all retightenedand while the author was in occupation theywere checked regularly but never came slackagain. What was holding the trusses together?Only the Bulldog connectors’ teeth embeddedinto the timbers.

The trussed rafter

The point of including the Belfast truss inFigure 6.10 was to show that the main tim-bers of the truss were joined together and the

1 A node is the point on any frame where timbers, bars,lines, etc. meet and join.

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PurlinrestPurlin

rest

Ridge

Wall plate

A

B

C

D

Joint A

Ridge slot

Joint B

Purlin

Joint CRaftersTies

Spacer between rafter and ties

Bulldog connector

12.5 hex bolt, 50 square washers both sides and hex nut

Ties

Purlin strutLong strut

Joint D

Fig. 6.11 TRADA trusses.

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Roof Structure 155

truss stiffened with thin boarding nailed eachside – very effective. So it is not a surprise tolearn that the idea of putting wooden ‘plates’on each side of a node is a good, cheap way tomake up trusses. With cheap plywood avail-able, this can also be used and often with muchless waste; certainly the plywood can be a lotthinner and so reduces the bulk at the nodepoints. Pattern nailing is required – thin nailsand lots of them and long enough to pene-trate almost through to the opposite side of thetimbers. Of course we are only talking aboutsingle layers of timber, not the multiple lay-ers used in the TRADA trusses. The processesare simple but labour intensive, and before theadvent of stress graded timber and the vari-ability of the workmanship at each node, therewas no way of making any great saving in tim-ber. Everything had to be overdesigned to givean excessive margin of safety.

This changed rapidly with the advent ofboth stress grading and the invention of thegang nailing plate. Stress grading allowed en-gineers to treat timber in much the same wayas metals had been treated; the performanceof timber could now be predicted with someaccuracy. This meant that smaller sections oftimber could be used. The gang nail plate asa method of jointing had a number of advan-tages over nails and bolts:

� Only two pieces of metal are involved in anyone joint

� There are no holes to drill� There is nothing to tighten up� There is no manual labour – other than plac-

ing components – involved in making thejoints

� The results of using the plates are highlypredictable

� There are no notches etc. to cut on timbers –only square or splayed cuts

� Timber is cut in a jig and assembled in a jig� There are no glues, heat or other compo-

nents involved.

Figure 6.12 is a sketch of a single gang nailplate and Figure 6.13 is a photograph of a nodeon a roof truss. The plates are pressed in eachside simultaneously using a hydraulic press.The result is that the little spikes on the plate

A plate of steel is punched with a

pattern and then thepointed pieces are

pushed through untilthey stand at right

angles to the generalsurface

The plate now lookssomething like this and is

hot dipped galvanised

Fig. 6.12 Gang nail plate schematic.

all penetrate at the same time, under an evenpressure. Different sizes of plate are availableto suit the conditions which will prevail at anynode in a structure.

The manufacturers of the plates provide acomplete service to the industry. They don’tmake trusses themselves but they supply allthe machinery necessary:

� The jigs in which to cut as well as set out thetimbers and the joining plates

� The hydraulic presses� Computer software to design the trusses� The plates.

Fig. 6.13 Gang nail plate.

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Fig. 6.14 Trussed rafter roof.

What is very different about these trusses isthe fact that they don’t support purlins whichsupport common rafters etc. These trussessupport a section of roof which extendshalfway to the next truss either side. So weare putting up a whole series of lightweighttrusses and the roof covering is laid immedi-ately on these.

Figure 6.14 is a photograph of some newlyerected trussed rafters on a two-storey house.They are spaced out at 600 centres and areset on and fixed to a wall plate bedded andfastened down to the inner leaf at the wallhead. Figure 6.15 shows the wall plate bed-ded down and two of the straps which hold itdown can be seen on top of the plate. In Fig-ure 6.14 the ends of these straps can be seen

Fig. 6.15 Wall plate secured to wall head.

Fig. 6.16 Trussed rafters secured to wall plate.

running down the wall at the far side. Theyare masonry-nailed to the blockwork.

Figure 6.16 shows the detail at the eaves ofthe roof. The ends of the trussed rafters sit onthe wall plate and a fastener can be seen nailedto the wall plate and to the trussed rafter – ev-ery trussed rafter. The long board fixed to theends of the trussed rafters is the eaves fasciaboard. It is simply nailed to the ends of theprojecting rafters and finishes off that verti-cal face. Supports for the guttering are fixedto it. Figure 6.17 is a photograph taken downthrough the centre of the trussed rafters. No-tice that the ties of the trussed rafters are intwo pieces and are joined with a gang nailplate.

With these four photographs we have cov-ered a lot of construction and probably raiseda number of questions in the reader’s mind.A little further down we will attempt to an-swer what some of these questions might be.Before that, there is one important thing toknow about the trussed rafters. They can bemade up in a variety of shapes using varyingsizes of timber according to the loads beingimposed.

The shape and layout of the trussed raftersin the photographs is known as a Fink trussand is easily recognised because of the dis-tinctive W shape of the internal strutting.

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Roof Structure 157

Fig. 6.17 View down centre of trussed rafters.

Figure 6.18 illustrates the Fink layout and afew other common arrangements. More areshown in Figure 6.19, including the mansardtruss and the attic truss. The term panel inrelation to these truss shapes refers to the un-supported length of rafter in each. The first,Fink truss is of four panels – each rafter is sup-ported by the struts at the centre of its length,dividing each rafter into two, therefore fourpanels.

A book, which is really meant for the timberengineer but which is well worth referring to,is the Timber Designers’ Manual by Ozelton andBaird, now in its third edition and publishedby Blackwell Publishing. There is a wealth ofinformation, over and above the design calcu-lations and methodology, which is not avail-able from other sources.

Once a roof structure has been started withthe erection of the trussed rafters, there is stillthe need to tie masonry walls to it at both thetie level and the rafter level. This is done inexactly the same way as floors are tied to ma-sonry walls, but nowhere in my travels haveI been able to find a correctly carried out ex-ample to photograph. Figures 6.20 and 6.21show badly carried out examples. Figure 6.20

Four panel Fink truss

Six panel fan truss

Two panel monopitch truss

Three panel monopitchtruss

Four panel Pratt truss

Fig. 6.18 Various truss shapes (1).

Four panel Howe truss

Six panel Howe truss

Mansard truss

Attic truss

Fig. 6.19 Various truss shapes (2).

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Fig. 6.20 Wall to roof stability (1).

shows the correct style of metal strap but theblockwork has been hacked down to accom-modate it, and it is nailed to a thin strap ofwood laid and nailed over the top of the ties.The hole hacked in the blockwork breachesthe integrity of a mutual wall, which is nowno longer up to fire resistant standard. Thenail hold for the strap into the thin timbersmust be questioned. There are no noggings be-tween the ties and no solid packing or wedgesbetween the last tie and the wall.

Figure 6.21 shows a correct style of strap,this time fixed to a fairly heavy horizontalbearer which in turn is fixed along the faceof the struts of the trussed rafters just underthe rafters. So far so good, but there is no wallin sight; it is obviously a case of get the tie

Fig. 6.21 Wall to roof stability (2).

in now and we’ll build the wall to it later.That would be all right, but on this particularsite there must be grave doubt that the ma-sonry even came close. The scope of the pho-tograph in Figure 6.21 has been deliberatelykept fairly wide to show up one or two otherpoints. At the very bottom left it is possible tosee the diagonally opposite corner of this roofwith another strap projecting out waiting formasonry. Also, note how small the gang nailplates are at the top of the row of struts fromthe centre and running down to the right ofthe photograph. These struts are in compres-sion and so only need restraining in positionwhile being erected etc.

The first part of the prefabrication of the roofwas done when the trussed rafters were made.They have been erected now, and fixed downsecurely to a wall plate which in turn is fixed tothe masonry wall below it. In Figure 6.21 hor-izontal and diagonal bracing can be seen andwe have already criticised the arrangementsfor wall restraint.

The next part of the prefabrication is donefor the formation of the verges. A verge ladderis made, so called from its appearance not be-cause it is used for climbing. Each verge hasa ladder and Figure 6.22 is a good example.This is on a timber frame house but the restof the roof is not made from trussed rafters; ituses three trussed purlins.

Figure 6.23 has drawings of a verge ladderand how it is fitted. Note that the fully prefab-ricated version has the finish to the edge andto the underside. These panels are frequently

Fig. 6.22 Roof ladders.

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Roof Structure 159

Timber frame gablehaffit

Timber frame panelcladding and breather

Rafter

Roof ladder section

Masonry cladding built up to soffit plate on ladder

Ladder nailed hereand here

Soffit plate

Vergeboard

Outrigger

Render on blockwork

Roof ladder ends splayed to match pitch of roof

Fig. 6.23 Roof ladder details.

painted up to undercoat stage and many aremade with PVC verge boards and soffit plates.The sketches explain the terminology. In tra-ditionally constructed stick built roofs thesetimbers – and some more – were all cut andfitted individually from the scaffold.

The verge ladder has four components:

(1) One side of the ladder is the verge board.(2) The other side of the ladder is a simple

timber plate but more often a length of 9or 12 plywood.

(3) Between (1) and (2) are nailed a numberof short lengths of timber, the outriggers.

(4) To finish the underside of the projection ofthe verges there is a soffit plate which islet into a groove on the back of the vergeboard and nailed to the outriggers.

To erect the verge ladder is simple. Hoist it intoplace and nail the plywood ‘side’ of the ladderto the last trussed rafter at the gable end. Sidenail the outriggers to the runner in the top ofthe timber frame panel. Job done. The endsof the ladder are cut to a splay which matchesthe pitch of the roof so that there will be aneasy join to the eaves finish, which is put onnext.

The eaves finish can be prefabricated aswell. First of all the ends of the rafters of thetrussed rafters must be cut to the correct angleand length. The ends must be in a straight line.The easiest way to achieve this is to stretch achalk line across the top of the rafter projec-tions the correct distance from the wall face,pull the line tight and lift and release – snapthe line – against the rafter ends. A chalk line

Tie

Rafter Fascia plate

Soffitplate

Masonrywall

Render onblockwork

Timber frame wall panel

Bracket

Fig. 6.24 Eaves detail.

is made on a rafter – which will be in a straightline with all the others. Cut at the correct an-gle and the work is ready to receive the eavesboxing. Figure 6.24 shows one way of doingthis.

Once the rafters have all been cut, the fas-cia and soffit plates can be put in place. Pre-fabricated, they come with the soffit plateglued into the groove on the fascia and with afew rectangular off-cuts of timber pinned andglued inside to keep the soffit and fascia atright angles. The whole unit is offered up tothe rafter ends and nailed into the ends of therafters. A more traditional approach would beto fix the brackets to the rafters, fix the fasciain place on the rafter ends, then put the soffitin the groove and nail it to the brackets.

Verges meet eaves

The tricky bit for someone trying to draw thedetail is what happens where the verge detailmeets the eaves detail?

Figure 6.25 shows a solution which has longbeen used on traditional roofs. In it the vergesoffit is left short of the join between the vergeboard and the fascia plate. The verge board isjoined to the back of the fascia plate. The eavessoffit is cut to match the width of the vergesoffit, and the eke piece, a triangular piece oftimber matching the verge board, is grooved

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Fascia plateand

soffit plateextend to

meet vergeboard

Masonrywall

Render onblockwork

Outline of timber frame wall corner

and gable

Outline of bottom of verge board

Section down roof ladder

Verge/fasciajoint

Verge soffit plate

Eaves soffit plateMasonry quoin

Ekepiece

Short lengthof soffit plate

material

Fish plate joiningeke piece toverge board

Fig. 6.25 Eaves and verge junction.

on one edge to support the eaves soffit. Theeke piece is also grooved on the adjacent edgeto take a short length of soffit plate materialwith a width matching the verge soffit. It ismade a tight fit between the verge soffit andthe eaves soffit and is wedged in the otherdirection between the eke piece groove andthe masonry. To hold the eke piece in place, aplate of plywood is screwed to the back andallowed to project. This allows the joiner tosecure the eke piece to the verge board.

Roof bracing

The roof structure is not complete without be-ing properly braced, and gable walls have tobe attached to the roof structure. The latterwas dealt with when we looked at some badlyexecuted strapping work next to blockworkgables in Figures 6.20 and 6.21 and also inChapter 4, the section ‘Support of masonry

walls’, where similar principles apply. Prop-erly done, these would be more than ade-quate. What we have not dealt with is howto connect timber frame panel gables to theroof structure. Figure 6.21 gives a clue. Whilethe photograph was taken on a site with allmasonry walls, note that there is a long con-tinuous timber fixed to the top of the trussedrafter struts and it has a galvanised strap fixedto it. That timber is part of the roof bracing andin that figure was showing that it could alsobe used as an anchor point for the tie-in to thegables. That is what happens in timber frameconstruction. The brace extends into the depthof the panel and is secured either directly tothe studs of the panel or to a dwang or noggingsecured between the pair of nearest studs.

The author has seen many direct connec-tions and few if any have overcome the minorproblem of the different angles of the braceand the studs, the latter vertical and the for-mer tilted over with the struts in the trussedrafter. What usually happens is the carpentertakes an axe to the end of the brace and chops itroughly into line with the stud, and then ham-mers a large nail or two through the thinnedend – which splits! If noggings are fitted theyare done badly and not properly secured tothe studs.

Figure 6.26 shows a method of dealing withboth situations which might take a little longerto execute but would improve the final resultbeyond all measure.

The other braces within a trussed rafter roofare:

� Diagonal braces across the underside of allthe rafters in each roof plane

� Horizontal braces on the outside, both sidesof the peaks of the trusses

� Horizontal brace along the upper edge ofthe ties.

Figure 6.27 illustrates these.The last part of the structure is whether

or not the rafters are covered with board-ing of timbers or sheet material such as ply-wood, OSB or some other fibre-based board.Tradition varies across the UK. With the oldbituminous felts as underlays below the slates

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Roof Structure 161

StudStud

Stud

Stud

Horizontalbrace

Horizontalbrace

Blocking nailedto stud

Dwang ornogging

Blockings from one piece of

timber nailed to nogging and

Line ofnailing

Fig. 6.26 Horizontal brace to gable panel detail.

or tiles and the manner of fixing slates andtiles regionally, there were arguments for andagainst boarding. With the advent of alterna-tives to the bituminous felts, it is largely amatter of how the slates or tiles are fixed andwhether the boarding plays some structuralfunction.

The traditional boarding used is a sawn andnow treated, plain edged board 150 × 12.5,150 × 15 or 150 × 20. This boarding is calledsarking. It is nailed to each rafter with two65 galvanised nails, pressing the edges closetogether as nailing is done. If there are to bebattens fixed to the roof, then 12.5 or 15 thickboards are adequate depending on the spanfrom rafter to rafter. If there are slates to befixed direct to the sarking, at least 20 thickboards should be used.

Where no sarking is to be fixed, then anunderlay which can be stretched across therafters without tearing must be used. The

Positions of the horizontalbracings

Plan view of roof showing alternativediagonal bracing positions

Rid

ge b

raci

ng

Diagon

al br

acing

Diagonal bracing

Diagonal bracing

Trussed rafters

Wall outline

3

22

1

1

1

1

Fig. 6.27 Roof bracings schematic.

old bituminous felts had a reinforced variety,the reinforcing being a mesh of hessian yarn.It was sometimes known as sarking felt.These felts, while being impervious to solidwater, did not allow air to pass through andthis is an important feature of modern roofconstruction. Roof ventilation will be coveredin Chapter 7, Roof Coverings. The modernequivalent of the old felts is generally a plasticsheet; one is made that has micro-perforationsat about 50 centres each way. This is marketedunder the brand name Tilene by VisqueenBuilding Products. Figure 6.28 shows how itworks and how it is impervious to solid waterand yet allows air to pass each way. Others aremade from felted polypropylene fibres, suchas Tyvek. The felted polypropylene fibre cov-erings are impervious to solid water but arebreathable.

Many of the roofs examined recently areboarded with 12 sheathing plywood, then a

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50

50

Water dropletHole raised above

general level

Plastic sheet is perforated with a needle which firstraises a blister and then pushes through. This

work hardens the plastic which retains the volcanoshape. It is this as well as the extremely small

hole which repels solid water

Fig. 6.28 Breathable plastic underslating felts.

breathable membrane such as Tilene is laidand battens are laid to which the tiles are fixed.Slated roofs are boarded with timber sark-ing or thicker sheathing plywood and coveredwith one of the polypropylene fibre felts. Theslating in the author’s home area is direct ontothe sarking boards.

Natural timber sarking imbued a certainamount of bracing or stiffening of the roofstructure to which it was fixed. This is alsotrue of the sheet materials used in its placeand these now become a necessary part of thestructure in tightly designed roofs in timberframe construction.

Our discussion so far has followed a pro-gression which has ended in the concentra-tion on trussed rafter roofs as if they were theonly thing built today. While this is not thecase, trussed rafters account for the majorityof roofs erected now.

Flat roofs in timber

Flat roofs deserve some attention for, althoughin a minority in new work, they are still pop-ular for extensions.

There are few of these roofs which are trulyflat, i.e. the surface is horizontal. The majorityhave a fall or falls to take the rainwater offto one side or to a collection point. We willdiscuss a truly horizontal roof a little later.

What do we have to do to introduce a fall(s)in a flat roof? Before we start, the timberswhich provide the main structural support areproperly termed roof joists although we willend up calling them simply joists – in the cur-rent context this is fine.

There are various ways of giving a fall to aflat roof built in timber:

(1) The roof joists which support it are laid toa slope, which in itself can be done in twoways:(a) the joists slope end for end(b) the joists are horizontal but each is set

at a different level to the adjacent joists(2) The joists are laid horizontally and addi-

tional taper cut timbers are placed:(a) on top of them to introduce a slope –

firring pieces(b) across them to introduce a slope –

declivity pieces(3) The upper surface of the joists is cut to a

slope(4) The structure is built horizontally but the

insulation applied to the bearing surfacevaries in thickness to give a fall(s).

Figure 6.29 shows how these techniques canbe used for roofs with a single slope to oneedge of the roof. Only an indication is shown –these are not detail drawings.

When a roof requires more than one slope –it is laid to falls – then the joists are usu-ally laid horizontally and the added timbersare arranged to give the multiple slopes re-quired. Modern requirements for insulationof these roofs has brought in the idea thatthe roof shown in (4) of Figure 6.29 is a goodway to achieve multiple slopes. The insula-tion is designed using a CAD2 system and the

2 A CAD (computer aided design) system will allow thedesigner to prepare a plan of the roof with all the keydimensions (sometimes referred to as CAAD – com-puter aided architectural design.) This can be input toanother dedicated design system which calculates theshape of a whole series of blocks of insulation requiredto cover the roof and which will give multiple falls onthe roof. Each block is cut by a machine which takes itsdata from the dedicated design system. Each block isnumbered and a drawing is prepared which shows thelaying sequence.

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Roof Structure 163

1a Joists slope end

for end

1b Joists are horizontal but set atdifferent levels. Note how tops of the

joists are cut to the roof slope

2a Joists set horizontally with tapered cut timber on top called a firring piece

2b Joists set horizontally but taper cuttimbers set at right angles now known as

declivity pieces. Note thin end still hassome thickness to take load from deck

3 The upper edge

of the joist is cut to a slope

4 Everything built horizontally andinsulation layer over deck is cut to taper

Deck

Fig. 6.29 Flat roof – options for structure.

insulation is cut using numerically controlledmachinery.

At the start of the chapter we defined a flatroof as one with a slope not exceeding 10◦

and that slope for flat roofs was usually givenas the fall in millimetres per 1000 millimetres

Table 6.1 Roof angles and rise in horizontal distancein millimetres.

Angle in Roof slopewhole degrees Natural tangent in mm/1000

1 0.01746 172 0.03492 353 0.05241 524 0.06993 705 0.08749 876 0.10510 1057 0.12278 1238 0.14054 1419 0.15838 158

10 0.17633 176

horizontally. That ratio of vertical height tohorizontal distance is better known as the nat-ural tangent of the included angle, i.e. be-tween the slope and the horizontal. The natu-ral tangents for the whole degree angles from1 to 10 are listed in Table 6.1, together with theflat roof slope to give the reader some instantappreciation of what these slopes are reallylike.

There are a lot of different ideas about howa flat roof might be constructed to provide afall(s). The illustrations in Figure 6.29 showa deck, and in an all-timber roof this has tobe of timber as well. Most traditional mate-rial was a tongued and grooved boarding –usually flooring boards simply nailed to theroof joists. Depending on the centres of thejoists, 20–22 thick boarding was usually ade-quate. On top of this deck a waterproof layerwas built up by first nailing roofing felt to thetimber and then sticking further layers ontothe first with hot bitumen. These old felt roofswhen laid by experts had a surprisingly longlife but only if the pitch was kept as high aspossible. Not flat roofs, but the author knowsof several roofs laid in the 1960s which lastedwell over 20 years with only minor repairs. Infact the company that laid them gave a 20 yearwarranty on their work. That is not done to-day despite the ‘advances’ made in bitumi-nous felt technology – polymerised bitumensand all the rest.

The old roofs were not insulated, and all thatwas between the occupier and the great British

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climate was the felt on the deck, then a gapwhere the joists ran, and a simple ceiling. Asheating fuel rose in cost, insulation was givensome thought – although not a lot, for manyof us spent hours sorting out the problemscaused by the insertion of insulation in newand existing roofs. The problem had a lot todo with the fact that putting a layer of insula-tion into a structure causes one side to becomewarmer and humid and the opposite side tobe colder, and if the moisture gets through theinsulation it also becomes wet because of con-densation. High humidity levels cause rot andincreased insect attack in timber, so flat roofswere rotting down all over the place.

Insulation, vapour control layers andvoids and ventilation

Figure 6.30 shows a series of flat roof sectionswith insulation, a deck and waterproof layerand a ceiling. They are in different positions

Layer ALayer B

Layer CLayer D

Layer E

Layer C

Layer D

Layer A

Layer B

Layer FLayer G

Bituminous felt

Deck

Ventilation by design

Layer ALayer B

Layer CLayer D

Situation A Situation B

Situation C

Situation D Situation E

Polystyrene tiles

Ventilationby accident,not design

Fig. 6.30 Flat roof – options for insulation and venti-lation.

relative to each other and the provision of aVCL may cure or kill the roof.

Taking each in turn, situation A shows whatwas the traditional approach to the flat roof.Any water vapour-laden air passing into thevoid between ceiling and deck had somechance of escaping along the joist void andout at the eaves or head of the roof, such wasthe looseness of the build. Ventilation was ac-cidental, not planned and allowed for. Never-theless, in times of cold weather and increasedheating activity below the ceiling, a lot of thewater vapour would condense out on the un-derside of the deck – Layer B. Layer C alsobecame wet due to solid water dripping offLayer B. Moisture would pass through thedeck but could not pass through the water-proof layer and so condensed within the deckstructure or material at Layer A. With the co-pious amounts of heat being pushed throughthe ceiling, the ceiling itself would remain dryuntil the heating was turned off and it cooledrapidly, leading to condensation at Layers Cand D. Being in kitchen and bathroom exten-sions, there was plenty of moisture around.

Situation B was the next step. Insulationwas stuffed into the joist void by taking downthe ceiling, insulating and replacing it. An al-ternative is shown at situation C. Here thekeen DIY man has covered the entire ceil-ing with polystyrene tiles, only 10 or at themost 12.5 thick so as insulation almost in-effective and a waste of time. The adhesiveused was a strong wallpaper paste and thetiles were of polystyrene beadboard. Neitherwould present any real resistance to the pas-sage of water vapour. In both these situationsthere was occasionally some attempt to in-crease ventilation flow rates down the joistvoid. Unfortunately many people includingtradesmen did not realise that each joist voidwas a sealed cell on its own and required in-dividual ventilation. The effect was to furtherincrease the temperature gradient across theroof, so not only was there condensation atlayers A and B but also on top of the insulationat layer E. If the latter became serious enoughthe water could get back down to layer C andthe insulation could be rendered ineffective –except as a sponge!

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The polystyrene tile solution was no real so-lution; besides, there was a hazard using suchtiles as, decorated or not, they presented a realfire hazard. The author spent some time on‘fire report’ duty with a company that owneda number of houses. Monday morning wasthe busy day, with drunks setting fire to theirkitchens with an unattended chip pan, andin the 1960s the whole thing was exacerbatedby the use of polystyrene tiles on the ceil-ings, which melted, dripped and caught fire,so spreading the fire across the room. SituationB could have been salvaged if the tradesmenDIY experts had only realised that stoppingthe water vapour getting into the joist voidwas as important as putting in the insulation.The key element missing was the vapour con-trol layer (VCL), a simple layer of polythenefixed to the underside of the joists before theplasterboard went on.

This layer could not be 100% vapourproof;there were too many things to go wrong forthat. First, there were a few hundred staplesor tacks holding the polythene in place beforethe plasterboard went up. Then lighting ca-bles had to come through, perhaps a pipe ortwo, a ventilation stack for plumbing or an ex-tract fan, and finally a few hundred nails heldup the plasterboard. Many experts argue thatdriven nails and staples present no problemas the plastic stretches round the nail shankand seals to it. Certainly the cables, pipes andducts do present a problem and there are sim-ple ways to secure the plastic to these itemswith gaffer tape etc., which should reduce theflow. But, at the end of the day some watervapour is going to get through and we don’twant to have it condensing above the insula-tion layer at Layers A and B.

This is where the ventilation comes intoplay. There has to be sufficient ventilation tohave a flow of air which can carry any wa-ter vapour away – ideally before it condenseson Layer B, but that is not always a possibil-ity. The flow should ensure that Layer B willdry out rapidly and so prevent a build up andcondensation at Layer A. The felt layer(s) areleft exposed to the weather and to ultra-violetradiation. UV does break bitumen down, andthe improved bitumens now used have gone

some way to improving the resistance to de-cay caused by UV. The weather also has itseffect on the felts. The constant wetting anddrying and heating and cooling cause the feltto expand and contract far beyond the scopeof the material to continue to move for itsexpected life span. The result is blisters andcracks and splits in the layers which have to bepatched, and there are more thicknesses oversome of the roof giving rise to more inconsis-tent layering which makes the whole situationworse.

One novel approach which met with greatacclaim for a short period was to make the wa-terproof layer absolutely horizontal and puthigh upstands round all the edge of the roof.Rainwater outlets were arranged at high levelin these upstands, or rainwater disposal pipeswere brought through the felt and terminatedhigh above it. The result was that the roofwould be permanently under a volume of wa-ter and a depth of about 300 was supposed tobe ideal. If there was no rain and the pond-ing was evaporating away, the owners weresupposed to fill it up with a hose! The wa-ter protected the felt from UV radiation – afew millimetres are enough. The mass of waterwould also help to regulate the temperatureof the felt layers and keep it from fluctuatingwildly. Problems solved!

The snags came immediately the roof wasconstructed – it was very expensive becausethe structure had to be super strong to holdup about 300 litres/m2 of water and its ownweight. Then there was the workmanship as-pect. Some of the roofs leaked from day one.Others started to disintegrate at the one pointthat wasn’t protected by the water – the up-stand above the water run-off level. The wa-ter certainly kept the main felt cool and UVfree, but the little strip round the edge wasvery vulnerable. The cost of pumping out theroof – no-one had thought about building in adrain down facility – on top of the repair costat the upstand without any guarantees, putpaid to the idea. It was to be at least 20 yearsbefore a better method of protecting the feltwas found.

Situation B is commonly referred to as a coldroof construction – the insulation is at ceiling

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finish level. Situations D and E are warm roofconstructions and were seen as the answer toa lot of problems, including one or two left bysituation B and the ponding disaster.

Situation D came first and simply moved theinsulation to the top of the deck. This meantthat the deck stayed warm and so there wasno condensation at this layer. The insulationused was either extruded polystyrene with aplywood or cork layer bonded on top, or apolyurethane layer with aluminium foil oneside and kraft paper the other. The idea of theplywood or cork bonded to the polystyrenewas to protect the polystyrene from the heatused to lay the felt. Polystyrene melts veryeasily at a low temperature. The extruded ver-sion has a very high resistance to the passageof water vapour. Layer F in that situation isa cold bituminous emulsion – a water-basedmixture of bitumens – which is used to stickthe polystyrene down. Solvent-based adhe-sives would dissolve the polystyrene. By nowthe reader will be wondering why we botherto use polystyrene. Well, the problems withpolyurethane can be just as bad: incompati-bility with adhesives, not so bad with heat-ing but a terribly friable material even whenplaced between paper and foil layers. There isalso a cost difference in favour of polystyrene,as well as its very much lower vapour trans-mission rate. Polyurethane must have a foilface.

So polystyrene it is. And on top of the ply-wood or cork, a layer(s) of felt can be bondedin hot bitumen. The bitumens were still im-proving but were being challenged by singleskin materials of synthetic rubber and someplastics. But no matter what the covering, itwas still exposed to weather and UV. It wasnow that the idea of forming falls in the insu-lation was introduced.

Then along came situation E – a simple ideabut instead of using bituminous emulsion tostick down the polystyrene, use the whole wa-terproof layer, bed the polystyrene into the bi-tumens – cork or plywood down – and thefelt layers are kept at a constant temperatureunder the insulation, and the UV radiation canonly get at the insulation. High winds might

be a problem – foamed plastics can be liter-ally shredded by high winds. The slightestimperfection in the surface is enough for thewind to get a grip, and soon there is a biggerhole and then a bigger hole and soon lumpsof plastic are scudding through the sky likeFrostie the snowman on speed! The answerwas again simple: weigh it down. Gravel wastried but on high level roofs could be blown offto shower people below with pebbles. There isno point in using a layer of concrete as it pre-vents inspection and maintenance of the in-sulation layer and ultimately the felt. The an-swer was to simply to make the stones biggerand/or to couple their use with laying pavingslabs for access by maintenance workers andin areas with higher exposure ratings.

In both situations D and E, the bitumen layerunder the insulation acts as a primary vapourcontrol layer. The use of extruded polystyreneacts as a secondary VCL.

How these roofs are faring now the authorhas no idea, but he would like to hear of any-one’s experience with them.

To return to the earlier mention of ventila-tion – a vexing question for the energy conser-vationist. Who needs a cold draught blowingdown the joist voids between the ceiling andthe insulation? Some conservationists haveadvocated ventilating these voids to the build-ing itself, even the room covered by the roof,but if it were a bathroom or kitchen that wouldhardly be suitable. They cannot be ventilatedinto other voids such as wall cavities and soon because of the risk of fire spreading. So itwould appear there is no alternative to pro-viding holes or slots in the roof edge to al-low air to move down these voids. But whatis not wanted is a howling gale which willcarry away all that expensive heat.

An old-fashioned Victorian idea comes tomind which is still found in extant buildingsof that vintage, and most of them schoolroomsor lecture halls etc. – the Tobin tube. This isa hole part way into a wall with a ventilat-ing grid on the outside; the hole turns up inthe wall for about 1200 and then turns intothe building with another adjustable grille setabout 1400–1500 above floor level. Sometimes

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a flap valve is fitted near the bottom of thetube, with linkage to a lever on the grille in-side the building. The effect of this device wasto induce a flow of air without the flow gettingout of hand even on quite windy days. It can-not be beyond the technology of our presentcivilisation to put this idea into practice fittedwith some automatic valve operator such asone finds on greenhouse windows and so on.

Traditional roofs

The chapter would not be complete with-out a brief look at the stick built traditionalroof. This is best illustrated with a few pho-tographs, some of them very recent – the onesof the pole plate roof are only three years oldand yet the technique was first shown to theauthor some 45 years ago and was very oldthen.

Figures 6.31, 6.32, 6.33 and 6.34 are of a fairlyold roof (1961). It had a 54◦ pitch – very steep –and three rooms in the attic space formed. Theceiling ties were laid in and supported partlyon brick partitions and partly on an RSJ as theyformed the attic rooms’ floor. A timber about35 square was let into a groove along the pointwhere the rafters would meet the ends of theties. The ends of the rafters were splayed off atthe correct angle and lengths, and a notch wascut in one end to go over the timber let into theties. Then the rafters were erected in pairs atintervals along the roof with the ridge board.

Fig. 6.31 Stick built roof (1).

Fig. 6.32 Roof with traditional sarking boards.

Collars were put in to support a ceiling in theattic rooms and hangers were finally added tobreak the span of the lower part of the rafters.They bore down on a plate nailed to the topof the ties. The ties were simply nailed to thewall plate, which was only bedded in mortaron top of the brick walls.

Temporary bracing was fixed inside therafters, and sarking boards were nailed to theoverall roof surface which was finally cov-ered with underslating felt and concrete in-terlocking tiles. The photograph of the roofwith the sarking shows some boards missing.These were put in place as the boarding wenton but they were not nailed into place. Oncetwo more boards had been nailed above themthese boards were taken out and nailed tem-porarily to the boards below. This gap gavea foothold for the carpenters to step up andcomplete more of the boarding. Once the last


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board at the ridge had been fixed in position,the carpenters came down one step and fixedthe previous boards into the step just left. Andso on down the roof.

These steps you see in the photograph wereleft out so that the plumbers could go up andfix the lead round the dormer window. Theyhad been used earlier by the carpenters whowere making the dormer window frameworkand cladding. With the exception of the ties,the roof timbers were all 100 × 50 sawn, andtimber was selected from a slight overorder inquantity in order to get straight grained tim-bers with not too many knots for all criticaltimbers.

Figure 6.35 is not about the roof so much.It is, after all, a trussed rafter roof, which hasbeen shown earlier. It is more about the wallwhich sticks out at you in the centre of the

Fig. 6.35 Corbel at eaves verge junction.

Fig. 6.36 Pole plate (1).

picture. This is the gable wall which runs awayto the left of the picture. Notice how the de-signer has used a corbel ‘stone’ – a concreteprojecting block – to carry the wall out be-yond the front wall of the building and thusfacilitate the joint of the verges to the eaves. Asketch of how this detail might have looked isgiven in Figure 6.38.

Figures 6.36 and 6.37 show the rafter feet totie junction on a pole plate roof. The problemwith rafters is that they tend to spread theirfeet out when under the load of tiles and windand snow and so forth. Many roofs have therafters joined to the ties by making a halvingon the rafters and nailing this to the side of thetie. The joint is entirely reliant on the nailing.In a pole plate roof, the pole plate itself acts as

Fig. 6.37 Pole plate (2).

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Roof Structure 169

Outline of gable wall

Fasciaboard

Eavessoffitboard

Corbel stone in gable wall

Rafte

r

Tie

Sprocket p

iece

Rafter

Rafter

TieTie

Wall plate Wall plate

Pole plate

Pole plate

Fig. 6.38 Roof details – corbel and pole plates.

a key between the rafter feet and the ends ofthe tie and prevents the rafter feet sliding off.

Figure 6.38 includes a detail of how the crit-ical junctions on both these pole plate roofswere done. Also included is how the sprocketpiece was used to finish and support the eavesconstruction on the 1960s house.

Roof insulation

Insulation is even more important in roofsthan ever before. Now that we have a bet-ter understanding of the inter-relationship be-tween the need to insulate and the control ofvapour passing through a structure, the possi-bilities for condensation and the need for ven-tilation, it is possible to design warm or coldroofs, whether flat or pitched, to suit everycondition we could hope to meet in practice.One or two points merit mentioning here.

Ventilation of roofs is vital. While masonrywalls can be built as breathable structures andthere is now serious doubt about the need forvapour control layers (VCLs) in them, this is

not the case in a roof where the structure ispredominantly of wood and wood products.The ideal must be to keep the wood prod-ucts on the warm side of the insulation andto keep them protected from moisture com-ing through the ceiling finish by placement ofa VCL immediately behind the ceiling finish.There has to be a void to carry ventilating airin and moisture laden air out, and whether theroof is a cold roof or a warm roof will deter-mine where that void will be. Looking backat Figure 6.30, situation B is a cold roof so thevoid is to the outside of the insulating layer.Situations D and E show warm roofs and thevoid is to the inside of the insulating layer. Nomatter what else happens in the constructionof these roofs, the Regulations require a mini-mum 50 deep gap between the insulation andthe deck or ceiling.

Pitched roofs can also be built as warmor cold roofs. Figure 6.39 shows schematicsfor both warm and cold roofs. Cold roof A

Cold roof A

Warm roof

Cold roof B50 gap here

Fig. 6.39 Warm and cold pitched roofs.

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Counter battens

Underslating membrane

Insulating extrusion Rafters

Rafter

Insulating extrusion

Cuts

Fig. 6.40 Preformed plastic foam insulation for awarm roof.

requires ventilation at or just above the eaveslevel, and at the ridge. At eaves level it is usualto perforate or form a continuous slot in theeaves soffit and ensure that there is open pas-sage for air over the top of the wall into theroof space. At the ridge, special tiles or roofventilators can be fitted. The warm roof hasinsulation under the sarking and if not used

for forming rooms there is a need for someventilation, with control over the amount butnot the means to cut it off completely. Thecold roof B is common in speculative housing,and the insulation of the framing forming theroom in the roof space is often so spectacu-larly badly done that it is a waste of moneyputting it in. Note that there has to be a min-imum 50 gap at the slope on the attic ceiling.The slope is called a coomb. Ventilation is re-quired top and bottom of the roof.

Figure 6.40 shows a detail for a warm roofusing a purpose-made foamed plastic extru-sion. This product has a rebate on each edgewhich fits over the rafter. The slots either sideof the extrusion run the complete length. Asthe extrusion is forced between the raftersthese slots close, ensuring that the plastic isa tight fit between the timbers. An underslat-ing membrane is placed over this and counterbattens nailed through the foamed plastic intothe rafters.

The plastic used is the ubiquitous poly-styrene, this time in bead board form but afairly dense grade. Tiling or slating can be car-ried out over the counter battens just as in aconventional roof.

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7 Roof Coverings

Tile and slate materials 171Slates 175Plain tiles 175Interlocking tiles 176Timber shingles 176

Bituminous shingles 176Pan tiles 177Spanish and Roman tiles 177Edges and abutments 178

In the previous chapter on roof structure, wereached a stage in construction where therewas a waterproof layer over the structure –the underslating1 felt or membrane. This is nota long-term solution to keeping the buildingwatertight but is frequently used short-termto allow internal work to proceed while out-door work continues, dependent on weatherconditions, at the same time. This makes gooduse of that scarce commodity, time. Eventu-ally the underslating membrane will have tobe covered over with something a little morepermanent, and that is what this chapter isabout – a short discussion on the materialswhich can be used, how to apply the morecommon types and how they are finished atedges, junctions and changes in direction. Asa finale, complete details of basic roof edgeswill be given. We will not consider metal orplastics roof coverings in this text except forthe necessary leadwork at abutments.

1 Some in the industry always refer to this layer as under-tiling felt or membrane (even if it isn’t felt); others al-ways refer to it as an underslating felt or membrane.One could be pedantic and always apply the correctterm in relation to the final materials used. Is it a bitu-minous felt or a plastic membrane? Is it slate or tileswhich are being laid? It makes little difference to whatis actually used. We will use underslating membraneirrespective.

Tile and slate materials

Mock-ups are used extensively in this chap-ter, with cardboard slate and tiles, to aroundone-fifth scale. They may not be as life-like butthey have the benefit of using less manual ef-fort to set up and allow annotation to be addedwith a marker pen. We hope they are useful.

Figure 7.1 is a photograph of several ter-raced houses with roofs at different levels.Note how the roofs are covered with a mem-brane, and that membrane is held down bytimber strapping running up the roof over therafters. These are the counterbattens. Thereare also at least two horizontal straps, againto help hold the membrane down. A roof lightcan be seen above the prominent doorway andthe membrane is fitted to that to provide a wa-tertight junction. This roof is ready to have thetiles or slates fitted.

Materials natural and man-made, sizes,colours, techniques for all sorts of situations –they all abound in this one area of construc-tion. The easiest way to come to terms withit is to set out much of this information in atable, Table 7.1.

When laying any of these units it is obviousthat they must overlap adjacent units all theway down the roof slope, forcing rainwater toflow from one waterproof surface to the next.

171

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Fig. 7.1 Houses ready for roof tiles.

So we can group these units according to thetype or number of laps they require.

Cutting across the material differences, thefirst grouping can be taken as those unitswhich require to be laid with double laps andthose with single laps to obtain a watertightcovering. Slates, timber shingles and plaintiles require a double lap; the interlocking tiles

Table 7.1 Tiles and slates – materials, fixing and laying method.

Fixed direct to Fixed toMaterial Man-made/natural Product sarking battens Lap

Slate Natural Slates from Wales,Cornwall, LakeDistrict, Scotland,Spain and China

Yes Yes Double

Artificial slate Man-made of asbestosor silica fibre inOPC

Slates which arerecognisable asartificial

Yes Yes Double

Reconstitutedslate

Man-made of slateparticles in resin

Slates which attemptto imitate thenatural product

Yes Yes Double

Clay Natural Fired clay tiles; plainor interlocking. Canbe obtained glazed

No Yes Plain tiles double;interlockingtiles single

Concrete Man-made Pressed concrete tiles;plain orinterlocking.Colour and textureadded

No Yes Plain tiles double;interlockingtiles single

Timber Natural Shingles Yes No DoubleBituminous

feltMan-made Shingles Yes No Single

Clay Man-made Pan tiles No Yes Single withspecial side laparrangements

Clay Man-made Spanish and Romantiles

Yes and can bebedded inmortar on aconcrete slab

Yes Single withspecial side laparrangements

and bituminous shingles only require a singlelap. The double lap is only necessary in thedirection of the slope; laterally all units relyon a single lap. To help illustrate these laps ashort series of photographs has been preparedwhere the units were modelled by rectanglesof card, something which students can do forthemselves to help their understanding andvisualisation of double lap joints in particular.

Figure 7.2 illustrates the double lap fixingof plain tiles or slates. The work always startsat the eaves and works up the roof, generallyfrom right to left, forming verges and abut-ments as course upon course of tile or slateis laid. We will refer to the tiles or slates ofwhatever type as units.

So the bottom of the mock units is the bot-tom of the work – the eaves. The first course,

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Roof Coverings 173

Fig. 7.2 Double lap roof unit – method of setting out.

A, is a course of short units either specially cut,as in the case of natural materials, or specifi-cally made for man-made units. If there werebattens for fixing, the first batten would be un-der the bottom of this course and the secondbatten would be under the head of these units.The course is termed an eaves course or a cuttile/slate course.

The first full unit course is course B, and itmust be observed that the units in this courseare displaced horizontally by half the width ofa unit. Run the eye up the stepped ends of thecourses to be aware of this effect. The coursesare laid to break bond just like laying bricksor blocks. The object here, however, is to keepwater out of the house.

The third course, course C, overlaps theheads of the units in course A by a specifiedamount, the lap. It is because C overlaps bothcourses A and B, that the system is termeddouble lap. Lap varies according to the type ofunit and the pitch of the roof – less pitch, morelap. The associated term gauge is the distancefrom the top of one course to the next immedi-ate course and is therefore the centres at whichany battens used for the units are fixed, or formarking out where no battens are used.

Without the double lap, rainwater runningoff the unit would simply go down betweenthe two units below. With the double lap, thatrainwater is caught and drained away by themiddle unit in the group of three that make up

Fig. 7.3 Double lap roof unit – showing lap with al-ternate courses.

the double lap. This is illustrated in Figure 7.3where a unit has been removed in course Cto expose the heads of two tiles in course B,which have been folded back to reveal thehead of the cut unit in course A.

This all looks quite simple but there is onething missing in the little mock-up – unitthickness. Thickness is not of great concernwhen using Welsh slate, which is thin, or eventhe modern Spanish and Chinese slate, whichare of similar dimensions, but it is of concernwhen using thicker units such as clay or con-crete tiles. Even slate from other UK sourcestends to be slightly thick but just manageablewithout too much trouble.

Units are generally nailed at the head, twoholes being provided. Not every tile has tohave two nails. In sheltered areas it is com-mon practice to nail with one nail entirely oreven omit nails for tiles in the centres of largeareas. The manufacturers are the best guide tonailing requirements – see also the commenton interlocking tiles later in this chapter.

Figure 7.4 uses some books to illustrate whathappens when thick units are piled up threehigh at the overlap. The cut course is droop-ing and a gap shows between it and the nextfull course. This is a point of potential weak-ness. Eaves, like other edges of a roof, are veryvulnerable in high winds, and a unit cockedup like that second course would be caught ina gust and ripped out. Apart from any weak-

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Fig. 7.4 Eaves tile ‘cocking up’.

ness, it is also unsightly and we will return tothis aspect when we examine single lap fixing.

The answer to the problem is quite simple:cock the first course up. This is done by restingit on the top of the fascia board which is raisedabove the general level of fixing for the units,be that direct onto sarking boards or battens.This will be seen in the details in the last sec-tion of this chapter.

Slates, natural and manufactured, must allbe nailed to the roof. Manufacturers of thicktiles as opposed to natural slates have over-come the problem simply by introducing aconvex curve to the upper surface of the tile.This effectively lifts the centre of the tile upand away from the intervening head of thecourse immediately below and ensures thatthe tile sits down on the batten at the head. Inaddition, tiles may also be curved across theirwidth, ensuring that all four corners sit downon the underlying surface. There is anothertheory put forward regarding the compoundcurvature. Perfectly flat tiles have a tendencyto lift and fall in a high wind on an exposedroof. They make a noise, which has led tothe term chattering being used to describe thephenomenon. The noise is one thing but theincessant hammering of tile upon tile startsto chip the edges and gradually larger piecescome away and so on. The double curvatureis supposed to stop chatter.

Figure 7.5 illustrates single lap fixing, andFigure 7.6 is a photograph of the underside ofa typical, concrete, interlocking tile.

In Figure 7.5 we show a schematic of in-terlocking tiles, the section being taken alongthe roof slope. The photograph in Figure 7.6

Counter batten

Tile batten

Sarking board with

underslatingmembrane

Tile nails

Lap

Gauge

Fig. 7.5 Interlocking tiles on battens and counterbattens.

shows the tile top at the top. One featurecan be seen that is common to all man-madetiles – nibs which hook over the tile battenand prevent the tile sliding down the roof.There are ridges running horizontally acrossthe back of the tile. These are water stops andprevent water being blown up under the tilefrom the surface of the one below.

Figure 7.7 is a schematic view of the ridgesand rebate on the side of these tiles. This iswhat stops the water passing down betweenthe tiles onto the surface below.

On a roof not every such tile need be nailedor otherwise fixed to the battens. Rules for fix-ing vary from manufacturer to manufacturer

Fig. 7.6 Underside of an interlocking tile.

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Roof Coverings 175

Upper surface of tile

Nib Nib

View looking up the tile

Fig. 7.7 Schematic of side interlocking joint.

but a reasonable guide would be to nail thefirst three courses at the eaves, the last threecourses at the ridge and three tiles in at everyverge or abutment. In addition, every thirdor fourth course has to be nailed for its com-plete length. It used to be normal for all tilessupplied to have two nail holes preformed intheir head. Many tiles are now made withoutholes as the manufacturers wish to have theirtiles fixed with their own patent clips. See themanufacturers’ websites.

Having set the scene it is time to look ateach individual roof unit and consider thoseaspects of it and its use which make it unique.

Slates

Natural or man-made, slates are produced asrelatively thin sheets of material in a widerange of sizes. Roofs are usually coveredwith same size slates, although diminishingcourses are sometimes used for effect. Thereis no practical advantage to the technique. Inessence it means that the length of the slatesreduces as the courses go up the roof.

Slates must all be nailed to the roof timbers.They can be nailed direct to roof boarding,although this should be fairly thick naturaltimber rather than thin plywood or OSB. Al-ternatively they can be nailed to battens.

As well as the cut course at the eaves, therehas to be a cut course at the ridges to main-tain the double lapping. The cut edges are con-cealed with a ridge covering of clay or metal.

Slates can be head nailed or centre nailed.Head nailing means exactly that – two nailholes are made in the slate about 20 from the

top edge and used to take slate nails (see Ap-pendix G).

Centre nailing is not exactly as it sounds.When the lap and gauge have been calculatedfor a specific size of slate at a particular pitch,it is possible to further calculate exactly wherethe head of the slate below is going to be. Theslates are then pierced each side just above thispoint. Look at Figure 7.3 and in particular atunit B. Just above the B there is a dotted linewhich indicates the top of the cut course be-low. Centre nailing would be done just abovethis line – certainly more than halfway upthe slate.

We have already mentioned the fact thatwind will tend to lift units off a roof. The loosebottoms of the units are most vulnerable andas they lift they lever out the nails at the top ofa unit. However, if the unit is centre nailed, thenails are in a better position to resist that liftingforce. On the other hand, head nailing allowsthe slater repairing a roof to ease slates up andgive access to the nails of the damaged slatebelow, cut the shanks and replace the slate.

Some slaters will not double nail slates andonly head nail. The advantage here is all forthe maintenance slater. As before, head nail-ing eases the access problem, and single headnailing further allows the slates to be swungaside, pivoting on the single nail. This giveseven easier access for replacing slates or eveninserting pipes, cables and so on into the roofstructure.

Slates for the cut course are cut from wholeslates. A minimum pitch for slates is around25◦. Head lap varies with the lap and gaugeand the overall size of the slate, but generally75 is a minimum.

Plain tiles

Plain tiles come as clay or concrete. Clay isusually a warm terracotta colour, while con-crete tiles are usually coloured and also tex-tured by the impression of coloured sandsinto the surface of the concrete. Laying themis done in exactly the same way as slatingbut always on battens. The correct term is tile

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hanging. Tiles can be hung simply by placingthe nibs over the battens; however, the follow-ing rules regarding nailing must be applied.

Whether one or two nails are used dependsmuch on local practice. The tiles generallyhave two nail holes. Double nailing of verges,eaves and at hips should be insisted uponwhere the roof is exposed, together with sin-gle nailing of all other tiles. Sheltered expo-sures allow single nailing of only two coursesat eaves and ridge and single nailing of twotiles in from the verge, plus single nailing ofevery fourth course over the rest of the roof.

Moderate exposures allow double nailing oftwo or three courses at eaves and ridge anddouble nailing of at least three tiles in fromthe verge, plus single nailing of every third orfourth course over the rest of the roof. Tiles forthe first cut course are cut from whole tiles. Aswell as the cut course at the eaves, there hasto be a cut course at the ridges to maintainthe double lapping. The cut edges are con-cealed with a ridge covering of clay beddedin mortar. A minimum head lap of around 75is required. A minimum pitch for plain tiles isaround 25◦. Look at the websites quoted in thefollowing sections and read what the manu-facturers have to say about fastening tiles.

Interlocking tiles

Interlocking tiles are only manufactured inconcrete and can be coloured and textured likeplain concrete tiles; indeed manufacturers co-ordinate the colour and texture of at least someof their plain and interlocking tiles.

Figure 7.7 is a schematic view of the sidejunction arrangements on an interlocking tile.The ridges and gaps channel water down thegrooves as well as reducing the air pressureas the wind blows across the tile face.

Laying is always done on battens, althoughfixing can be with nails or patent clips. Nail-ing is always through preformed holes nearthe head of the tile. Clipping is only done onone side of the tile but can be slightly furtherdown the side of the tile from the head. Thedifference is so slight that it is hard to see anyreal advantage.

Like the slates and the plain tiles, interlock-ing tiles are laid to a pattern which breaksbond across the roof. There is no need for acut course of tiles at the eaves but the bot-tom course can droop unless the eaves detailis correctly formed. There is no need for a cutcourse at the ridge but the ridge is concealedwith concrete ridge tiles or metal ridge covers.

A minimum head lap of 65 is common butsome low pitch types may require more. Pitchcan vary from around 20◦ to the vertical.

For the best in technical information ontiles try the following websites: http://www.marley.com and http://www.lafarge.co.uk

Both sites cover clay and concrete, plain andinterlocking tiles in a wide range of profilesand suitable for a variety of pitch angles.

Timber shingles

Timber shingles are not often seen in the UKbut are a very practical form of roof coveringand with the right timber can give as good alife as clay tiles. The main timbers used areoak and western red cedar. The shingles aresplit for logs to give a rectangular shape inwhich length is constant but width varies. Allone length are used on one roof. Thicknessvaries, being thinner at the head of the shingleand thicker at the bottom or tail. This allowsshingles to bed down on top of one anotherwhen being laid.

Shingles are always laid direct onto sark-ing and follow the double lap rules for giv-ing a watertight finish. Nailing is generallycopper but stainless steel could be an op-tion now. Compatibility with the timber beingused must be taken into account. Cut coursesat eaves and ridge are cut from whole shinglesas required.

Bituminous shingles

These shingles are made from a mineral sur-faces roofing felt, a Type E felt being the mostcommon, although an asbestos fibre felt mighthave to be used for fire regulatory purposes,i.e. proximity to another building, boundary,

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Roof Coverings 177

etc. They are made in strips of around sixshingles, only the tails of the shingles beingseparated to make it look as though there areindividual shingles.

The tails each have an adhesive bitumen doton the underside protected by a peelable pa-per cover. As the strip of tiles is nailed intoposition, the strip is removed and the adhe-sive dot sticks to the course below.

Bit shingles are nailed to timber board di-rect, preferably a plywood or OSB sheet ma-terial. A strip of heavy roofing felt is laid ateaves as a cut course. This is well nailed to thesarking. Nails used are galvanised steel feltnails (see Appendix G).

These tiles are not particularly long lastingand should only be used in situations or onbuildings where a relatively short life is ex-pected, 12–15 years maximum.

Pan tiles

Up and down the east coast of the UK therehas been a long tradition of trade with the LowCountries. Many ships left the UK with hides,salt, and so on and returned with other goodsand sometimes in ballast with a cargo of roof-ing tiles – pan tiles. These were the originalsingle lap roofing units with a special arrange-ment at the side junctions to keep the roof wa-tertight. They also had a nib for hanging thetile on a batten. They were heavy so nailingwas not considered necessary and thereforenail holes were not formed. In time, potter-ies on the east coast sprang up wherever asupply of clay and fuel could be found closetogether, and one of the first roofing tiles tobe manufactured was the pan tile. Pan tilesare still made today but while local manufac-turers abounded until the 1950s, only one re-mains today. For all the technical informationand some great photographs try the web siteat http://www.sandtoft.co.uk

Modern pan tiles are lighter than the origi-nals and are now nailed or clipped in much thesame way as interlocking tiles. One originalfeature about the nailing is that the nail holeis not in the face of the tile but through the nib.This means that the next tile up sits right down

into the curve of the previous tile, there beingno nail head to create a pressure point whichwould induce cracking in the upper tile.

Pan tiles are designed and laid to overlapon all four edges, which means that thereis the possibility of a four-layer thickness oftile at every corner. This just does not work.Tiles laid this way do not bed down properly.The solution to this problem must have beenarrived at many centuries ago and involvedmaking the tiles with two opposite corners cutoff at an angle of 45◦ – the solution is still usedtoday. The website shows how it works.

The minimum pitch angle for pan tiles is 30◦

and the head lap should be 75.

Spanish and Roman tiles

These are frequently called over and undertiles due to the shapes used and the mannerof laying them. The shapes are illustrated inFigure 7.8.

Dow

n roof slope

Spanish under tile Roman under tile

Nail hole

Over tile

Under tiles laid like this.Little chutes discharging into one another all the

way down the roof

Once the under tiles are in place, the over tiles arelaid on top, wide end downto cover the gap between

the under tiles

Fig. 7.8 Spanish and Roman over and under tiles.

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The under tiles can come holed but only inthe Roman version, which being flat lends it-self to nailing to a timber roof board. Thesetiles are heavy so a thick sarking board is nec-essary. In the Spanish version there is only oneshape and the manufacturers will not makehalf with holes and half without.

Although we are talking about Roman andSpanish tiles, they can be seen in the UK. ACooperative supermarket in a little town inFife has clay Roman tiles on its roof and theNational Museum of Scotland in Edinburghis roofed with clay Roman tiles. They presentsuch a pleasant pattern of rounded ridges anddeep channels that interlocking tile manufac-turers make a mock Roman tile profile in allsorts of colours other than the original ter-racotta. Lest the reader think that these tworoofs mentioned above are covered in concretemock tiles, both buildings are far older thanconcrete interlocking tiles.

Millions of these tiles must be laid on roofsin Spain every month. And modern practiceis to cast the pitched roof slab in concrete orclay partition panels and then lay a mortarscreed. On top of this two coats of bituminousemulsion are brushed and the under tiles arebedded into a mortar layer. The over tiles arethen bedded over the under tiles and the mor-tar joint pressed down and flushed off.

Edges and abutments

Learning about the different units used overdomestic timber roofs is fine and we canquickly appreciate the differences and simi-larities in technique required to lay them overthe general roof area. The tricky part is deal-ing with the edges of the roof surfaces andwhere these surfaces meet walls, other verti-cal tiled or slated areas, hips and valleys, andso on. The comprehensive text on the subjecthas yet to be written and is certainly not goingto happen here. But this is not to say that read-ers could not work out for themselves how tocope with Blogg’s Patent Interlocking Tile at averge, a ridge or an abutment if they had seena detail or even a photograph of the work done

in slate or plain tiles. The same techniques canbe applied or adapted in this trade as in anyother.

So the figures which follow are a selectionof how to cope with a particular roofing unitin a precise situation. These details includenot just the roof covering but also the roofstructure. They are complete architectural, tra-ditional details (using mortar for beddingthe tiles). Much modern tiling now uses drysystems for finishing eaves, verges and ridgesand every manufacturer has their own sys-tem. All the websites mentioned above havedownloads using Adobe Acrobat to allow theuser to print out complete catalogues of all thedetailing, or just the pages which interest you.All three sites have good details. From therethe roof tiling world belongs to the readers.

Figure 7.9 is a fairly complete detail of aneaves, with masonry wall and trussed rafterroof with sarking. Rainwater disposal is animportant feature of all buildings in the UK,far more so than in many other countrieswhere there is a higher rainfall. Here the fasciaboard provides a mounting for a half roundgutter in uPVC. The down pipes are nevershown on these details and barely feature onthe elevation drawings, only to indicate theirposition. Gutters are laid to a slight slope,going down to the outlet into the rainwaterpipework. Setting out the gutters like this iscalled crowning.

Figure 7.10 shows the detail of a ridge.Again the roof is a trussed rafter construc-tion with sarking board and interlocking tileson battens and counterbattens. Note that themembrane is taken up one face of the roof andplaced over the ridge. Then the opposite faceis covered and the membrane again taken overthe ridge. The ridge tile is bedded onto theroof tiles with cement mortar which is usuallytinted to match the tiles. The space betweenthe tiles is also pointed with the same tintedmortar.

Figure 7.11 shows a verge. The detail is asection across the verge projection, the line ofsight being parallel to the slope of the roof sur-face. The verge is on that same roof but withthe wide projections at eaves and matching

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Roof Coverings 179

Soffitbracket

Concrete interlockingtiles on 38 x 25 sawn

treated tile battens

38 x 20 sawn treatedcounterbattens on tiling

membrane

Trussed rafter @600 centres

12 sheathing plywoodsarking

100 x 50 sawn treatedwall plate with 35 x 4

galv. hold down strapsnailed to blockwork

Trussed rafters clippedand nailed to wall plate

Brick on edge cavityclosure

Eavesventilator

Soffitventilator

Fascia

Tiltingfillet

Line of roof insulation

Counter batten cutshort and roofing

membrane brought outover fascia

Half rounduPVC gutter onfascia brackets

Half brick10065Common brick with 15

render

12.5 plasterboardon plaster dabs

40 polystyrene bead-board part cavity fill

insulation

Fig. 7.9 Eaves detail.

verge; the idea of bedding the verge in cementmortar is not really feasible. It was generallyonly done when the projection was about 75to 100 and an undercloak of asbestos cementsheet or of plain tiles was bedded on top ofthe masonry and then the roof tiles beddedon top of that. Here there is a wide projectionsupported on short lengths of timber – outrig-gers – nailed to the side of the first/last rafter.There is a soffit plate and barge board, theequivalent of the eaves fascia. The soffit plateis fixed to a batten nailed to the underside of

the outriggers and in a groove in the back ofthe barge board – sometimes referred to as aberge board. A further timber plate is cut tofit the underside of the projection of the roof-ing tiles – called scribing – and then nailed tothe barge board with a mastic fillet on top. Asit is fixed in position, the roofing membraneshould be brought out and stuck between theplate and the tiles.

Figure 7.12 is a photograph of a gable peakjust before the masonry skin was built roundthe house. The roof ladder can be clearly seen;

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Mortar bedding down ridge tile

Mortar bedding down ridge tile

Concrete half roundridge tile Double layer of

tiling membrane over ridge

Diagonalbracing of trussed

rafters

No horizontal bracing at ridgeas the roof is boarded out completely

Tiles, battens, counterbattens and roofing membrane asFigure 7.9

Fig. 7.10 Ridge detail.

the barge board and soffit plate have been pre-finished in this timber frame kit. The roof cov-ering edge is finished with a dry verge system,a series of plastic sections which are clippedonto the ends of the tiles – no mastic, no mor-tar, just push the plastic bits on and the job isdone. What could go wrong with that? Well,quite a lot if the timberwork is not square orwell finished; the roof tiler will have to followthe timber and then the plastic sections mightnot fit quite so well.

Figure 7.13 shows a detail of an abutment.The view is taken parallel to the sloping roofsurface, just like the verge. The roof is still thesame and it is only when the tiles are cut to fitjust next to the wall that there is a lot going onunder their ends. The heavy lines represent

Walls, insulation,tiles, battens,

counterbattens and roofing membrane

all as Figure 7.9

Trussed rafter OutriggerBarge board

Soffit plate

Scribingplate

Roof tiles Masticpointing

CounterbattenTile batten

Roofing membrane taken over fascia plate and

bedded into mastic onscribing plate

Fig. 7.11 Verge detail.

Fig. 7.12 Verge tiled.

sheet lead. It is there to form a double gut-ter and is held into that shape by the counterbatten on the sarking board, and then nearerthe wall another length of tile batten is placedover the rafter. The lead is then dressed up theface of the angle fillet and up the timber platewhere it is fixed with copper tacks. That formsthe gutter. But rain could get down behind thelead in that upstand and this requires a pieceof lead to cover the join.

A groove has been cut in the masonry wall –a raggle. Raggles are cut for all sorts of thingssuch as concealing pipes, cables conduits, etc.as well as for tucking in ends of sheet materi-als such as the lead flashing. Before the leadflashing goes in, narrow lengths of lead stripare set into the groove, then the flashing is setin and rolls of lead strip are hammered into thegroove, trapping the little strips and the flash-ing. The strips of lead are called lead tacks,not to be confused with the copper tacks. Thelittle rolls of lead sheet are called wedges andas they are hammered into place they trap thetacks and the flashing. The raggle should then

Masonry wall with render

Bellcast on render

Roofing tiles

Tile battens

CounterbattenDouble lead gutter

Leadflashing and clips

Backplate and angle filletfixed only to sarking and

left free of the wall

SarkingRafter Rafter

Fig. 7.13 Abutment detail.

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Roof Coverings 181

be pointed up with mortar but when there isa render to apply to the wall, it will be ren-der which is forced into the gap. The flashingis beaten down over the lead upstand so thatthey are neatly finished tight together and theprotruding ends of the lead tacks are turned

up against the flashing and hammered downflat.

The idea of putting in a double gutter is toprovide an overflow channel in case the maingutter gets blocked by leaves or other debris –not much seen nowadays but a good idea.

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8 Doors

Functions of doors and windows – obviousand not so obvious 182

Types of door 184Ledged and braced doors 185Bound lining doors 185Flush panel doors 187Panelled doors 188

Pressed panel doors 18915 pane doors 190

Hanging a door 190Fire resistant doors 193

Smoke seals 195Glazing 196

Ironmongery 196

Functions of doors and windows –obvious and not so obvious

Rather than repeat all the material from thissection when we study windows, the com-ments will be made once here and a cross ref-erence included in the next chapter.

Doors and windows fulfil some fairly ob-vious functions. There are statutory require-ments for both doors and windows in theBuilding Regulations. These and some of theless obvious functions will be highlighted asthe text unfolds.

Doors and windows are always more ex-pensive than the portions of wall which theyreplace – a point we will return to on severaloccasions. According to a current edition of aprice book, a fairly basic timber casement win-dow with double glazing is 3.75 – 4 times moreexpensive than a brick/block cavity wall. Asummary of the specifications is shown inTable 8.1. The rates may not be quite up todate or typical of your area but the compari-son is fairly valid anywhere in the UK. Sim-ilar exercises can be carried out for all partsof a building, costing out alternative forms ofconstruction or the use of alternative compo-nents and/or techniques. This is an important

part of any building manager’s, building sur-veyor’s and quantity surveyor’s work, al-though the latter is the acknowledged expertin this field.

To meet the maximum economic require-ment of a client, the total area of doors andwindows must therefore be kept to the statu-tory minimum. Aesthetics can dictate other-wise but always at a cost.

There is no requirement to have more thanone entrance door to any house. Internallythere does not appear to be any general re-quirement to have doors at all – holes in thewalls would be sufficient. However, the needfor privacy in bathrooms and bedrooms doesdictate otherwise.

To provide natural lighting, most habitablerooms in a house require to have a doorand/or window(s) with an aggregate glazedarea either in wall or roof equivalent to onefifteenth of the floor area.

Generally, the provision of natural ventila-tion can be satisfied by use of a trickle ventila-tor. These can be nothing more than a thin slitcut into the top of a window frame although,like most simple things, lots of people have‘invented’ the perfect trickle vent completewith insect screening and adjustable opening

182

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which is put in the slit in the timber. Theseare usually placed in the window frame. TheRegulations do not require windows to openfor the purpose of ventilation. Otherwise aventilator with an opening area not less than4000 mm2 is required. Note that kitchens andbathrooms require special treatment.

Most obviously, doors are required to allowaccess to a building or compartments withina building. External doors must exclude theinfluence of the external climate – wind, rain,heat and cold – and for that reason many ex-ternal doors today incorporate a certain levelof insulation. External doors must provide se-curity, both when the property is unoccupiedand at night when the occupants are asleep,but also a means of escape in case of fire. Se-curity can be enhanced by using a steel faceon the door but this is all for nought if theframe, hinges and lock(s) are not up to thesame standard.

Internal doors provide privacy and perhapssecurity within compartments. They are fre-quently required to provide resistance to fire.Internal doors can be used to control the modi-fication of heating or cooling regimes betweencompartments, and they can modify the pas-sage of sound between compartments.

All doors can be used to provide or as-sist with natural ventilation. More usually, thejunctions between external doors and framesare provided with seals to reduce the naturalinfiltration of air and therefore only ‘ventilate’when they are opened. Internal doors are sel-dom sealed at jamb and head but frequentlyhave draught excluders fitted at the threshold.

Doors can be fully or partly glazed to pro-vide a source of natural light or to allow a viewto the other side.

The provision of doors and windows posesa large number of problems, which oftenmake conflicting demands on the solutionadopted by the designer. For example, thedesire to have a large area of glass to obtain aview must be tempered by the very obviousincrease in heat loss occasioned by large areasof glass. On the other hand, triple glazingcould reduce that loss to acceptable limits butat a large increase in construction cost. Widerwindows in particular are more expensive

than taller ones of the same area, but the widerwindow may suit the client’s requirements.

Glazing of doors and windows decreasesthe general level of security. Double glaz-ing with sealed units is supposed to increasethe level. Triple glazing should therefore in-crease it even further. The use of toughenedor laminated glass or polycarbonate sheetwill increase the level even more. The use ofthese special glasses may be necessary on thegrounds of safety.

Other problems arise with regard to main-taining the integrity of the wall structure at thejunction with the frames to doors and win-dows. Poor detailing at junctions will allowrain and wind to penetrate the fabric of thewall to its detriment, and even into the interiorof the compartment itself to the discomfort ofthe occupants.

Positioning of the openings in a wall cangive rise to problems with the structural in-tegrity of the wall. Instability can occur ifopenings are placed too near corners or inheavily loaded portions of the wall. Addi-tional wall ties must always be built into cav-ity walls at all door and window openings.

The portions of wall over openings for doorsand windows will require support. This is pro-vided by building in lintels made from a vari-ety of materials: precast concrete, prestressedprecast concrete, and hot or cold rolled mildsteel section, which may also be galvanisedand/or coated with plastics. Adequate sup-port for ends of lintels is vital. The provisionof this support has already been studied inChapter 5, Openings in Masonry Walls.

Types of door

Having looked at some of the functions ofdoors and windows, we will look now atthe various types of doors available. Only themore commonly used types will be listed; thenumber of different kinds is very large, as aquick look at manufacturers’ literature willshow. Try these websites:http://www.magnet.co.ukhttp://www.johncarr.co.ukhttp://www.jewson.co.uk

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Doors 185

Doors can be grouped as:

� External doors� Internal doors.

They can be further grouped as:

� Entrance doors� Pass doors – doors allowing passage from

one compartment of a building to another.

They can also be grouped as:

� Doors with a specific resistance to fire� Doors with no particular resistance to fire.

We will first study the construction of doorswithout any requirement for a specific fire re-sistance.

No matter what the construction of thedoors, they are always hung in a frame; onhinges; with a latch or lock to keep the doorclosed, with stops against which the doorcloses; sometimes with weather stripping ordraught proofing round the edges; architravesto cover the joint between frame and wall;handles or ‘furniture’ with which to operatethe latch and open and close the door; andmore often than not now, a threshold plate ora weather bar.

That is a lot to remember. Taking it a bit ata time we will look at vertical and horizontalsections through some common door types,and then frames, adding more complexity un-til all is explained. Wall interfaces are partlyincluded in Chapter 5 Openings in MasonryWalls, but this will be expanded on here.

The first question which comes to mind is‘What sizes do these doors come in?’ Doorscan be made any size you wish but bespokejoinery is expensive and there are so manymanufacturers making a good range of sizesand types of doors at competitive prices froma variety of materials that few buildings incor-porate bespoke doors and especially housing.

Doors are still made in both imperial andmetric sizes. The more usual sizes for internaldoors are: 1 ft 6 in × 6 ft 6 in, 1 ft 9 in × 6 ft6 in, 2 ft 0 in × 6 ft 6 in, 2 ft 3 in × 6 ft 6 in,2 ft 6 in × 6 ft 6 in, 2 ft 9 in × 6 ft 6 in and626 mm × 2040 mm, 726 mm × 2040 mm,

826 mm × 2040 mm and 926 mm × 2040 mm.For external doors: 2 ft 6 in × 6 ft 6 in , 2 ft 8 in ×6 ft 8 in , 2 ft 9 in × 6 ft 9 in and 807 mm ×2000 mm.

Types of door we will discuss now are:

� Ledged and braced doors� Bound lining doors� Flush panel doors� Panelled doors including 15 pane glazed

doors� Pressed panel doors.

Ledged and braced doors

The face of this door is made from tonguedand grooved boarding nailed vertically to atleast three rails, the ledges, and with two di-agonal braces toe jointed into the rails. Matchor floor boarding is frequently used from 12to 20 mm thick. Boarding should be nailedto the ledges and braces with round or ovalbrads long enough to penetrate fully and beclenched over.

A ledged and brace door is illustrated inFigure 8.1. These doors are only suitable forsheds, outhouses, garden wall gates and ancil-lary buildings with minimal security require-ments.

To hang these doors, usually T or crook andband hinges are used screwed to a woodenframe or with one end of the hinge built di-rect to masonry jambs. Note to which edge ofthe door the hinges and fasteners are fixed.The bracing is put in to prevent the lock edgeof the door from dropping. Fasteners can beslip or tower bolts, Norfolk or Suffolk latches,hook and eye or hasp and staple. Some basicironmongery – hinges, locks and latches – isillustrated in the last section of this chapter.

Bound lining doors

Bound lining doors look to the uninitiatedmuch like ledged and braced doors, but thereare a number of crucial differences:

� A frame is used all round the outside edgeof the door.

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Lock rail

Bottom rail

Rails finish 12--15 short of edge of door

Brace

Boarding nailed to rails and braces

t&g V jointed t&g beaded joint

Diagonal brace

Lock, latch or hook for

fastening on this edge

Run of grain

Door hinged on this edge

Top rail

Doo

r ed

ge

Fig. 8.1 Ledged and braced door.

� The t&g boarding is 10–12 mm thick and isalways V-jointed or similarly shaped, and itis nailed to the frame all round the edge.

� The edge frame is generally made of 38–45 mm thick timber and diagonal bracesare set between the three horizontal ‘rails’which form part of the frame.

� The joints of the frame are ‘through mor-ticed and tenoned’.

� The braces are morticed into the top, mid-dle and bottom rails rather than plain toejointed.

These are much more robust doors and withgood ironmongery will fulfil many require-ments where basic security is an issue. It wasthe art of constructing such frames and fillingthem with light boarding or panel work whichgave the craftsmen who made them the titlejoiners. This was a secret known only to theold guild members and distinguished themfrom the carpenters who made things fromsolid timber.

To fully understand the frame of thisand panelled doors the student must knowabout morticed and tenoned joints (see Ap-pendix C).

For external doors it was customary to coatthe mating faces of the joints with white leador red lead paint in the belief that this would

preserve the timber in the joint from rotting.The lead paint dried out fairly rapidly andthe timber still rotted. Doors with bare jointsallow the wood to swell and generally givea watertight joint – and still rot. Timber cannow be treated to prevent rot but the timberwill still take up moisture and swell to keepjoints tight.

There may be some confusion in the stu-dent’s mind on reading of this door beingmade with a frame and then hung in a frame.These two frames are required; the first is anintegral part of the door, the second is made,supplied and fixed quite separately.

Figure 8.2 illustrates a bound lining door.Note that the lock and bottom rails are thin-ner than the other frame members to accom-modate the thickness of the lining. The liningis nailed with round or oval brads to all theframe members. Nails do not penetrate theframe and are driven at an angle to the face ofthe door The heads are punched below thesurface and filled over before decoration.

The door is hung on one or one and a halfpairs1 of hinges, a single pair being 150 mmand a pair and a half being 100 mm.

1 Hinges are always bought, specified, talked about, fixedetc. in pairs. Three hinges are therefore a pair and a half.The terminology apples to all types of hinges.

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Doors 187

m&t joint

m&t joint

LiningStile

Run of grain

Vertical and horizontal section

External face Internal face

Lining finishes below

the bottom rail

Bottom rail

Lock rail

Lining

Lock stile

Top railBrace

Lining

Hinge stiles

Lock stile

Top railLining

Fig. 8.2 Bound lining door.

Note that the position of the brace deter-mines which is the lock stile and which thehinge stile. For a door of the opposite hand,the brace must be fitted the other way round.Most types of lock or latch can be fitted to thisdoor but the mortice and tenoned joint of thelock rail should be avoided when fitting anylock or latch, but particularly mortice locks.

Flush panel doors

These doors are made from two sheets of prin-cipally plywood or hardboard (although anysheet material can be used) separated by acore. Cores can be solid or cellular. Solid coresare made of timber strip or sheets of particleboard, various densities being used. Cellularcores are made from an ‘egg box’ constructionof timber strips or cardboard. They can alsobe made from sheets of particle board whichhave holes running the height of the door.

The flush panels are glued only to the corematerial. Wire staples are occasionally usedin the core but never in the panel face mater-ial. Cellular cores make the lightest doors but

offer little security and can only be used aspass doors2. Solid cores make the heaviestdoors, which do offer security and are usedfor external doors if the glues used are weath-erproof.

The long edges of flush panel doors arenearly always covered with a thin strip ofhardwood. The short edges can also be cov-ered if required. This cover is known as alipping and it is always glued on without anynails or screws.

Doors with cellular cores of egg crate con-struction have an extra edge of solid timber –normally softwood – all round between thetwo panels. This timber is simply butt jointedat the corners. A solid block of softwood isalso included at both sides of the door wherethe lock or latch would usually be fixed. Somemanufacturers fix only one lock block in placeand mark the edge of that door. Sometimesthey mark the wrong edge! Better manufactur-ers fit two lock blocks, then it doesn’t matter –but it can be good for renovation work as the

2 A pass door is another term for an internal door, gen-erally without any security arrangements.

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Core

Perforated core

Face materialS/w frame

Lipping

Lipping

Lock blocks

Letter plate block

S/w frame

Solid timber coreHoneycomb core

Fig. 8.3 Flush panel door.

door can be hung the other way round with-out turning it upside down. The edging andthe blocks allow ironmongery to be secured.

Figure 8.3 shows the construction of flushpanel doors.

Honeycomb cores are made of thin woodenstrip or cardboard strip, half checked at alljoints. Cores are glued to the S/w ‘frame’and to the face materials. Timber honeycombcores are frequently ‘let into’ slots in the S/w‘frame’.

The S/w ‘frame’ is only butt jointed andglued at the corners. S/w ‘frames’ are not usedwith solid timber cored doors.

Lock and letter plate blocks are only gluedin. Light staples may be used to hold them inposition during manufacture.

Lippings are generally of hardwood and areglued in place. Plywood and hardboard facematerial can be left for painting and plywoodcan be veneered, pre-finished if required.

Panelled doors

Panelled doors are constructed by forming aframe generally of 38 to 45 mm thick timber

inside which thinner layers of timber, ply-wood or veneered particle board or even glassor mirror are set into grooves cut in the in-side edge of the frame. Most usually the edgesof the frame, where the panels are let in, aremoulded to improve the appearance of thedoor. The most common shape is the ovolomoulding, shown in Figure 8.5, although youmay find older doors have variations on theovolo or the cavetto moulding.

The panels can be any thickness up to thefull frame thickness but are usually 10–15 mmthick in modern construction. The faces of thepanels can be carved. The number of panelscan vary but two, four or six is usual. Panelscan be shaped by reducing the thickness at theedges. The reduction can be a simple rebate ora splayed rebate. The back of the panel can beleft plain or can be shaped differently from orin the same way as the face. It is not uncom-mon to have the upper panel(s) glazed andthe lower panel(s) solid.

The joint between frame and panel is some-times covered by the addition of a shaped or‘moulded’ length of matching timber. This iscalled a Bolection moulding. There is no one

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Doors 189

These edges of

frame usually moulded

Top rail

Stile

PanelUpper rail

Muntin

Lock or mid rail

Muntin

Bottomrail

Stile

Victorian 4 panel door Colonial 6 panel doorSection showing different

panel shapes

Fig. 8.4 Panelled doors.

profile which bears this name, only the factthat it is rebated on the back to fit over bothframe and panel. Bolection mouldings can befitted on one or both sides of the door, al-though one side is normal.

Figures 8.4 and 8.5 show panel doors of dif-ferent types and their construction.

The vertical members called stiles are fur-ther differentiated when the door is hung,being called hinge stile and lock stile respec-tively. Members are morticed and tenoned

PanelBolection moulding

Stile, rail or muntin

Ovolo moulding on edge of

frame

Panel

Note: Panels do not extend to bottom of groove. This allows panel to expand/contract independently

of the frame

Fig. 8.5 Fitting panels into frame. Bolection moulding.

together in good work, dowelled in cheapwork but now very common. In high classwork all the mortices are ‘blind’. Muntins arestub tenoned into the rails.

Pressed panel doors

Pressed panel doors are similar to flush paneldoors in that they have a S/w ‘frame’, to eachside of which is glued a sheet of hardboardwhich has been hot pressed into the shape of afour or six panel door. The ‘frame’ is generallyjust large enough to accommodate the screwsfor the hinges, i.e. no more than 25 long. Afurther feature is the impressing of a timbergrain into the surface of the hardboard. It isdone quite well with the grain running in thecorrect direction in what are supposed to beseparate timbers.

Lock blocks are fitted, though not necessar-ily both sides, with the same consequencesas before. Only glue is used in assembly andthere are no lippings. Cheap and cheerful, it isusually supplied with a coat of white primerbut some are not always easy to decorate.

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Nocore

material

Pressedhardboard face

material 4.5 thick

S/w frameroundedge

No lippings

Fig. 8.6 Pressed hardboard mock panel doors.

Figure 8.6 illustrates a pressed panel door.

15 pane doors

15 pane doors comprise a frame of two stiles,a bottom rail and a top rail, the space insidethat frame being divided into 15 equal panes,5 × 3, by the use of astragals or glazing bars.The construction of the frame is similar tothe panelled doors already described, withovolo mouldings generally used on one sideof the frame edges, with a plain rebate andloose beads on the other side to receive theglass.

Glazing can be in putty, glazing compound,velvet, wash leather or silicone glazing com-pound. The use of compounds is based onthe idea that a small amount of compoundis placed in the rebate in the frame or astragaland the glass is pressed into it. A little morecompound is added and the beads fixed in po-sition with glazing pins – small, fine, roundwire, lost head nails. These nails can be ham-mered in but the proximity of the glass hasbrought in a simple invention, the pin push,which makes broken panes a thing of the past.It is a piston with a magnetic tip running ina cylinder just large enough to take the glaz-ing pin, and the whole thing mounted under alarge round push pad. Pop a nail into the cylin-der, put the end of the cylinder on the beadand push. The bead is nailed in place in afew pushes. A word in favour of silicone com-pound here: it sets and sticks to both glass andtimber, and it doesn’t shrink or dry out so theglass never comes loose and rattles when thedoor is opened and closed.

Run of grain

Astragals

Stile or rail

Ovolo

Ovolo glazing

bead

Glass

Ovolo glazing

bead

Astragal

Stub tenon

Astragal

MitreMitres

Halving joint

Glazing compound

Fig. 8.7 15 pane glazed door.

The astragals are made with double ovolomouldings and double rebates as shown. Theends of the astragals are reduced to rectangu-lar section tenons and let into mortices cut intothe stiles and rails. The mouldings on the as-tragals match those on the stiles and rails andthese are mitred at the morticed and tenonedjoints. Junctions of astragals are generally halfchecked, with the mouldings mitred.

Figure 8.7 illustrates a 15 pane door.

Hanging a door

A door is not fixed or fitted or built in, itis hung, generally on hinges, sometimes inpatent devices for sliding, folding etc. doors.Ledged and braced doors are generally hungon T hinges and if of heavy construction oncrook and band hinges. The other doors wehave described are usually hung on butt oredge hinges.

The independent frame in which the door ishung can be made up in a number of differentways:

� Plain with a planted stop. This type of frameis often referred to as a casing. In olderforms of construction a sawn timber framewas fixed into the opening and the casing

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fastened to that but that practice has longbeen dropped largely due to the cost of theadditional timber.

� Rebated to provide an integral stop.

The joints can be plain housings or they canbe morticed and tenoned. A threshold plateis common and always supplied when buyingin ready-made frames. Figure 8.8 illustratesboth types of frame.

Door frames rebated to provide a stop forthe door are generally associated with goodquality work but are more frequently usedfor external doors where the frame is fixedaccording to English practice.

Head Housed joint

Horns

Mortice and tenoned joint

Jambs

Threshold plate

Door

Frame withfull widthplantedstop

Frame with shortplanted stop

Door

Frame rebated to form stop

for door

Fig. 8.8 Door frame construction.

Planted stops are more commonly usedfor internal work, short stops being usedin cheaper forms of construction. Use of aplanted stop for external door frames set ina rebated or checked reveal is shown in Fig-ure 5.27. There the stop also becomes a doorfacing and is generally termed a stop/facing.

Figure 8.9 illustrates the use of a full widthcasing and stop at an internal door, while Fig-ure 8.10 is a photograph of the opposite sideof that door with a more decorative architravewhich covers the joint of the frame with thewall finish and the complete door frame.

The door is hung on two or three hinges.Hinges are always referred to as being in‘pairs’ so a door is hung on a ‘pair’ or a‘pair and a half’ of hinges. Hinges are alwaysscrewed to door and frame. Joiners will al-ways sink edge or butt hinges into the edge

11 plaster

Timber plugs

94 x 15 architrave

15 stop

122 x 45 frame

Door

100 blockwork

DoorBlock wall

Full widthplanted stop

Architrave

SkirtingThreshold plate

Plaster

FrameHinge

Skirting orplinth block

Plan section through hinge stile ofdoor, frame, stops and architraves

Fig. 8.9 Hanging a door – arrangement of all thetimbers.

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Fig. 8.10 A hung door.

of the door. They are sunk into both door andframe in good quality work.

In Figure 8.9 the frame is shown fixed in theopening to a set of plugs or dooks driven intothe blockwork. The width of the frame is setby measuring the thickness of the blockworkand adding the nominal thickness of the plas-ter both sides. The frame is fixed in place, theplaster is applied and finishes applied to theframe, then the door is hung and the stops andarchitraves added.

Figure 8.11 shows the fixing of a frame insuch an opening:

� The frame is made up as the left hand draw-ing; it is squared and braced.

� The plugs are driven into the mortar jointsand left protruding from the masonry.

� The door frame is offered up to the opening,i.e. it is held against the ends of the plugs,the jambs are set plumb with a builder’s

Horn on frame removed

before fixing into opening Head

Stiles

Temporary bracing

Threshold plate

Four plugseach side

Outline of opening in masonry

A billgate

Fig. 8.11 Fixing door frame using plugs or billgates.

level and the plugs marked with a pencilat the back of the frame.

� Plugs are cut off on the pencil mark.� Door frame pushed between the ends of the

plugs and nailed to them once it is plumbwith the wall faces each side.

An alternative – but very inferior – fixing tothe driven plugs is the use of billgates. Piecesof wood 10 thick and the same width as thewall and about one brick long are built into thewall and left protruding. Fitting the frame tothem is the same as for plugs. Problems arisewhen the billgate dries out and shrinks, losingits grip in the wall. Modern fixings tend to useframing anchors and packing pieces to keepthe frame plumb inside the opening.

One pair of hinges is generally sufficientfor light internal doors. For external or heavydoors a pair and a half should be used. Hingesshould be 100 mm for all pass doors and maybe 100 or 150 for external doors. Considersolid brass or stainless steel hinges for externaldoors. Plated hinges are not really suitable forexternal doors. On heavy doors, brass hingesshould be ‘washered’.

Hanging a pass door on a pair of hingesallows the joiner to set the alignment of thehinge pins so that as the door opens, the lockstile rises, allowing it to clear carpeting or

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Doors 193

Door leaf

Door leaf

FFL

Bottom hinge

Line of frame

Frame

Frame

Door point lifts when opened

Top hinge with half of hinge pin exposed

Bottom hinge with whole of hinge pin exposed

Fig. 8.12 Making the door rise to clear a floor covering.

other floor covering. On moderately heavydoors this will also give a measure of ‘self clos-ing’. It is not really feasible to try this withmore than a pair of hinges, nor is the selfclosing feature a substitute for a proper doorcloser where that is required.

Hinges are described as being ‘full’ or ‘halfball’. Figure 8.12 illustrates the idea.

Fire resistant doors

Doors with a designed resistance to fire arerequired in quite specific circumstances:

� An external quality door would be used ina common entrance stair

� An external quality door would be used be-tween a house and a garage attached to thehouse

� An internal quality door would be used ina three-storey house for each room openingoff the staircase.

That list is not exhaustive but serves to il-lustrate where fire resistant doors might beused. Fire resistance is measured as the timefor which the door, its frame and stops willresist the passage of flame, hot gas and heattransfer.

Before going any further, one materialwhich is new to fire resistant technology mustbe explained and that is intumescent mater-ial. Intumescent material is a plastic whichwhen heated will foam up to fill a gap orprovide a protective layer over another vul-nerable material. For example, many exposedsteel frames are now painted with an intu-mescent paint. In the event of fire, the plas-tic skin foams up and insulates the steel fromthe heat of the fire and delays any distor-tion it might suffer and which could lead tofailure of the structure. In doors and doorframes, strips of intumescent plastic can beinserted into grooves in the components andin the event of fire they swell up, sealing offthe gap round the door. Two types of thisseal are made. One exerts pressure on bothdoor and frame, keeping the door in placeand also preventing it from warping under theheat. One merely seals and insulates betweenthe two surfaces. The longer periods of resis-tance required benefit from the use of pressureseals.

So what makes a door fireproof? Well, thereis nothing which is fireproof. That is an abso-lute which cannot be achieved and is why wetalk of fire resistance, and measure that resis-tance in terms of the time delay from the startof the fire to the time when smoke, hot gasses,etc. will break through the door to the dangerof life – a breach.

Doors are rated FD20, FD30, FD45, FD60,FD90 and FD120, which all fall within therequirements of BS 8214, Fire door assemblieswith non-metallic leaves. FD means fire door

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and the number is the rating in minutes. Facedwith a door one can tell what its resistance isby looking on the edge of the door for a colourcoded plastic plug driven into the timber, orsome other form of label attached to the door.The plug, for example, has two colours: a ba-sic or background colour with a centre corecolour. Core colours can be red, green or blue.Background colours are white, yellow, pink,blue, brown and black.

The British Standard has an explanation ofthe coding system in Table 1 and from this canbe gleaned the facts that:

� Intumescent seals should be fitted for cer-tain ratings at the time of hanging the door –red core with all the ratings above given aunique background colour

� Intumescent seals should be fitted for cer-tain ratings at the time of manufacture ofthe door – green core with all the ratingsabove given a unique background colour

� The blue core only has two ratings:� no intumescent seal fitted – blue core,

white background, giving an FD20 rating;� intumescent seal fitted – blue core, white

background, giving an FD30 rating.

While it is certainly not the little plastic plug,what gives a door the required resistance? itis its construction using materials which havebuilt in fire resistance and the way the frameand stops are built into the building, and fi-nally the way the door is hung and securedin a closed position by hinges and other iron-mongery.

The door

Materials of construction cover the face mater-ials, the frame materials, core materials andglues:

� Face materials can be any timber-basedsheet material. The Standard states that anyface material is likely to be consumed fairlyrapidly in a fire.

� Frame materials refer to the structural frameof the actual door and although nothing isgiven in the Standard, it can be assumedthat similar criteria will apply to it as theydo to the frame in which the door is hung –straight grained timber free from knots,shakes, splits and wane. More about thiswhen we look at the frame. Nowhere doesthe standard show a preference for soft-wood or hardwood, although one must as-sume, on the basis of the evidence for otherframes, that hardwood would be used indoors with longer ratings.

� Core materials can be any timber or fibre-based sheet material from flax board to solidtimber laminations, and these will gener-ally achieve ratings up to FD60. For ratingsabove FD60 and sometimes for FD60 itself,the core must incorporate inorganic mater-ials such as cement bound mineral fibresor plasterboard layers. In every case the in-tegrity of the core is dependent upon theglues used within it as well as attaching itto the face material. The core thickness is,of course, increased to give an appropriaterating, with doors of up to 55 thick not un-common.

� Glues such as PVA and similar glues mayprove to be effective in doors rated up to30 minutes but beyond that time a WBP glueshould be used. The glue is important, foronce a face has been burned away, the glueline between the remaining face and the coreis responsible for keeping the core in place.

The method of construction must ensure thatthe component parts are a good fit, for if gapsare left between the face materials or there arefaults in frame or core, there will be burn-through.

The frame and door stops

For timber ratings FD20 and FD30 there isgenerally no need to use anything other thansoftwood but it must be straight grained andfree from knots, shakes, splits and wane. ForFD60 and beyond, hardwood would seem to

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be preferred. In each case the density of thetimber is the important factor, softwood of atleast 420 kg/m3 and for hardwood 650 kg/m3.Straight grained timber is less likely to warpand twist in the heat of a fire.

Defects in timber – knots, splits, shakes andwane – introduce holes in the general con-struction, and holes are weak points. Fire canbreak through at these points much earlierthan the general rating expected from the as-sembly – a breach.

The method of construction must ensurethat component parts are a good fit withoutgaps and voids. Selection of good quality tim-ber is the important factor in this.

With the introduction of intumescent seals,the necessity to have deeply rebated solidframes or extra thick planted stops screwedin place all seems to have gone. Now stopsof 12 or 15 material may be planted and fixedwith nails. The stops material should matchthe frame material. The fit to the wall in whichthe frame is fixed is more important than thestops.

Architraves now take over as an importantfeature. They should match the frame in termsof material and should overlap the frame andthe adjacent wall by at least 15. This applies toboth masonry and timber stud walls coveredwith plasterboard. If there is a gap of morethan a few millimetres between back of frameand masonry wall, this should be filled withmineral wool or glass fibre quilt. No densityis given but it is imagined that it should bepacked in quite tightly. Architraves can be dis-pensed with if the masonry opening is a goodfit to the frame, but the gap requires sealingwith an intumescent mastic both sides.

On the matter of rebates in door frames andplanted stops, there is one more thing whichcan be done to improve the integrity of theassembly and that is to double rebate or over-rebate the door. Figure 8.13 shows how this isdone.

All of these precautions are carried outround the perimeter of the door except on thethreshold. Testing has long ago ascertainedthat the threshold is far less susceptible tothe effects of fire than the rest of the door

Frame and stop

Door

Fig. 8.13 Over-rebated door in frame.

and frame. Gaps between door and thresholdshould however be kept down to 3. Smokeseals might have to be fitted in the bottomedge of the door.

Smoke seals

Smoke seals are different from the intumes-cent seals. They are about containing thespread of smoke from one compartment to an-other. For example, there might be a commonstairway rising through a block of flats. Eachcorridor joining that stairway would have afire door, but not only would these doorshave a fire rating, they would also have tohave smoke seals even if there were intumes-cent seals fitted to make them comply withtheir rating. Smoke seals are about stoppingcold smoke or smoke at ambient temperaturespreading into escape areas etc. In our myth-ical block of flats, fire on one floor or in oneflat would not break through the fire door forsome time, but smoke could pass if the intu-mescent seals were not activated. This smokecould prevent escape down the stair or harmpeople attempting to escape.

Smoke seals can be of the wiper style or thebrush style. Both are made of synthetic mater-ials. Figure 8.14 illustrates both.

In narrow doors it might not be possible toaccommodate both a smoke seal and an intu-mescent seal in the edge of the door, but thereis nothing to prevent one being in the doorand the other in the frame.

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Wiper blade flexes against opposing

surface

Other end of seal fits into groove in door or frame

Brush seal --man-made fibres set in a hard plastic

backing strip

Fig. 8.14 Wiper and brush smoke seals.

Glazing

So far the discussion has centred on doors anddoor frames, but the Standard goes on to dis-cuss glazed portions of the door and glazedsidelights and so on – the whole thing be-coming quite complex. In summary, glazingin a fire door needs a lot of care and attention.When a hole is cut into a door the core is ex-posed and this core edge requires protectionfrom the effects of fire. Also, core materials donot often give ideal conditions for mechani-cal fixing – nails and screws – so that beadstyle solutions to fixing glass in an opening aredoomed to fail. There are proprietary systemsof high resistance rated materials which can beused to hold glass in such openings, and they

are frequently used in conjunction with a solidtimber sub-frame set between the face materi-als. The glass itself has little resistance to fireand early attempts centred on reinforcing thepanes with a wire mesh. Modern glasses areavailable which do resist the effects of fire. Ofcourse, one of the effects of glass is to allowradiant heat to pass through the door and thiscan be so intense that it can set inflammablematerials alight.

Ironmongery

This could make a book in itself, and a fewhave been published. If you can ever lay handson an ironmonger’s catalogue take it withthanks; we mean, of course, an architecturalor builders’ ironmonger.

Among the many items made and sold ina huge variety of styles, materials, coloursand complexity, a few have been singled outhere for mention, more in the hope that thecorrect terminology can be learned than thatthe complete range be described. As the par-ticular requirements for fire resistant doorscome up, the reader’s attention will be drawnto the point being made. Before getting intothe list, however, it should be mentioned thatironmongery is always fixed with matchingscrews. They may be countersunk, raised orround-headed, but the finish on them alwaysmatches the ironmongery.

For doors we might require:

� Hinges� Latch or lock or both� Bolts� Hinge bolts� Letter plate� Weather bar� Door furniture.

Hinges

Hinges come in a variety of styles but forthe doors described earlier we would useT-hinges, edge or butt hinges and crook andband hinges.

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T-hinges come in a range of sizes which de-scribe the length of the leg of the T. They canbe black japanned finished or hot dipped gal-vanised. A 300 hinge would be adequate forthe ledged and braced door of light construc-tion.

Crook and band hinges are only usedfor heavy doors. They also come as blackjapanned or hot dipped galvanised. The crookpart can be supplied for building into ma-sonry, or driving into a predrilled hole in ma-sonry or timber, with a bolt end, nut andwasher for fixing through timber and with aplate for screw fixing to timber.

Edge or butt hinges are the choice for en-trance doors and pass doors. They come ina range of sizes from the impossibly smallto the quite large, 20–150, the size being thelength of the plate screwed to door or frame.Butt hinges are preferred on fire doors butthe screw nails should be a minimum of 38long. No recommendations regarding gaugeof screw are made but 4.5 mm or No. 8 screwswould be advisable.

Figure 8.15 illustrates these hinges and Fig-ure 8.16 is a photograph of a few: T-hinges,galvanised and black japanned, and the edgeor butt hinges in 50, 100 and 125 sizes. Thewhite hinge on the bottom left is a rising butthinge. This hinge has one leaf which raises upthe cam shape you see round the pin as thedoor is opened. This hinge is in white nylonwith a stainless steel pin.

Latches and locks

Latches and locks can be confusing terms, es-pecially when both functions are combinedin a single unit. Basically, asking for a latchwill give you a unit which pushes a boltinto a keeper in the frame when the door isslammed, and to release it you turn the doorknob or depress the lever handle. A dead lockrequires that the door be pushed shut and thena key used to push a bolt into a keeper in theframe. To release it a key is required to pullthe bolt out of the keeper. A lock combinesboth these features. There is a bolt operatedby a key both to open and to close, and there

is a bolt which will slam shut but requires thehandle to be turned or the lever depressed torelease it.

Locks which use keys can be of two types:lever operated or cylinder operated. Thecylinder operated types divide into two moresub-species – the Yale style or the Euro cylin-der style. Of all of these the lever operatedstyle is the oldest but unless there are at leastfive levers, it lacks security. The key style isfamiliar to everyone – a long(ish) bar or rodwith a handle at one end and a knobbly bit atthe other. The knobbly bit is cut into a distinctpattern, the idea being that the different levelsof knob each lift a lever by a discrete amount.If all the levers are lifted by the correct amountthe bolt will be moved as the householder con-tinues to turn the key. Five levers make it diffi-cult for burglars to lift all the levers and movethe bolt to gain access to the property.

Cylinder locks operate on a somewhat sim-ilar principle. The key is a flat bar with flutesdown the side and a saw tooth pattern on oneedge. The fluted pattern on the side of the flatbar is designed to prevent other manufactur-ers’ key blanks from being used but it doescontribute to the security by making thingsawkward for any lock picker. Without a keya series of spring loaded pins in the cylinderstick out into corresponding holes in the bodyof the lock. As the key is inserted the saw toothpattern pushes up the spring loaded pins asmall amount which depends on the size ofthe tooth or gap under them. If the correct keyis inserted, the pins align in such a way thatthey are free of the mating hole and the cylin-der will turn. Four, five and six pin cylindersare available, but five or six are recommendedfor security.

Yale-style cylinders are forever associatedwith the night latch, which at its most basicwas a latch on the way out of the door – justpull the door and the latch slammed shut, butneeded a key to get back in – unless someonehad pushed the little lever or knob on the latchcasing which stopped the bolt moving! Thesedevices are installed in millions of homes butnow they are considered a security risk ratherthan an asset. One benefit they do have: in theevent of fire there is no need to hunt around

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S

S

S

S is the size of the hinge

Crook for buildling into masonry

Crook for bolting through timber. The square shank is pulled into the hole

drilled and stops crook from turning

Crook for screwing to timber door frame

Crook for driving into a hole in masonry or timber

Band of a crook and band hinge. The squarehole takes the head of a coach bolt

Butt or edge hinge

T- hinge

From 16--25 diameter

Fig. 8.15 Some hinges.

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Doors 199

Fig. 8.16 More hinges.

for a key to make your escape through thedoor. A lot of people have died looking for thedoor key and Fire Officers everywhere nowrecommend that any lock fitted to your en-trance door should have a thumb turn fittedon the inside. Fitting a key-operated dead lockdoes not allow this but fitting a Euro cylinderlock does.

Figure 8.17 shows a variety of locks. Startingat the top left and moving along the top rowof units there is a bunch of cylinder keys forthe Euro cylinder and Euro lock to the right.To the right of the Euro lock is a cylinder byLegge, and the two items below that are thekeeper and night latch which are used with thecylinder. Both the night latch and the keeperare face mounted, on the door and door framerespectively. The hole for these cylinders isstandardised at 32. At the extreme right of the

Fig. 8.17 Some locks and latches.

photograph are a tubular latch and its keeper.This simple latch is used on pass doors. Sim-ply bore a hole in the edge of the door, takeout a little rectangular recess for the front plateand screw the latch into position. If you re-membered to drill a hole through the door forthe spindle, fit a handle. The keeper is pressedsteel and has to be fitted to the door frame witha recess for the latch bolt. All metal versionsof this cheap latch can be very effective andquite long lasting, those with plastic in theirconstruction much less so.

On the other side of the night latch is a fivelever mortice lock which uses a conventionalkey. The hole for the latch spindle is clearlyseen, as are the dead bolt for the lock andthe latch bolt. This lock and the Euro lock aremorticed into the door with holes cut for thecylinder or conventional key to pass through,and the spindle for the latch. Above the fivelever lock is a fore-end plate and above thatthe keeper. All mortice locks and dead lockshave similar fore-end plates. The keepers ofhigher security locks are of welded box con-struction and are fixed into a mortice in thedoor frame, and the cover plate is put overthe exposed edge of the lock.

During all this discussion we are assumingthat these dead locks and ordinary latches areof the mortice variety – i.e. a slot or hole iscut in the edge of the door and the unit slidinto place and secured with screw nails intothe edge of the door. Older locks were calledrim locks and were fitted to the inside face ofthe door and the adjacent edge. It is still pos-sible to purchase these older style locks butnot nearly so large as the one illustrated inFigure 8.18. It is of heavy brass, 270 long ×150 high and 40 thick. It incorporates a lock, alatch and a sliding bolt, which is only opera-ble from inside the house. It has one lever, soin terms of modern security requirements it isvery much below par. A complicated ward 3 is

3 A ward is a system of metal strips inside the cover ofthe lock case, which prevents a solid key from turningin the lock unless the key has a matching pattcrn cutin its blade. Wards are still used today together withanything from three to six levers to provide security;five lever locks are recommended for external doorswith or without wards.

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Fig. 8.18 An old rim lock.

incorporated, which was a security feature atthe time it was made – around 1820 – that wassupposed to stop the use of picks. Another in-teresting feature is the round head screw nailsused to fix the lock to the door. They look likebrass but it is only a cover of brass squeezedtight onto a heavy round headed steel screw.Once this cover is fitted, the slot is cut in bothbrass and steel. That may seem to be a lot oftrouble, but there are a couple of points whichfollow. First, steel screws are much strongerand the slot is less likely to be damaged insert-ing a heavy screw if it is made from steel. Sec-ond, brass plating wasn’t possible 180 yearsago so covering with brass was the only wayto get screws to match the ironmongery. Thekey has long since disappeared but it wouldhave been massive and not at all easy on thetrouser pocket.

A further feature of locks is that there isfrequently a separate piece of ironmongeryto reinforce the edge of the timber round thekeyhole. It is called an escutcheon plate. Thisreinforcement can also incorporate a hingedflap to reduce draughts and provide privacy,especially when fitted on the room side of thedoor. With mortice locks it is usual to fit an es-cutcheon plate on the inside of the door witha flap, and one without on the outside. Some-times this is unnecessary if the handles fittedhave a large back plate which includes a rein-forcement for the keyhole cut-out. Not manyof these incorporate a flap. If you look closelyat the photograph in Figure 8.18 you will see

Fig. 8.19 The escutcheon plate for the lock inFigure 8.18.

that the lock itself has a built in cover forthe keyhole opening, thus preserving the oc-cupants’ privacy. There is a plain escutcheonplate on the outside ( Figure 8-19) which mea-sures 80 from top to bottom.

Bolts

Bolts are frequently used to bolster securityof a door, but only if the occupier is insidethe house! With double leaf doors, they areused to secure one leaf while a lock is used forthe other leaf. They are available in a varietyof types to fix to the face of the door or theedge of the door. They have keepers for thebolt or shoot. Frequently the keeper is only ahole drilled into the frame or into the floor orslip step. Bolts are also used in smaller sizeson WC and bathroom compartment doors, toensure privacy for the occupant.

Bolts are available in black japanned steel,galvanised steel, brass, aluminium and evenplastics. Sizes are indicative of the overalllength and range, from around 25 to 600. Onfire doors, bolts which hold the door leaf shutmust be of metal and be screwed to the face ofthe door, never on the edge. A minimum screwlength of 38 is recommended. Screw manu-facturers have standardised on 40 so 40 × 4.5should be the recommendation.

Figure 8.20 shows some of the more ‘nor-mal’ bolts used in domestic work. From thetop they are:

� A 225 galvanised tower bolt

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Doors 201

Fig. 8.20 Tower and slip bolts.

� A 180 black japanned tower bolt� Two 50 slip bolts, chromium plated steel on

the left and satin anodised aluminium onthe right.

Hinge bolts are a security device and de-spite their name have no moving or slidingparts. They are passive – until someone tries tobeak down the door. Mating holes are drilledin frame and door edge as close to the topand bottom hinges as is reasonable but be-tween those hinges. The ‘bolt’ is hammeredinto the hole on the door and the keeper plateis screwed to the frame. The hole under it iswidened slightly to allow easy movement ofthe bolt when the door swings open and shut.Any attempt to force an entry has to overcomenot just the resistance of the hinges but also thehinge bolts set much deeper into both doorand frame. Figure 8.21 shows a sketch of ahinge bolt in position, and the two compo-nents. Bolts are normally around 12 o.d. (out-side diameter).

Letter plates

Letter plates are a must on the front door, andthe only thing to say about these has to do withtheir insertion in a fire door such as one wouldfind in a common stairway. British Standard2911 applies to these items, and one with amaximum aperture of 250 × 38 should befitted. It should have a strong spring or goodgravity action and be made of steel or solid

Countersunk hole for wood

screw

Oval hole for bolt

Edg

e of

doo

r

This enddriven into pre-bored

hole

This end goes into hole in door frame

Door frame

Door

Door bolt

Keeper

Hole in frame

Hinge

Fig. 8.21 Hinge bolt.

brass. Internal as well as external flaps are amust, but all of these precautions will give arating of 30 minutes. Note that plastic flapsare not even considered.

Weather bars

Weather bars are the constant subject of the‘invent a better mouse trap brigade’. Thereare at least six companies in the UK and Eirewhich make their own styles of weather bar –not just one style – one company makingdozens of different styles. Weather bars areonly used on external doors or doors in com-mon stairways. A common form is a hingedflap attached to the door, and as the door isclosed a simple cam mechanism forces the flapdown so that a flexible seal on the flap pressesdown on the threshold. These fail primarily atthe cam mechanism for it has to operate at thehinge end of the door, and enormous forcesare at work to shut the flap frequently at anoblique angle to the cam mechanism. A fur-ther failure can occur with the flexible stripor cushion, but that is usually easily replacedin the better-designed products. The secondgeneral type is the one piece, no moving parts

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Door

Slip step

OutsideInside

Weather bar of anodised

aluminium alloy

Drainage holes

Bar bedded in mastic and

plugged and screwed to slip

stepSynthetic rubber seal slides into groove in

weather bar

Bottom of doorcut off at angle and painted

before hanging

FFL

Fig. 8.22 Threshold/weather bar schematic.

type, usually an anodised4, aluminium extru-sion fitted to the threshold. The door closesagainst an upstand of the threshold and thegap is taken up with a flexible seal of eitherthe wiper type or the cushion type.

Figure 8.22 is a sketch of a cushion type sealin a one-piece threshold/weather bar. Notehow, as the door is closed, the cushion top onthe seal is squeezed forward and seals againstthe sloping bottom of the door. It is impor-tant to paint or otherwise seal the bottom ofthe door before it is finally hung in place. Trythis website, which will give you a lot of in-formation about the various types available:http://www.stormguard.co.uk

Door furniture is the term used to describethe knobs and levers, pull handles and pushhandles with which a door is opened andclosed and which operate the latches. Eschew-ing the architectural, the usual domestic set-up is to use either knob or lever furniture.Knobs and levers of course come in a varietyof shapes and patterns. They also come in avariety of materials and finishes.

4 Anodising is a process applied to aluminium and itsalloys whereby the metal is oxidised within a controlledenvironment and the oxide layer formed can be left witha natural colour or dyed to give a gold, silver or brassappearance. The oxide layer can prevent further deteri-oration of the metal substrate.

Fig. 8.23 Knobs and levers.

Materials include cast iron, brasses, stain-less steel, wrought iron, aluminium, plastics,wood and porcelain. Finishes include enam-els or japanning (applied to cast iron andwrought iron), BMA (bronze metal antique –applied to brass ironmongery), SAA (satin an-odised aluminium) and plain old-fashionedpolishing and lacquering (applied to brassironmongery).

To operate, all latches have a square spin-dle which locates in a square hole on the latchmechanism. They can be secured there by a va-riety of devices, ranging from soft steel pinsdriven into place, to nuts on the spindle, grubscrews, cotter pins, split spring washers andso on, some good, some poor but there is notthe space to discuss the merits of all the sys-tems used. Faced with a choice, a spindle witha thread run on it and a nut or handle withthread internally is possibly the most secure –we are not talking security, but the least likelyto come apart in the user’s hand. Grub screwsare ‘iffy’; those tightened with a screwdrivertend to have the slot or cross head badly dis-torted when someone tries to fit it or retightenit. Those which use an Allen5 key are muchbetter. Figure 8.23 shows a selection of knobsand levers with which to operate the latchmechanisms of locks and latches. From left toright there is a brass lever handle, Bakelite ballhandles, wooden ball handles and brass ballhandles. The brass handles are hollow.

5 An Allen key is an L shaped piece of hexagonal sectionhardened steel. Either the short or long leg can be in-serted into a hexagonal section blind hole in the end ofthe grub screw. It is a very positive method of connect-ing tool to screw or bolt.

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Doors 203

In general terms, for fire resistant doors, thefitting of ironmongery can degrade its poten-tial seriously if the holes cut are too large andleave spaces within the door core; or if thereis too much metal conducting heat into the

core and therefore burning out the core roundthe metal and so on. Fitting ironmongery tothese doors has to be done with precision, andany void left must be filled with incombustiblecore material or an intumescent putty.

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9 Windows

Timber casement windows 205Depth and height of glazing rebates 206Timber for casement windows 206Draught stripping materials 206Hanging the casements 207Joining the frame and casement

members 209

Timber sash and case windows 211The case 212The sashes and case together 214

Vertical sliding sash windows 214Glazing 218

For ordinary glazing work 218

Windows are made from a variety of materi-als, in a variety of types and in a variety ofconfigurations:

� Materials – wood, plastics and metals� Types – casement, sash and case, pivot and

sliding� Configurations – plain, mullioned, oriel,

bay, etc.

Our detailed study will cover only timberwindows configured as casement windowsand sash and case windows. Other materialsfor these two types will be discussed briefly.Only plain windows will be discussed as theseare the basis upon which all other types arebuilt.

The British Standard 6375 Parts 1 and 2 isstill current in the Regulations in relation towindows but has been partially supersededby three BS EN documents: 12207:2000 onAir permeability; 12210:2000 on Resistanceto wind loading – Classification; 12208:2000on Water tightness – Classification. Generallythese specifications of windows are on a per-formance basis and are not of the type whichgive precise minimum dimensions for com-ponent parts or minimum standards for ma-terials and workmanship in the manufactureof the window. These Standards are a speci-

fication for the operation and strength of thewindows, which is also performance-based.

A wide range of tests is given which win-dows must pass to be deemed to be up to Stan-dard. This leaves the specifier free to choose aproduct from a manufacturer who will designand make windows which either meet or ex-ceed the performance requirements; thus thechoice is made on performance and cost.

Windows have one thing in common –glazing. This can be done in a variety of waysand will be dealt with in the last section ofthis chapter when we have discussed the ac-tual windows.

Alternative materials used for the manufac-ture of windows are:

� Plastics, particularly uPVC, which may bewhite or coloured and even impressed witha timber ‘grain’. The sections are hollow andmay be of a single skin or more likely withinternal ribs. Sizes of sections are similar totimber sections. Sections are mitred at thecorners and ‘welded’ using RF (radio fre-quency, i.e. microwave) radiation. Metal re-inforcement may be introduced at the cor-ners or even the full length of the sectionswith the metal ‘crimped’ at the corners aswell as having the plastic ‘welded’.

204

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Windows 205

� Metals such as steel and aluminium can beused. Steel windows use hot rolled sectionsand these are now hot dipped galvanisedafter fabrication, i.e. cutting to length withmitred, electric arc welded joints. Alumini-um windows are now made using extrudedhollow sections. Some sections may be of asingle skin construction or may have inter-nal ribs. Corner joints can be made by insert-ing an L-shaped corner piece and crimpingthe sections to it or by drilling and screwingwith plated steel self-tapping screws.

� Metal windows can be self finished, coatedwith plastics, e.g. powder coating, in a va-riety of colours, including metallic colours.Additionally, aluminium windows can be‘anodised’ – oxidising the surface of the alu-minium and dying and polishing the oxidecoating.

� Plastic coated timber. Here the timber is ma-chined to shape and a thin skin of PVC isdrawn over the section prior to fabrication.

� The author did see casement windows ofprecast reinforced concrete displayed at theNational Building Exhibition some 20 yearsago. They were not a success!

All these windows can be draught strippedand can be glazed in the same ways as timberwindows, although glazing in beads or gas-kets are the most common methods.

Timber casement windows

Traditionally, casement windows of whatevermaterial are made with a ‘frame’ and a num-ber of fixed or opening casements, these case-ments being nothing more than a simpleframe fitted into the main overall frame, orthe main frame may be glazed directly, whichis more often the case for those portions ofmodern windows without an opening sash.

Fixed sashes are screwed into the frame. Theopening sashes are usually hinged (hung) onone vertical side, although it is common prac-tice to include one or two top hung sashesfor use as ventilators, sometimes opening in,sometimes outwards. The majority of case-

Jamb

Sill

Head

Jamb

Mullion

Sill

Head

Glazingdirect into the frame

Glazing direct into the frame

Glazingdirect

into the frame

Glazing direct

into the frame

Transom

Transom

Mullions

Sas

h hu

ng o

n th

is e

dge

Upper sash hinged attop

Sas

h hu

ng o

n th

is e

dge

Sas

h hu

ng o

n th

is e

dge S

ash hung on this edgeS

ash hung on this edge

Upper sashes hinged attop and opening out

Upper sashes hinged at bottom and openinginwards also called hopper hung

Fig. 9.1 Typical layouts of fixed and opening sashesfor casement windows.

ment windows used in the UK have the sidehung sashes opening out whereas on the Con-tinent inward opening sashes are more nor-mal.

Figure 9.1 shows some typical layouts offixed and opening sashes for casement win-dows. Note the conventional symbolism re-garding the opening sashes and the side onwhich they are hinged, and the proper namesof the members of the casement windowframe.

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Planted drip

Head

Sashframe

Sashframe

Sashframe

Transom

Frame sill

Sashframe

Sashframe

Framemullion

Framejamb

Sashframe

Groove for timberinner sill

Sash or casement frameswould be from 65 x 65 timbers,jambs and heads of frame from65 x 85 timbers, mullions and transoms from 80 x 80 timbersand sills from 145 x 80 timbers

Glazing shown is single glazingin putty or glazing compound

Semicircular grooves cut in the mating faces of casements

and frame break the air pressurebuilt up when the wind blows

across or against the window. The splayed faces prevent the casement binding on the frame

when opened. The splayedfaces also reduce the capillaryattraction across the junction

Section through jambs and mullion of window

Section through head,transom and sill of window

Glazing

Fig. 9.2 Typical casement window sections.

Figure 9.2 shows typical sections for a basiccasement window frame and casements.

There are a few points to make about thesefigures:

� The timber sizes given are fairly typical ofmodern joinery practice and are not nec-essarily used by different manufacturers.Older windows may have heavier timbersections.

� The depth of rebates used to take the glasscan be of different sizes to allow single, dou-ble or triple glazing.

� The method of glazing also affects the depthof the rebates, e.g. in putty or with beads orin gaskets.

� The height of the rebates remains at 12–15 mm

Depth suitable for singleglazing

Height

Same height

Depth suitable for double glazing

Weather rebateof sash to frame

Weatherrebate ofsash toframe

Fig. 9.3 Ordinary and deep rebates in casementsashes.

Depth and height of glazing rebates

The sketch in Figure 9.3 shows what is meantby height and depth of glazing rebates.

Timber for casement windows

Timber used may be softwood or hardwood.The majority of casement windows made inthe UK are of softwood, European redwoodbeing the most common. It is now common totreat the timber used by the vac-vac processor similar – see Appendix C – after the timberhas been machined to shape for the variouscasement members.

The rebates used at the opening sashes/frame junctions are the minimum required toexclude weather and should not be reducedeven if draught stripping is used. Draughtstripping involves the introduction of a flex-ible or compressive strip of material which‘seals’ the joint. Figure 9.4 shows examples ofthis, some of which we have discussed in ear-lier chapters, some of which are new.

Draught stripping materials

Figure 9.4 shows that draught proofing mater-ials can be classified as compression or wiping

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Windows 207

Frame

Brush glued into groove in frame or sash. Can be used in edges of sliding

sashes such as sashand case windows

Foam strip glued to frame or sash. Foammay be exposed orcovered with a skin of plastic or rubber. Use in compression

only

Flexible plastic extrusion wedged into sawcut in edgeof frame or sashrebate. Can be used for sliding surfaces or incompression

Plastic or rubbersection pinned or stapled to frame or sash. Only use in compression

Bronze or nylon strip pinned to frame, usually used in compression but would

operate on sliding edges

This is usually used for existing windows (or door) and would operate in compression

or with sliding surfaces

Brush slidinto groove

Hard plastic strippinned to frame

Sash

A selection of draught stripping materials shown fitted to timber window sections

Fig. 9.4 Brush, wipe and compression seals.

depending on where they are placed in rela-tion to sash and frame. Foam strips are bestlimited to compression seals whereas brushand plastic or metal strips can be used in eitherway. Fitted against the sash opening, plastic ormetal strips allow wind to penetrate and canjam the sash in a closed position or make itdifficult to open. Fitted with the sash open-ing, they make the sash difficult or impossi-ble to close and can be noisy in high winds ifloosely fitted, acting like a reed in a musicalinstrument.

Hanging the casements

Fixed sashes are retained in the frame byplacing screws through pre-drilled holes inthe glazing rebate. Mastic bedding is fre-quently introduced. Opening sashes are hungon hinges and have fasteners to retain themin the closed and open positions. Fixed sashes

add considerably to the overall cost of thewindow and a cheaper alternative is to glazethe frame direct. Glazing direct interruptsthe line of the glazing across multiple sashwindows.

A particular requirement of the BuildingRegulations is the facility to clean windowsfrom inside the building. Hinges can be de-signed to throw the sash clear of the frame tofacilitate this. An early form of hinge madeto throw a sash clear of the frame is shownin Figure 9.5. The clearance was minimal andnot very effective.

Modern hinges work on a pantographicprinciple and can allow a tilt and turn facilityin a vertical plane. The hinges are made of gal-vanised or sherardised steel, stainless steel orcast in an aluminium alloy. Screws match theironmongery being fitted. Figure 9.5 showshow the sash is projected clear of the framein the open position.

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This part is screwedto the sash

This part is screwedto the frame

This part projectsbeyond the face of the sash or frame

Vertical frame

member

Vertical frame

member

Sash in closed position Hinge

Hinge pin

Sash in open

position

Fig. 9.5 Easy clean hinges.

Fixing casement windows into openingsin walls varies according to the position ofthe window in relation to the various lay-ers of the wall structure, as well as the tra-ditional building practice in the wider areasof the UK. Whether windows are built for-ward in the opening or to the back of theouter leaf but built in as the walls are builtup, window ‘cramps’ are fastened to the backof the frame. The back of the frame is the edgenearest the masonry. Cramps are of galvanisedsteel, with holes for at least two screws, andfishtailed for building into a mortar bed.They should be twice fixed with screws, notnailed.

Fixing casement windows into openings canbe achieved in other ways. There is a traditionthat casements should be wedged over the

Wall

Casement

window

Cramp screwedto back of

frame

The casement outline is diagrammatic only

Fig. 9.6 Building in a casement window.

frame stiles into openings with checked re-veals and head. The sill is bedded onto themasonry sill – traditionally with a haired1 limemortar – but in recent times in a cement/limemortar. It is impossible to nail the wedges intoposition to prevent them coming loose due tovibration, so a little glue should be placed onthe mating surfaces prior to hammering themhome. The glue not only secures them in po-sition but also lubricates the moving surfaceswhen putting them in place. With plain re-veals and head, the frame is nailed or screwedto plugs in the reveals, but more modern prac-tice uses frame fasteners and packing pieces.The number of fasteners put through eachjamb into the reveal depends on the height ofthe casement window. Two would be a mini-mum but it could be up to four.

These methods are illustrated in Figures 9.6,9.7 and 9.8. What is not shown on any ofthe figures is the possibility of filling the gapbetween wall and frame with self foamingplastic. We have pointed out before that thisreduces draughts and cold bridging as well

1 Cow hair was most commonly used. It was readilyavailable, being a by-product of the tanning industry.

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Windows 209

Foldingwedges

Mortar bed

Elevation of casement window

Wedges placed overstiles of frame

Masonry sill

Fig. 9.7 Wedging in a window.

as adhering to both masonry and frame, soincreasing the efficacy of holding the framein position. To obtain maximum adhesion alight spray of water on both surfaces is ad-vantageous.

Plain masonry reveal

Framestile

Sashstile

Sashstile

Nail or screw through rebate

of frame stile into

plug

Plug

Packer -- offcutof plywood ortimber

Frame fastenerthrough rebateof frame stile

Fig. 9.8 Plugging in and framing anchors.

Joining the frame and casementmembers

The sections of all the members of a casementhave been shown in the figures above, butwhat has not been covered is how they arejoined together. Many casements in the pasthave had all the joints carried out as mor-tice and tenon joints with the rounded andsplayed edges mitred. With modern machin-ery cutting mating faces and the joints them-selves, variations of the finger joint are nowcommonplace and gluing is the favoured ‘fas-tener’. Figure 9.9 shows a typical joint of twocasement sections. The joint of jambs to sill ofthe frame can be plain housings, glued andnailed from the back, or they can be fingerjointed but the sill is not cut to its full depth.

Fig. 9.9 Finger joints in a casement.

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Fig. 9.10 Typical casement window.

Normal length fingers are cut on the stiles, andblind or through sockets are cut in the sill. Theglue does the rest. The cutter used cuts boththe fingers and the scribe to match the adja-cent timber.

Figures 9.10 to 9.14 are of a typical casementwindow. Figure 9.10 is a picture of two case-ment windows, one leaning against the other.The outermost one shows only the openingcasement with the frame stile on the left and amullion on the right. Note that glazing beadsare pinned into the rebates in the left and bot-tom rebates. Figure 9.11 is a picture of thejunction of frame stile to the frame sill. Notethat the sill is in two pieces: the stile is fingerjointed to the sill with through fingers, andthere is a groove on the back of the sill to takethe rebate on the timber inner sill.

Figure 9.12 is a picture of the joint of theframe head to the frame stile. Note that this

Fig. 9.11 Casement sill and frame stile joint.

is a finger joint and in particular note howthe fingers on the right-hand side incorpor-ate a scribe to fit the ovolo moulding whichis run on the frame members. There is also aweather moulding planted on the frame headand it appears to be only glued in place. Thisis not usual. It is usual to have a rebate on themoulding to allow fitting it into a groove in thehead.

Figure 9.13 is a picture of the junction of themullion to the frame sill. It has been done witha through tenon on the mullion into a morticein the sill. Note on the left of the mullion thereis the opening for the casement, and the re-bate has been fitted with a P-type compressionweather seal. The light-coloured block on the

Fig. 9.12 Casement head and frame stile joint.

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Windows 211

Fig. 9.13 Mullion and frame sill joint.

right-hand side of the mullion is a soft plas-tic setting block for the double glazed unitswhich will be used. Finally, Figure 9.14 is a pic-ture of the corner of the casement showing avery deep rebate, and it is on the outside. Thewhole window was set up for glazing witha sealed unit in beads from the outside. Thehouseholder will get a window which has thesecurity value of a sealed double glazed unitcompletely negated by having it glazed fromthe outside.

Fig. 9.14 Casement joint.

Timber sash and case windows

Sometimes known as vertical sliding sashwindows, this latter term should only be ap-plied when the frame holding the sashes is notforming a case to contain the counterweightsfor the sashes. This will become clearer as thesketches are studied.

The British Standards previously listed canbe used to set standards for sash and case win-dows. The list of alternative materials to woodgiven at the beginning of this chapter also ap-plies to this style of window, although the win-dows are usually vertical sliding sash type,there being no case and no counterweights oncords.

Although they can be manufactured inhardwood, the traditional material for sashand case or vertical sliding sash windows isEuropean redwood, which in modern practiceis treated with preservative using a vac-vacprocess or similar after the timbers have beenmachined.

The most basic window comprises:

� A frame with head, sill and stiles, the stilesforming open-backed boxes with frontalgrooves in which the sashes slide

� Two sashes, upper and lower, which slidepast each other in grooves in the frameand are counterbalanced by weights on sashcords or chains over pulleys set in the stilesof the frame.

Weights are commonly cast iron although leadcan be used. They are made up in a widerange of standard weights, and small lead‘makeweights’ from 250 g to 1 kg are also usedto adjust the counterweights for their partic-ular situation.

A sash is counterbalanced by a pair ofweights connected to the sash over a pairof pulleys by two lengths of sash cord. Thesash cord core should be of hemp, or nowpolypropylene, with a braided cotton outercover. It is made in a number of strengths andqualities. For extremely heavy sashes a chaincan be substituted for the cord. The chain issimilar in style to a bicycle chain. Obviously

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Not to scale

Not in proportion

Top rail

Stile

StileStile

Stile Stile

Stile

Sill

Bottom sashB

otto

m s

ash

Top

sash

Top sash

Meetingrails

Outside

Inside

Vertical sectionthrough sashes

Horizontal sectionthrough sashes Elevation of outside

Alternative arrangementof meeting rails with rebated, splayed joint

Note: On bottom sash,glass is set in a groove

on the meeting rail. The groove is filled withglazing compound and

the glass pushed up intoit then swung into the

rebates.

Fig. 9.15 Schematic of sashes in sash and case win-dow.

the pulleys for chain are different from thoseused for sash cord and should have a metalsheave on a stout metal axle.

The sashes shown in Figure 9.15 are madein pairs to fit into a single case. Points to note:

� The meeting rails can be splayed or splayedand rebated, the latter being a superiorfinish.

� The sketches show the sashes in their rel-ative positions when closed. Note that thetop sash is the outermost sash. They are re-tained in these positions by the componentsof the case and the counterweights

� While the system of rebates and beads inthe case provides weather proofing for thecompleted window, it is modern practice toinclude seals, similar to the ones we havealready seen in Figure 9.4.

� It is common practice to have an ovolomoulding on the internal exposed arris ofsash members. Decorative mouldings havebeen omitted from these drawings for sim-plicity. They are shown in later sketches.

� To keep the upper sash closed, i.e. at thetop of the case, the counterweights shouldbe a half to one kilogram heavier in totalthan the weight of the sash and its glaz-ing. To keep the lower sash down, i.e. in the

closed position, the counterweights shouldbe a half to one kilo lighter than the sash andits glazing. The sashes are weighed by thejoiner, who then adds the calculated weightof the glass.

The case

The case is made up with a sill to which are at-tached a pair of vertical members for each sideor jamb of the window. A head is fixed acrossthe top of the jambs. The jambs and headhave the general shape shown in Figure 9.16.Note that this shape is made up from five sep-arate timbers.

The sill of the case can be formed from asingle piece of timber, or two or three piecestongued and grooved together to give an over-all shape much as shown in Figure 9.17. Thedouble rebated shape where the lower sashsits is required for weather resistance.

The head and jambs of the case are madefrom five pieces of timber, which are joined inthe specific ways shown in Figure 9.18. Thesketch shows a typical jamb and head. Notehow the inner facings of the jambs have tobe made wider than the outer facings whenthe masonry opening has splayed reveals, adetail common on old thick stone walls.

Figure 9.19 shows three ways in which thesill can be made – in one, two or three pieces.The three piece is the least desirable option.Note the ovolo moulding on sash rails.

Insi

de

Out

side

Varies on whether ahead or jamb detail

Groove inwhich lower

sashengages

Groove inwhich upper

sashengages

Fig. 9.16 Outline of case head and stiles.

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Windows 213

Masonry sill

Groove for innerwindow

sill

Throating

Case sill

Lower sash sill

Grooves forweather bar

Fig. 9.17 Outline of case sill and sash sill in relation tomasonry.

Inner lintelOuterlintel

Inner facingBackplate

Outer facing

PartingbeadBatten rod

Outer facing

Counterweightfor upper sash

Batten rod

Partingbead

Stile backplate

Counterweightfor lower sash

Innerfacing

Spl

ayed

, che

cked

rev

eal

Detail at head

Detail at jamb

Fig. 9.18 Detail of case head and stile in splayed,checked reveal.

Sill of sash

Sill of sash

Sill of sash

One piece sillGenerally used where the sill does not

extend beyond the outer facing of the case

Two piece sillUsed to give extra depth to the rebate for thesash sill and so increase the weatherproofing

effect. The additional depth of the double splayed rebate is beyond the capacity

of some four-sider machines – seeAppendix C

Three piece sillUsed where the sill needs extra depth as well as width beyond the outer facing of the case. The vertical joint in the sill is not a good feature as it could allow waterto penetrate. The joint should be glued with a WBP,

thermosetting glue

Fig. 9.19 Alternative sill build ups.

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The sashes and case together

We must know how the pieces of the sashesand the case are joined together:

� Stiles, sill and rails of sashes are morticedand tenoned together with a single shoul-dered tenon, or they are finger jointed, mod-ern practice using a waterproof glue andwooden dowel or metal star dowel.

� Case facings are tongued, grooved, gluedand nailed to the backplates with ovalbrads.

� Case sills are trenched for the backplates,and these are glued and nailed (and some-times wedged) into the sill.

� Parting beads and batten rods are nailedand glued to the head backplate.

� Parting beads are a friction fit into thegroove in the stile backplates.

� Batten rods are usually nailed to one stileback plate and screwed to the other, thusallowing removal of one batten rod. Withthis removed the lower sash can be swunginto the room. Removal of the parting beadallows the upper sash to be swung into theroom.

� To take the weight of the sashes, duplexhinges are screwed to the fixed batten rod,and round head screws fitted to the face ofthe sash stile, which is then lowered intothe slots in the duplex hinges. With the sashcords trapped in the cord grips, the lowersash can be swung inwards.

Figure 9.20 shows a schematic of this opera-tion.

Figure 9.21 shows details of the upper partof a sash and case window, showing how thecounterweights, pulleys and sashes all fit inthe case. Typical sizes of components for mod-ern sash and case windows are:

� Backplates – 20 mm thick, width dependson circumstance but 185 mm would matchminimum sill

� Facings – 15 mm thick, width depends oncircumstance

� Batten rods – 15 mm thick, 36 to 45 mm wide� Parting beads – 15 mm thick, 19 mm deep

Case backplate

Parting bead

Sash

Round head wood screw

Duplex hinge

Batten rod

Masonry

DPCMasticpointing

This flap screwedto fixed batten rod

The RH flap folds against sash and thensash is lowered so that screw

engages in the slot

SashBack

ofcase

Batten rod

Duplex hinge

LH side of lower sashand case window

RH side of window showingscrew fixing of batten rod

Fig. 9.20 Duplex hinges and operating schematic.

� Case sill, two piece� Sash head – 65 × 65 mm overall� Sash stiles – 65 × 65 mm overall� Meeting rails – 80 × 45 mm overall� Sash sill – 95 mm thick, 185 wide, minimum.

Vertical sliding sash windows

The basic difference between these and thesash and case window is the use of a ‘spiralbalance’ on each side of both sashes to supportthe sashes in either the open or closed posi-tion. The spiral balances are made from alloytube with one end having a hole for screw fix-ing to the case stile. From the other end of thetube there protrudes a flat metal strip whichhas been twisted into a spiral. The end of thetube is finished with a rectangular slot suchthat when the spiral is withdrawn or pushedin, the spiral will revolve. The hidden end ofthe spiral is attached to a strong spring fixed

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Windows 215

DPCMastic

Outerfacing

Stilebackplate

Sash counterweight ofcast iron or lead

Sash stile

Sash cord in groovein back of sash stile

Sash pulley lineInner facing

Stil

e ba

ckpl

ate

Sash cord attachedto stile of sash andcounterweight

Groove in sash stilefor sash cord

Sash pulley screwedinto mortice instile backplate

Ovolo mouldingmitred at joint

Star dowel

m&t joint of top rail to stile

Head backplate

Stile let into trenching inhead backplate

Case stile in splayed, checked revealshowing lower sash and counterweight

Section through stileand head of caseand upper sash.Batten rods and

parting beadomitted for

clarity

Fig. 9.21 Pulley stiles and counterweights.

inside the tube. As the spiral revolves, thespring is wound ever tighter and, if the freeend of the spiral were fixed to the sash, wouldsupport the weight of the sash.

Fixing of the balance to the sash is done witha metal bracket which allows the balance tobe released and refastened to the sash easilyand effectively. The balances are manufac-tured with a range of spring strengths whichaccommodate all but the largest sashes. Thecritical factor in many instances is the use ofdouble or triple glazing.

Figure 9.22 illustrates a typical stile detail ofa vertical sliding sash window and a sash bal-ance. The balance can be housed in a groove inthe sash – most commonly used when fittingbalances to old corded windows. In the de-tail shown of a new or replacement window,the balance is frequently housed in a groove inthe backplate, which is of much thicker timberthan a conventional sash and case window.

Note how the parting bead is a plastic ex-trusion with multiple blade wiper seals be-tween the sashes, and that the sashes havebrush seals between them and the backplate.

The only wood-on-wood contact is where thesashes touch the outer facing and batten rod.This reduces friction considerably and makesthe sash less likely to stick in damp conditions.Paint must not be allowed to clog any of theseseals.

The balance has a hole for an ordinary woodscrew fixing to the stile, and the spiral has a

Hooked end to engagebracket on sash

Steel spiral

Aluminiumtube with

spring

Hole for fixingscrew at bottomof balance

Upper sash

Lower sash

Batten rod

Partingbead

Balance

Balance

Brushweather

sealInner facing

Brushweather seal

Outer facing

Section through stile

Fig. 9.22 Spring balances.

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Balance

Brush weatherseal along meeting rails

Vertical brushweather seal

Vertical brushweather seal

Inner facing

Lower sash

Upper sash

Outer facing

Batten rod

Fig. 9.23 Hinge down sashes with spring balances.

split hook which fits into a bracket fastenedto the corner of the sash. A pair of balancesis used for each sash. Balances are purchasedwhich match the weight of the sash but whichare adjustable by twisting the spiral beforehooking it to the bracket.

Facings are the same sizes as for sash andcase windows; however, the backplates andheads are generally out of 50 or 65 thick tim-bers.

Spiral balances can be fitted to windowsmade from plastics and metals which donot lend themselves to the fitting of counter-weights, pulleys and cords.

An alternative arrangement for this type ofwindow is shown in Figure 9.23. Here the ideais to allow the traditional vertical movementof the sashes but to add the possibility of tilt-ing the sashes inwards (from the top of eachsash) for ease of cleaning. The system has toincorporate a pair of stays for each sash toprevent them tilting completely over, and bal-ances are available which incorporate them.These can be accommodated in rebates cutinto the backplates. The batten rods are hingedon both frame stiles and can fold back clear ofthe stiles. They are held in the operational po-sition by captive thumb screws. When the bat-ten rods are hinged back, first the lower sash

Fig. 9.24 General view of sash and case window.

and then the upper sash can be tilted inwardsfor cleaning.

Figures 9.24 to 9.30 are from various old andfairly new sash and case windows of tradi-tional pattern. Figure 9.24 is a general viewof the upper part of an old window withtwo sashes. Only the top sash opens and hassash cords and sash weights. Figure 9.25 is

Fig. 9.25 Sash pulleys and cord grip.

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Windows 217

Fig. 9.26 Batten rod screws.

taken from inside a more modern window andshows two pulleys in the case backplate. Theleft-hand cord runs through a cord grip whichallows the user to pull the cord and raise thesash weight, allowing the sash to be releasedfrom the cords. Figure 9.26 shows the battenrod removed, and the captive screws whichhold it in position can be clearly seen in therod and the plate they screw into in the back-plate. The parting bead is still in position. Fig-ure 9.27 shows the parting bead removed andthe pocket – the access plate to the weights –which is cut into the backplate. Note howthe pocket cover is cut in the groove for theparting bead. When the parting bead is fit-ted, this together with the lower sash keepsthe cover in place. Maintenance work can becarried out such as replacing cords, addingmake-weights, shortening off stretched cords,etc. Figure 9.28 is a picture of the joint of thesash stile and sash sill. The joint is a morticeand tenon joint with a dowel, which can beseen proud of the face under the paint. Fig-ure 9.29 is the same joint viewed from the side,and the end of the tenon can be seen with thewedges either side to hold it firmly in place.Figure 9.30 is the joint of sash stile to meeting

Fig. 9.27 Pocket in stile open.

Fig. 9.28 Sash sill to stile junction (1).

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Fig. 9.29 Sash sill to stile junction (2).

rail. The meeting rails on this old window hada hook arrangement machined on the matingfaces. The joint is a hand-cut finger joint andone finger and slot is dovetailed.

Glazing

The fitting of glass into openings in doors andwindows has already been mentioned on sev-eral occasions but without much explanationbeyond what is necessary to understand therequirements of our door and window discus-sions.

Fig. 9.30 Meeting rail to stile junction.

Bottom ofrebate

Glass size short

of rebateBack ofrebate

Fig. 9.31 Rebate nomenclature in glazing.

Glass is generally fitted into rebates in thetimber joinery work we have already descri-bed. The glass is always cut a little less thanthe opening measured to the bottom of the re-bates. This allows fitting without forcing theglass into position and so allows the glassto expand and contract without itself or thejoinery being put under stress. It also allowsthe joinery to move (moisture content, centralheating, etc.) without putting the glass understress. Figure 9.31 illustrates this point.

For ordinary glazing work

Glass was at one time measured by weight –ounces per square foot – but is now measuredby thickness in millimetres. There were a largenumber of different qualities of glass but thesehave now been cut down to just float glassand polished float glass. Float glass is usedfor general glazing such as we have previ-ously referred to. Polished glass is used formirrors, table tops, etc. Glass comes eitherclear or obscured, the degree of obscuritydepending on the pattern which has beenimpressed into the molten sheet during man-ufacture. There are not quite as many patternsavailable now.

The basic method of glazing is done in join-ery which is to be painted, and requires a sim-ple rebate primed to seal the timber and pre-vent it from absorbing the oils in the glazing

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Windows 219

Glass

Timber

Sight line

Putty finished below sightline. This is called

fore putty

Small oval wire brads areoften used but should be

galvanised

Compound

Edge of firmer chiselused to drive in

glazing sprig

This compound iscalled back putty

The compound is allowed to dry off and thenpaint is applied from the sight line on the glassand over the compound, over the timber

Surplus back puttycleaned off and surface

pressed down

1

3

2

4

5

The glazing in compoundsequence

Paint

Glazing sprig

Fig. 9.32 Glazing in compound.

materials. These can be linseed oil putty orglazing compound – an improved versionwhich does not dry out and crack quite soquickly. There are a number of distinct stepsdescribed below and illustrated in Figure 9.32:

(1) Place a generous bead of compound in therebate.

(2) Press the pane of glass into that bed ofcompound ensuring that it is evenly bed-ded in and that the compound is squeezedup between the back of the rebate and theglass.

(3) Drive glazing sprigs into bottom of rebateto secure glass in position.

(4) Apply more compound in front of theglass and smooth off to an even slope,pressing it in to give an even, firmdensity.

(5) Remove surplus compound from the in-ner face of the glass.

Once this is completed the glazier’s work isdone but the painter has to paint over the com-pound and up the front of the glass in order toseal the compound and the joint to the glassagainst weather.

Glazing in beads is commonly done wherethere is a need for greater security, the glassis very heavy, in an exposed situation or thetimber has to be varnished or polished. Some-times the beads and the glass are beddedin compound, sometimes in silicone glazingcompound, sometimes in wash leather2 stripsor velvet ribbon, and sometimes in a syntheticrubber or plastic gasket.

In compound, the steps are:

(1) Place compound in the rebate as backputty.

(2) Press glass into the back putty and presshome.

(3) Spread a little compound against glass asa fore putty.

(4) Press bead against the fore putty, squeez-ing it up the face of the glass but not trap-ping any under the bead. Fix beads withglazing pins or wood screws.

(5) Clean off surplus compound.

The effect is to surround the glass with com-pound and have the beads secure it all inposition.

In velvet or wash leather, the steps are:

(1) Place strip of velvet in rebate with enoughupstand to cover the back of the rebate.The pile of the velvet should face the glass.

(2) Place glass against the velvet and securewith two temporary sprigs on the sides.

(3) Turn velvet up the front of the glass onan edge without the sprigs and press beadinto place and secure with pins or screws.

2 Wash leather was originally natural leather from thechamois, an Alpine antelope, but was also manufac-tured from the leather made for other animal hideswhich were split to give the correct texture. There isa synthetic chamois leather available now.

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(4) Turn velvet up on an adjacent edge andsecure with the bead, having removed thetemporary sprig.

(5) Treat adjacent sprig-free edge.(6) Remove last sprig and treat last edge.

Note that this is an entirely dry exercise –no compound, glue, silicone or anything likethat. The glass is literally gripped firmly be-tween the pile of the velvet or the soft washleather. The velvet comes as a ribbon but thewash leather has to be cut with a sharp knifeinto strips.

Velvet or wash leather is no use for externalglazing and is used mainly for joinery workto cupboards and cabinets.

In silicone glazing compound, the steps are:

(1) Place spots of compound in the corner ofthe rebate. For a 15-pane door, four spotsper pane would be sufficient.

(2) Press glass into the rebate.(3) Press beads into the rebate, pushing the

compound off the back of the rebate andup against the glass. Fix beads with pinsor screws.

(4) Clean off surplus silicone.

This must be one of the favourite methods offixing the small panes in a 15-pane door. It isquick and easy, especially if pins are used tofix the beads and are driven home with a pinpush.

In gaskets with beads, the steps are:

(1) Cut gasket to length for each rebate andmitre the corners or cut length of gasket forcomplete glazing length and form mitredfold cut-outs.

(2) Place gasket around the edge of the glass.If an external window or door, bed thegasket onto the edge with a suitablecompound; usually a silicone-based com-pound is used.

(3) Place bedding compound layer on backand bottom of rebate.

(4) Bed the glass and gasket in place.(5) Fit beads against gasket, compressing the

gasket, and fix in place with pins orscrews.

The type and design of the gasket will deter-mine whether or not to apply a sealer at eachstage. There are gaskets which can be fittedentirely dry.

In gaskets without beads the gasket is intwo pieces. The first is pressed home into agroove in the rebate and has a front whichhinges over to allow the glass to be pressedinto place. Finally a locking strip is pressedinto the hinged portion which prevents thegasket hinging open and so retains the glass.The locking strip is a very tight fit to com-press the gasket against both the timber andthe glass.

This is very unusual in timber framed open-ings but is common enough in plastics andmetal windows where, instead of being set ina groove in a rebate, the gasket fits over a thinfin much like the glazing methods used in theautomobile industry.

From a security point of view, fixing withbeads and gaskets with locking strips can onlybe done if the rebate is on the inside of thedoor or window. There have been instances ofburglars gaining entry by simply unscrewingthe fixings for the beads and taking out thepane of glass.

Figure 9.33 shows some of the abovemethods.

Raised cs headwood screw

Bead

Compound

Outside


Bead

Only oninternaldoorsSilicone

spots

Outside


Bead

Gasket

Lockingstrip

Gasket

Glass

Timber

Outside

The illustrations of gaskets are schematic only

Fig. 9.33 Glazing alternatives.

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10 Stairs

Landings 222Steps 222Balustrades 223

Measurements 224Joining steps to stringer 225Winders 227

This chapter will be confined to simple domes-tic stairs within a single dwelling. While thislimits us to basic timber stairs it is nonethelessquite complex because of the Building Regula-tions and in terms of the joinery and precisionmeasurement required.

Like doors and windows, stairs in housingwere an early contender for the factory-builtapproach, and while bespoke stairs are stillbuilt in numbers, that is nothing compared tothe thousands built in factories up and downthe UK. But factory-built or not, built downto a price or not, stairs are still manufacturedin much the same way as they were 100 andmore years ago.

Some definitions to begin with. A stair isthe correct term for that which we use to gainaccess to a different level within our homes.A stair can comprise a flight or flights ofsteps, and if flights are involved there hasto be a landing(s). A flight is a series ofsteps supported each end by deep but rel-atively thin timbers called stringers. A sin-gle step comprises a horizontal board – theone you stand on – called a tread, which inturn is attached to a vertical piece of timbercalled a riser. The tread and riser are con-nected by a rebated and grooved joint, andthe ends of the step so formed are housedinto an L-shaped groove cut in each stringer,where they are glued and wedged into po-sition. Angle blockings are glued all round

the underside of the step between tread andriser, tread and stringer, and riser and stringerto reinforce the joints. Screws may be usedin these blocks but nails should not be usedin any part of a stair; when the timber driesand shrinks away from the nail it will squeakas people use the stair. The tread overlapsthe riser in each step and the projecting edgeof the tread is rounded off; there may alsobe a cavetto moulding applied at the joint.The projection of the tread is called a nos-ing. Figure 10.1 illustrates the points made sofar.

The strings support the steps and in turn re-quire support. This is provided by the joisting,trimmers and trimmed joists of the opening inthe upper floor and, at the bottom of the flight,by the joists etc. of the lower floor.

In narrow stairs that construction is usuallyadequate without further support, but wherethis is required one or even two supports maybe added. This comprises a strong piece ofsawn timber running parallel to the undersideof the stair flight, with shaped timber brack-ets fixed on alternate sides and pressing onthe underside of the tread. The ends of thetimber are supported by the floor structuresmentioned above

The compartment which contains the stair isthe staircase. Do not adopt the habit of refer-ring to the stair as the staircase – you cannotclimb a staircase, not without a stair.

221

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Tread

RiserGoing

Ris

e

Tread

Optionalcavetto

moulding

Nosing

Riser

Angle blockings The shaded tread and riser make up a step

NosingPitc

h line

Tread

Upper level flooror landing

Lower levelfloor or landing

String

StepA flight

Stringsupported

here

Stringsupported

here

Fig. 10.1 Illustration of some definitions associatedwith stairs.

Landings

Where a stair is in two or more flights forwhatever reason, there has to be a landing(s).These are intermediate platforms where theuser can rest or the stair will change direction.Landings obviously must be the same widthas the stair but must also project that sameamount in the direction travelled on the flight.Landings are constructed in the same way asfloors, with joists, a floor boarding and a ceil-ing finish. The joists and a trimmer joist on theopen edge are sometimes built into the stair-case walls, or runners are fixed to the wallsand the joists housed into the runners. A flightpressing against the open edge of a landingexerts considerable pressure and it is prudentto arrange the joists supporting the landing torun in the same direction as the travel on thestair. These joists then prevent the open edgeof the landing from being pushed back by theload on the stair. This is particularly importanton a half landing.

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

Half landing

Quarterlanding

Runninglanding

UP

1

2

3

45

6

7 8

9

10

11

12

13

14

Fig. 10.2 Flights and landings.

Landings may be quarter landings or halflandings (see Figure 10.2). The Regulationsonly allow 16 steps maximum in any flight.Should a stair be in excess of 16 steps, it mustbe broken into two or more flights with run-ning landings in between. The length of a run-ning landing must be at least the same as thewidth of the flight.

Steps

All steps in a stair must be of exactly the sameshape and dimensions.

There are conventions on how stair informa-tion is presented on drawings. The three mostimmediately important are:

� Steps only are always numbered. Landingsand main floor levels are not numbered

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Stairs 223

� The numbering of steps always begins withthe lowest step in the staircase

� The direction of travel up the staircase isshown at the lowest level on any one draw-ing by using an arrow pointing up the stairand with legend up attached.

Figure 10.2 illustrates the above points.What should be obvious to the more obser-

vant reader is that any flight starts and pro-ceeds with whole steps, until it reaches thenext level up, be it landing or upper floor. Atthis point an additional riser is inserted. Forexample, a flight of 7 steps will have 8 risersand 7 treads.

Stairs may be built into the staircase in sucha way that the strings may touch the wallson both sides, on one side, or be completelyunattached. Strings against the wall are calledwall strings; strings not touching the wall arefree-standing strings. Wall strings can be at-tached to the wall over their length and so canbe manufactured from thinner timber. Freestanding strings can be shaped so that theend of each step projects over the string. Suchstrings are termed open cut strings and theycan be done in many different ornamentalstyles. Figure 10.3 is a photograph of an opencut string stair being fixed into position. Notethe projection of the tread over the string andthe curved plate mitred to the riser. See alsoFigure 10.4.

Fig. 10.3 Open cut string stair being installed.

Riser face

Nosing

Nosing

MitrePlate mitred to riser

and planted on string

StringEnd oftread

End oftread

Fig. 10.4 Open cut string details.

Balustrades

With a free-standing string a rail is requiredto ensure the safety of the users. This iscalled a balustrade and comprises a handrailsupported on balusters, all exposed and fre-quently elaborately turned on a lathe. Alter-natively, a frame may be erected on the stringand panels placed in the frame, or it is boardedover with sheet material, matchboarding, etc.

With balustrades of either style there is aneed to support the ends of the handrail, andthis is done by fitting newel posts top andbottom of the flight. These are generally boltedat the bottom to the joist work of the floors orlandings, with coach bolts, the heads being setin counterbores and pelleted over.

Newels can also be used to support the free-standing string top and bottom of the flight.Mortices are cut into the newels, and tenonson the string are arranged to fit these ratherthan housings in the trimmer face of a landingetc.

With two wall strings only a handrail needsto be provided, and here the handrail is fixedto metal brackets screwed (and plugged) tothe wall. Sometimes a handrail profile is usedwhich can be screwed to the wall withoutbrackets. Handrails must be fitted in such away that catching a hand, fingers or clothingis not possible. Apart from the injuries whichcan be caused, this could also impede anyonetrying to use the stair as an escape route in the

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NewelHandrail

Balusters

String

Stringcapping

Trimmer

Joist orbearer

Floor

Landing

12 coachbolt

12 coach boltcounterbored and

pelleted over

Handrail housed

into newel

Handrail housedinto newel

String housed into mortice in

newel

String housed intomortice in newel

Count the steps, treads and risers

5

4

3

2

1

Total going

Tota

l ris

e

Fig. 10.5 Balustrade, newels, total rise and total going.

event of fire. Rounding off ends, turning endsinto the wall, leaving sufficient gap betweenwall and handrail and so on can all be used toease a potentially dangerous situation.

Figures 10.5 and 10.6 illustrate some of theabove points.

Measurements

Measurements on a stair are critical. If onedraws an imaginary line along the centre ofthe flight, touching the nosing of each step inturn, we have created the pitch line. The an-gle this line makes with the horizontal is thepitch angle. In domestic internal stair design,the pitch angle is kept to between 38◦ and 43◦.

Two more measurements are the going andthe rise, both of individual steps and of thecomplete flight, the latter being frequently

Handrail

Balusters housedinto handrail and

string cappingPellet

Counterbore

12 coachbolt

Pellet glued into counterbore

and ready to flush off

Stringcapping

String

Fig. 10.6 Balustrade detail, newel fixing detail.

referred to as total rise and total going (seeFigure 10.5). Going is measured from the faceof one riser to the face of the adjacent riser. Riseis measured from the surface of one tread tothe surface of the adjacent tread. The ratios ofrise and going are used to control the pitchangle under the Building Regulations.

When measuring up for, making and in-stalling a stair it makes quite a difference tothe ease of using it if the treads are not trulyhorizontal. It is better if they slope in the direc-tion of travel down the flight. The amount ofslope is miniscule – an eighth part the authorwas told – the reference being to one eighthof an inch or 3 millimetres across the treadwidth. The Regulations state that treadsshould be level, but the assumption has to bethat this means laterally – from string to string.

The relative dimensions of going and risefor individual steps in the Regulations forEngland and Wales are:

� Rise shall be no less than 155 nor greaterthan 220

� Going shall be no less than 245 nor greaterthan 260

� Selection of any sizes shall not make thepitch angle greater than 42◦.

The Scottish Regulations are quite similar butthere is a classification of stairs into privateand any other stair. Stairs should have a min-imum width of 1000 between handrails andbe free of obstructions; however, the stairs weare discussing here fall into the private cate-gory and again there is a choice. A private stairmay be as little as 900 between the handrailand clear of all obstructions, may be only 600when it serves only one compartment (but nota kitchen or living room) and/or sanitary ac-commodation, and 800 elsewhere. Maximumrise for a step in a private stair is 220 and min-imum going is 225 with a maximum pitch of42◦. Combining these extremes of rise and go-ing results in a pitch line in excess of the max-imum allowed – approximately 44◦ 20′.

Once one gets through all the civil servicespeak in the Regulations, there doesn’t seemto be much difference between them.

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Stairs 225

Joining steps to stringer

Earlier, methods of joining the steps to thestringer were mentioned, but they warrantsome further illustration and explanation. Fig-ures 10.7 and 10.8 are photographs of a stairsitting on its side in a house waiting to be in-stalled.

The string is around 32 thick, the tread isaround 25 thick, and both are of solid timber.The riser is of hardwood plywood and is only9 thick. The plywood is nailed and glued to theback edge of the tread and glued into a grooveon its underside. Blockings are fitted but areglued and power nailed. Despite appearanceson the side that shows, this is not a conven-tional stair by any means. Tread and string arefairly good but the riser of plywood and thenailing, even with a power nailer, are causefor concern with regard to a creaky staircase.That said, the nails could be of an improvedpattern and therefore less prone to slippage inthe timber.

On Figure 10.7 the joint of steps to stringeris illustrated. Remember that a step comprisesa tread and the riser immediately below it,glued and screwed together with triangularblockings. One can see the groove which hasbeen cut in the inside face of the string andinto which each step is placed – with glue.Once all the steps are in place they are joinedtogether by nailing the riser of the step to theback of the tread below – with more glue. Then

Fig. 10.7 Step to stringer joint.

Fig. 10.8 Ends of stair stringers.

long wedges are driven into the remainder ofthe groove, which forces the tread and riserof each step in turn to close up to the upperedge of the groove, and also puts the glue lineunder pressure. This is all done with the stairin the position shown in the photograph.

The next stage is to fit the other string. Glueis spread in the grooves and the string isbrought down over the ends of the steps. Sashcramps are placed across the stair and tight-ened, which forces the step ends into their re-spective grooves. The stair can be turned overnow and the wedges driven into the secondstring. Once the glue has set, the sash crampsare removed and the stair is ready for delivery.

A couple more points about that stair:

� When the steps were made, angle blockswere fitted between tread and riser but theywere nailed as well as glued. One can beseen in Figure 10.7 at the centre/top of thepicture

� The wedges used are quite wide and wouldprevent the placing of angle blocks at thejunction of tread to string and riser to string.

In Figure 10.8 we have a more general view:part of the upper surface of the stair, a view ofthe bottom end and a view of the two stringends. The string at the bottom of the picture

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1

2

3

4

5

678910

1

2

North wall

East wall

Plan of three winderstair

Alternative bottom steppossible with an open cut

stringer

String elevation onnorth wall

String elevation oneast wall

Schematic of arough carriage

Bottom edge of string

Bottom edge of string

Roughcarriage

Brackets on

alternatesides

Clearance

Bracket setat angle

under tread

Wedge glued in place Tread

Ris

er

Rough carriage

Alternativebracket fixing

7

8

9

6

5

4

3

6

5

4

3

Fig. 10.9 Winders and rough carriage.

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Stairs 227

has its end prepared for housing into a newelpost; the other string has plain ends angledoff to simply sit on the floor as this is a wallstring and will be fixed back to the wall for itssupport. The tread nearest the camera is thebottom-most tread of the flight and one wouldnormally expect to see a riser there. However,as one string is going into a newel, this riserhas been supplied loose for fitting when theflight goes into place. One wonders aboutthe blockings and the gluing of the riser intothe tread – the groove can be seen.

Winders

A single stair flight can change direction with-out being broken into two flights with a land-ing. The device used is the winder or kite step.The treads are of triangular shape and thestring round the outside of the turn is shapedto accommodate the wider ends of the kite

steps and the shallower pitch angle which thiscauses. At the inside of the turn the two piecesof straight string are normally housed into anewel post. This is what the stair in these pho-tographs will do.

Winders are covered in the Building Regul-ations. Rise presents no problems, but whereto measure the going? That is done along thecentre of the tread, and thereafter the samerules about ratios apply. One provision is thatthe winder steps shall not be less than 50 wideat the narrow end. Since tread measurementdoesn’t include the nosing projection, wecan only assume that it is not included inthe 50.

The usual number of winders is three butthe author has had a stair built with an opencut string and (complying with the Regula-tions) with four winders. It was wider thanthe minimum we now have of 900.

Figure 10.9 shows some more constructiondetails.



11 Mutual Walls

Transmission of sound 228Calculation of surface density 228

Wall types 229Fire resistance 231

Mutual walls are those walls which areshared between adjacent houses in a terrace –party walls, and occasionally known as mu-tual gables since the division is built completeto the very near underside of the roof cover-ing. The law everywhere has a lot to say aboutparty walls but here, thankfully, we are onlyconcerned with those aspects of safety andcomfort covered by the Building Regulations.

Two aspects are covered in the Regulations:

� Transmission of sound� Fire resistance.

We will attempt to distil some of the main re-quirements of the Regulations in order to givethe beginner to construction an insight intowhat will be finally required of him or her.

Transmission of sound

The transmission of sound has to be limitedotherwise neighbours would be complainingabout each other. This does still happen butonly when one party is extremely noisy, some-thing against which it is impossible to leg-islate. So, what do the Regulations require?They require that the amount of noise whichpasses from one house to another is modi-fied down to a particular level. The amountof sound is measured in decibels. The levelsrequired are not important in this text as weshall concentrate simply on the stock solutions

available. Modifying the level of sound trans-mitted is called attenuation.

Noise can be carried through the party wallor round the wall. The first is known as di-rect transmission and the second as flank-ing transmission. Just to complicate thingsfurther, sound can also be generated by im-pact upon the structure, e.g. footsteps, vac-uum cleaning, children playing, etc. and thiscan be directly transmitted or a flanking trans-mission. Then there is noise generated by peo-ple talking, radios, hi-fi systems and so on,which can also be transmitted as direct orflanking noise.

We will look at the more obvious permuta-tions a little later as we look at possible solu-tions to transmission problems.

Calculation of surface density

To attenuate direct transmission, the first ef-fort is usually placed on putting mass into thewall, and this should be relatively easy with amasonry construction. However, the Regula-tions are never quite straightforward and theyrequire minimum densities for the bricks orblocks and the mortar used in the wall’s con-struction. In the Regulations this is 375 kg/m2

for solid brick and 415 kg/m2 for solid con-crete block walls, and there is a procedure laiddown which has to be followed in calculatingthat mass – which may or may not include

228



Mutual Walls 229

any plaster finish or isolated panels of finishon each side. ‘Isolated’ means the panels willnot touch the masonry core and should onlybe fixed at floor and ceiling.

What is meant by a mass per square metre?This is termed a surface density and is theproduct of the volumetric (bulk) density ofthe material used and its thickness. For exam-ple, we wish to use a 200 thick concrete blockto build a party wall. The block has a volumet-ric (bulk) density of 1750 kg/m3. We proposeto finish the wall each side with 13 thick gyp-sum plaster. The Regulations allow the calcu-lation of plaster surface density as:

(1) Cement render 29 kg/m2

(2) Gypsum plaster 17 kg/m2

(3) Lightweight plaster 10 kg/m2

(4) Plasterboard 10 kg/m2

The Regulations provide formulae for calcu-lating the surface density from the above data.For composite walls – i.e. those with a finishapplied to a core – the formula is:

M = T(0.93D + 125) + NP

where M = the mass of 1 m2 of leaf in kg/m2

T = the thickness of unplasteredmasonry in metres

D = the bulk density for the masonryunits in kg/m3

N = the number of finished facesP = the mass of 1 m2 of wall finish

in kg/m2

So, if we put some of the above values into theformula we would have:

M = 0.200 × ((0.93 × 1750) + 125) + (2 × 17)So, M = 384.5

This proposal does not comply with the reg-ulatory surface density of 415 kg/m2. It is notfar short, so what can be done? The use of a ce-ment render would bring the surface densityto just over the limit of 415 kg/m2, as wouldthe use of a slightly denser concrete block. Forthe latter we could consider the use of a blockof 1800 kg/m3:

M = 0.200 × ((0.93 × 1800) + 125) + (2 × 17)M is now = 393.8

This is not a lot different and still fails tocomply. In fact, the best solution is to use acement-based render with the original blocks,or use a very much higher bulk density ofblock.

Wall types

Wall types are given in the Regulations (theScottish Regulations’ ’Deemed to Satisfy’ con-structions) and if followed will ensure compli-ance.

Wall Type 1 is a solid masonry wall of brickor concrete block with a minimum volumet-ric density of 1500 kg/m3. Wall finish eitherside is not required but would of course beusual. Obviously, from our calculation above,a wall with the minimum density of block,1500 kg/m3, would require a leaf of more than200.

Wall Type 2 is a cavity masonry wall. Thereare a number of possibilities:

� Half brick thick leaves with a 50 cavity be-tween and 13 plaster each face

� 100 concrete block leaves with a 50 cavityand 13 plaster each face

� Half brick thick leaves with a 50 cavity be-tween and 12.5 plasterboard lining – any fix-ing method, provided there is a step and/ora stagger in the alignment of the adjacenthouses

� 100 concrete block leaves with a 50 cavityand 12.5 plasterboard lining – any fixingmethod, provided there is a step and/ora stagger in the alignment of the adjacenthouses.

Figure 11.1 illustrates the construction ofTypes 1 and 2 and the idea of steps and stag-gers in schematic form.

Wall Type 3 is a masonry wall between iso-lated panels. The masonry core provides a cer-tain amount of mass but the isolating panelbrings in another method of attenuation, theflexible and absorbent layer.

Four alternatives are listed, each with a min-imum surface density and clad with one oftwo possible isolated panels:




Step

Stagger

Wall Type 1

No finish Plastered one or both sides

Wall Type 2

Plastered both sides

Wall Type 2

For walls with steps and/or staggers

12.5 plasterboard on plaster stripes

12.5 plasterboard on timber strappping

50min.

For minimum thicknesses of masonry, see text

Masonry

Fig. 11.1 Wall Types 1 and 2.

(1) Brickwork 300 kg/m2

(2) Concrete blockwork 300 kg/m2

(3) Lightweight aggregate blockwork200 kg/m2

(4) Aac blockwork 160 kg/m2

The panels:

(1) Two sheets of plasterboard joined with acellular core 18 kg/m2

(2) Two layers of plasterboard placed in posi-tion broken jointed and with or without aframe. If there is a frame, two sheets of 12.5plasterboard are sufficient, but without aframe the total thickness of plasterboardmust be 30, i.e. 2 × 15 thick.

Wall Type 4 is one example only of the typesof wall which can be used with timber framehousing. The type given is basically the tim-ber frame walls of each house separated by aclear space of 200 between their facings andwith a curtain of mineral wool suspended be-tween both timber frames. The mineral wool isabsorbent and so attenuates the transmissionof sound very well. It should have a density

of 12–36 kg/m2. The facings would normallybe of plasterboard in two layers, fixed brokenjointed and with a minimum thickness of 30.The inner face of the timber frames may becovered with plywood if there is any struc-tural requirement.

A masonry core can be built between thetwo timber frame panelled areas but it canonly be physically connected to one frame andits role is limited to one of structural support.It plays no part in the attenuation of sound. Itcan, however, play a part in the fire resistance.

Figure 11.2 illustrates the construction ofTypes 3 and 4 mutual walls.

In addition to the requirements discussedso far, the construction of these walls does re-quire attention to other details.

Solid connections across cavities or voidsmust be avoided. In masonry cavity wallsthis might pose a problem with the provi-sion of wall ties. However, tests have shownthat while solid galvanised steel wall ties arenot acceptable, butterfly wire ties are flexibleenough not to upset the precautions againstsound transmission. It must presumed that



Mutual Walls 231

200 min.

Mineral wool

Masonry

Joist in PS

hanger

Runner

Two x 12.5 plasterboardWall Type 3

Wall Type 4 -- Horizontalsection

Wall Type 4 -- Vertical section

Two x 12.5 plasterboard on timber frame

Two x 12.5 plasterboard

with cellular core

These finishes fixed only top and bottom

Noggings

Noggings

Noggings

Coach screw

Fig. 11.2 Wall Types 3 and 4.

flexible plastic wall ties might perform in asimilar manner although they may not be al-lowed from a fire resistance point of view.

Holes are not allowed in mutual walls suchas we are considering here. This of courseincludes the incidental formation of holesdue to incomplete bedding of masonry. Bedsand perpends must be fully filled. Holes andcracks may also form when building materi-als or components shrink and crack. Wherethere is a danger that this might happen, theuse of a flexible filling compound might beconsidered appropriate, and where the detailis combined with a need for fire resistance, in-tumescent compounds would be even better.

Equally, cavities which have not been de-signed in must be eliminated. For example,bricks with large frogs must be laid frog up,and perforated or cellular bricks, where mor-tar cannot penetrate into the cell or hole,should not be used even if laid cell facing up.

Much of what has already been written ap-plies to sound passing through the structure

directly to the next house, but flanking soundcan be structure borne as well as air borne.The Regulations give detailed guidance to therequirements necessary to avoid this.

Fire resistance

Having dealt fairly simply with building amutual wall which provides sufficient atten-uation of sound travelling across it we mustalso know how to resist the passage of fire.

Looking again at the various wall types, wecan see that Types 1, 2 and 3, with their ma-sonry core or leaves, are inherently fire resis-tant, and provided attention has been paid tothe sound-related problem of holes and crackswhich would allow fire to penetrate, thenthere should be no problem providing ade-quate fire resistance. Type 4 walls might seeminadequate when compared with the othersbut the idea of separating the frames of thehouses with a mineral wool quilt, or building




that into the panel structure of one house,is perfectly adequate, especially when takingthe effect of char on the substantial structuraltimbers. The mineral wool quilt is generallymade up with a galvanised wire mesh re-inforcement which holds the fibres together,preventing the quilt form disintegrating toosoon. In the Regulations the Type 4 wall isshown as two distinct sets of studs, each ina different plane. This is done to emphasisethe need to maintain the 200 gap between theface materials. However, provided that gap ismaintained there is nothing to prevent alter-nate studs in both frames from interlinking.Provided the noggings don’t touch the studsopposite and the mineral wool blanket can bewoven between the studs, it will be all right.Figure 11.2 shows the arrangement.

From a fire point of view, the support ofthe upper floor(s) is best done by the otherwalls, but where other design considerationstake over, then there are definite rules aboutwhat do to support the floor joists and othertimbers. Runners supported on brackets orRawlbolted to the wall are good, providedneither brackets nor bolts penetrate the skin

of masonry. Runners can also be used on tim-ber frame mutual walls, and although the de-tail cannot be backed up independently, theauthor would suggest that the runner shouldbe placed on top of the face material and notbreak the run of it down the timber frame wall.Heavy gauge joist brackets built into the ma-sonry wall are another solution. All of thesehave been shown earlier in Chapter 4, Tim-ber Upper Floors. The one thing that cannotbe done is to build the joists into the masonry,and it is for that reason that it would seemlogical to keep joists from getting close to atimber frame mutual wall. Figure 11.2 showshow the runner might work on a timber framemutual wall.

For an informative web site try http://www.british-gypsum.com, where you willfind up-to-date information on the use oftheir products in solutions for all types ofsound proofing and fire resistant construction.It would appear that Lafarge manufacture ahoneycomb core panel under the name Panel-plus, although their website does not mentionit. However, for more general information tryhttp://www.lafargeplasterboard.com

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12 Plumbing and Heating

Pipework 233Pipe fittings – couplings and connections 234

Range of fittings 239Valves and cocks 241Services generally 243Hot and cold water services 243Soil and ventilation stacks 246

Overflows 246Water supply from the main 246Equipment 247

Cold water storage cisterns 248Hot water storage cylinders 248Feed and expansion tanks 251

Central heating 252

Remembering that we are confining our studyto domestic walk-up construction, our text inthis chapter will be confined to a fairly basiclook at the current materials and techniquesapplied to the provision of hot and cold wa-ter supply, water-borne heating systems andsystems for waste disposal within one- andtwo-storey housing.

All water installations are subject to the pro-visions of the Model Water Byelaws. To dis-cuss the provisions of these in detail wouldbe impossible in this text; however, the workdescribed here will comply with these provi-sions. The byelaws are particularly concernedto prevent waste, misuse, undue consumptionor contamination of the water supply. Furtherbyelaws are published by the various supplycompanies or authorities across the UK, andthese supplement or expand on the Model Wa-ter Byelaws. It is important to comply withboth sets of byelaws. For further detail on thebyelaws refer to the website of the local sup-plier or authority.

Pipework

To begin, we will look at the materials andcomponents used. The most common material

is pipe and the fittings required to join pipeto pipe and pipe to appliances and equip-ment. Pipe is manufactured in a wide range ofmaterials, of which those listed in Table 12.1have relevance in modern plumbing.

On cost, the use of cast iron is confined tothose parts where the strength of the mater-ial is important, e.g. exposed lengths of pipewhich might be the subject of vandalism. PVCpipe can be damaged quite easily whereas castiron is much more resistant to attack. At thetime of writing, the cost of cast iron pipe wasroughly twice that of uPVC. Cast iron was alsoused for waste disposal, the size of pipe being2 in (50), with smaller bore copper pipe (32 and40) being used to connect sanitary appliancesto the 2 in cast iron. The advent of plasticswaste pipework at a much reduced cost hassuperseded the use of cast iron and copper inthis way. Just for comparison, 50 copper pipework is roughly three times as expensive as50 ABS pipework.

On top of the obvious cost savings in favourof using plastics for waste disposal pipework,there are two other factors involved. First, thefittings for cast iron and copper pipe work arevery expensive compared to plastic fittings,and second the plastic pipework and fittingscan be put together much more quickly and

233

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Table 12.1 Materials used for pipe.

Material Use Sizes used (mm)

uPVC (unplasticisedpolyvinyl chloride)

Disposal pipework from WCs andventilating stacks

100 nominal, 80 nominal

Ducts through walls for service entryDisposal pipework from sinks, WHBs,

showers and baths

32, 40 and 50

Cast iron Disposal pipework from WCs andventilating stacks

100 nominal

cPVC (chlorinated PVC) Hot and cold water supplyCentral heating pipe work

15, 22, 28

ABS (acrylonitrilebutadiene styrene)

Disposal pipework from sinks, WHBs,showers and baths

20, 32, 40 and 50

Polypropylene Disposal pipework from sinks, WHBs,showers and baths

20, 32, 40 and 50

PE-X (cross-linkedpolyethylene)

Hot and cold water supplyCentral heating pipework, particularly

underfloor heating∗

15, 22, 28

Polybutylene Hot and cold water supplyCentral heating pipework∗

15, 22, 28

Copper Hot and cold water supplyCentral heating pipeworkGas supply pipe

15, 22, 28

Stainless steel Hot and cold water supplyCentral heating pipeworkGas supply pipe

15, 22, 28

Black iron Central heating pipework 15, 22Galvanised iron Gas supply pipe 15, 22 and 23

∗When used for central heating pipework, ordinary plastics pipe will allow oxygen to diffuse in through thepipe wall. A special grade of pipe is manufactured with an oxygen barrier in the wall of the pipe.

without the use of heat. This is of prime im-portance to any developer who must keep acontract moving swiftly to a conclusion. Anypractice which takes up a lot of time has to beeliminated and the use of cast iron and copperfor waste disposal does fall into that category.

Stainless steel pipework has fallen out offavour for domestic pipework, largely due tothe cost of the special fittings required. Beinga much harder material, ordinary pipe fittingsused for copper pipe cannot be used on stain-less steel. Also, the price advantage that stain-less steel enjoyed has now disappeared.

Pipe fittings – couplingsand connections

When joining pipe to pipe, plumbers use cou-plings. When joining pipework to appliancesor equipment, they use connections.

Couplings fall into a number of categories,depending on the way the coupling worksand, occasionally, the type of pipe to be joined:� Compression fittings for copper, black iron

and plastics pipework� Capillary fittings for copper pipework

� End feed soldered joints� Integral solder ring joints

� Push fit fittings� For copper and plastics pipework� For plastics waste pipework

� Solvent welded fittings� Socket fusion and butt fusion joints� Electrofusion.

Compression fittings

Compression fittings are made of either brassor bronze, the latter being more expensivebut necessary to overcome ‘dezincification’

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Plumbing and Heating 235

caused in some areas with ‘aggressive’ water.Brass is an alloy of copper and zinc, and wherethe water in a supply is aggressive it will re-move the zinc from the alloy and leave behinda porous copper structure which will initiallyweep water then fail completely. Also, fittingson external supply piping which is buried inthe ground and has aggressive ground wa-ter, will be attacked. The resistant fittings aretermed DZR.

Compression fittings are, principally, of twotypes. The first is where the end of the pipebeing joined is manipulated in some way, themost usual method being flaring the pipe andsqueezing this flared end between the parts ofthe fitting. A variation on this is to insert a toolinto the end of the pipe which, when turned,raises a ridge on the pipe which gives thejoint mechanical strength. The second is a non-manipulative fitting. Here the pipe is merelycut square across and the fitting assembledon the ends, the mechanical strength and wa-ter seal being made by squeezing an internalring in the fitting on each pipe end. As a gen-eral rule, the manipulative fitting can be usedon copper pipe and the non-manipulative fit-ting on plastic and copper pipe. Care has to betaken to select a fitting which the pipe manu-facturer recommends.

Jointing compound or sealing tape must bewound round the metallic parts of the sealwhen joining copper pipe, and tape only isused with plastics pipe. The tape used is athin 12 wide tape of polytetrafluorethylene(PTFE). The tape is soft and stretches as it iswound into place. When pressure is applied asthe joint is tightened, it flows into any spacesand seals the joint completely. Note that ordi-nary PTFE tape is unsuitable if the pipeworkis to carry natural gas. A special tape is manu-factured for that purpose.

Figure 12.1 shows an exploded photographof a non-manipulative compression fitting be-ing assembled on copper piping on the left,and plastic piping on the right. Note the bodyof the fitting fitted over the end of the cop-per pipe and on each pipe the compressionring and the compression nut ready to slideup and squeeze the ring down onto the pipeonce the sealing compound or tape has been

Fig. 12.1 Compression fitting on copper and poly-butylene piping.

put in place. The end of the plastic pipe is leftexposed to show the stainless steel sleeve in-sert, another of which is shown separately justbelow. Below this again is a cutting of copperpipe taken from an old joint, showing the com-pression ring squeezed onto the pipe with thePTFE tape around it.

Non-manipulative compression fittingshave been used on black iron pipe for centralheating pipework. Although the pipe is madeby folding a strip into a tube and welding itclosed, the ridge on the outside of the pipe isground off, leaving a circular section whichcan be sealed by a fitting and either jointsealer or PTFE tape.

Both of the above joints are suitable for usewhere the pipework is under pressure, suchas hot and cold water services or gas supplypiping.

The non-manipulative compression fittingsused on plastics waste pipe (they can also beused on copper pipe of appropriate diameter)follow the same general design principles butwithout the need to withstand more than amodest pressure. The wedge-shaped sectiondesign of the sealing ring is only about ob-taining a seal and a modest grip on the pipe.A liner is not required as the plastics of thepipe is generally quite rigid.

Figure 12.2 shows, from left to right, astraight coupling with the pipe in the bodyof the coupling, then, on the pipe, the seal-ing ring, a hard fibre washer and the com-pression nut. The fibre washer provides a

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Fig. 12.2 Compression fitting on waste disposal pipe.

smooth surface between the compression nutand the sealing ring. The sealing ring, beingsoft synthetic rubber, would stick to the nutand would turn with it on tightening and be-come distorted, destroying the seal.

Capillary fittings

Capillary joints can be made on copper andstainless steel pipe; however, the remarkshere apply only to copper pipe. Capillaryjoints are soft-soldered joints made by push-ing the ends of the pipe into plain sockets onthe fitting and applying heat and/or solder.To solder successfully, the ends of the pipeand the pipe sockets must be clean1 and mustbe coated with flux. Flux is a chemical which,when the heating of the joint takes place,forms a layer over the metal which excludesthe air trying to oxidise it and also helps theflow of molten solder over the metal. Somefluxes are also chemical cleaners, removingthe oxidised layer which forms over the sur-face of the pipe and fittings between manufac-ture and final installation. It is still best to cleanthe pipe with steel wool. Heat is usually ap-plied using a gas torch fuelled from a dispos-able or refillable cartridge; there is, however, atool which clamps round the joint one socketat a time and is heated by electricity. The onlyproblem is the need to change the size ofthe clamp every time a different socket size

1 Cleaning is usually done by rubbing the end with steelwool.

is encountered. End feed fittings have plainsockets which fit closely round the pipe, andwhen the clean pipe is inserted and heated,the solder is introduced at the junction of pipeand fitting, the molten metal being drawn inbetween the two by capillary attraction.

Integral solder ring fittings are made withan annular groove in each close-fitting socketwhich, during manufacture, is filled with sol-der. All that is necessary is to clean and fluxthe pipe and fitting, push together, and heatuntil the solder appears as a bright ring at thejunction of pipe and fitting.

It is important to note that solders can bemade of alloys with a high lead content whichis unsuitable for use in hot and cold water ser-vices. The lead content of the solder can beleached out and so poison the users. Wherecapillary fittings with an integral solder ringare used in H&C services, they should bepotable water fittings which have tin-basedsolder. Tin-based solder must also be usedwith end feed capillary fittings in hot andcold water supply piping. All manufacturersmake fittings with both types of solder. Thereis nothing wrong in using lead-based solderin the installation of central heating, etc.

Although it might seem obvious, it has tomentioned that all the pipe ends must be sol-dered into any one fitting at the same time.While it is possible to solder in one pipe endand then solder in another at a later stage, thiswould melt the solder in the first joint and itwould not be possible to guarantee a water-proof or gasproof joint.

Figure 12.3 shows a capillary fitting, an el-bow, with integral solder ring joining two cop-per pipes together.

All these joints are suitable for use wherethe pipework is under pressure such as hotand cold water services or gas supply piping.

Push fit fittings

Push fit joints are of two types. The firsthas a simple sealing ring which is squeezedbetween the fitting and the pipe being in-serted. This type is only suitable for disposal

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Fig. 12.3 Capillary fitting joining two lengths of pipe.

pipework and is the most common type foundon large pipework for soil waste and on ven-tilation pipework, and is also found on thebores 32, 40 and 50 waste pipe.

The second type of joint has a sealing ringsqueezed between the pipe and the fitting, butalso a grab ring which resists any tendencyfor the joint to pull apart. There is no suchtendency with disposal pipework but there iswith any pressurised system such as hot andcold water supply and central heating. Pushfit joints are not used for gas supply pipework.

Figure 12.4 shows push fit fittings suit-able for waste disposal pipe. The seal showsas a black ring inside the socket. In the fit-ting shown it is circular in cross-section and

Fig. 12.4 Push fit fitting for waste disposal pipe.

Fig. 12.5 Push fit fittings for hot and cold supplypipework.

is known as an O-ring seal. Seals on largerdiameter pipes have been made of D-sectionmaterial and as a flat section with a taperedwiping seal attached resembling a T-section.

Figure 12.5 shows push fit fittings suitablefor hot and cold supply pipe work. On the leftis a copper pipe, on the right a plastic pipe andin the middle the body of the fitting. On thecopper pipe, starting from the left, there is anut – not a compression nut, but simply a nutto keep all the other components alongside itinside the body of the coupling. After the nutthere is the ‘O’ ring seal, a spacer of plastic,the grab ring of stainless steel (the ‘barbs’ onthe grab ring are clearly visible) and finallyanother plastic spacer. The grab ring is at thepoint on the pipe where it would be when allthe components were in the body of the pipeand the nut screwed back in place on the body.The components on the plastic pipe are exactlythe same and in the same order.

Solvent welded fittings

Solvent welded fittings are mainly used nowfor disposal pipework. The fittings are madewith sockets which fit closely round the pipefor which they are intended. There is no clean-ing to be done unless the fitting or pipe isvery dirty. Cut the pipe square, coat the socketwith solvent weld cement and push the pipeinto the socket with a quarter twist. Leavethe joint undisturbed for a few minutes un-til an initial set of the weld has taken place,and continue with the rest of the joints andpipework. The principle is quite simple. The

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Fig. 12.6 Solvent weld fitting for disposal piping.

‘cement’ comprises plastic in a plastic solvent.The dissolved material acts as a filler, the sol-vent literally softening the surface of both pipeand socket, and the three elements – pipe,socket and filler – then fuse together whenconfined in the space between socket andpipe, the quarter turn mixing all the dissolvedcomponents together. Figure 12.6 shows a sol-vent weld fitting for disposal piping.

Fusion joints

Socket fusion, butt fusion and electrofusionjointing are well beyond the scope of this text,as they are relatively expensive processes toemploy for simple domestic pipework. Theyare, however, widely used by the gas sup-ply industry, especially in the mains supply inthe street and to the house, where plastic gaspipework is now common and where mixedmaterials installations can lead to problemswith corrosion etc.

Joining pipes to appliancesand accessories

Having given a brief explanation of joiningpipes we must now look at the joining of pipeto appliances and accessories. In the domesticmarket there are three possibilities which arisewhen joining a pipe to an appliance:

� The appliance has an externally threadedend protruding – a male thread

� The appliance has a protruding socket withan internal thread – a female thread

� The appliance has no threads or sockets pro-truding but may or may not have a hole fora fitting.

In the first situation above, the most com-mon method of joining the pipe depends onthe bore of the protruding end. This may ormay not accept one of the standard diame-ters of copper or plastic supply piping, 15, 22and 28.

Should the end accept pipe, then a variationof the non-manipulative compression joint isused. This is common when connecting pipeto taps. Should the end not accept pipe, thena special connection must be used. This hasan end which accepts the pipe and can utiliseany method to accept the pipe – compressionfitting, grab ring, push fit fitting, capillaryfitting, and so on, but the other end is an inter-nally threaded socket – a female thread. Sucha fitting is described as a female iron to cop-per connection – ‘female iron’ – because it is afemale thread and is a form of thread first de-veloped to make threaded joint on iron pipe,and ‘to copper’ because it was to copper pipethat these compression type joints were firstdeveloped – even if it is a grab ring, push fitjoint.

The joint is, of course, mechanically strongto resist pressure and the seal of the threadedportion is made by using jointing compoundwith oakum or cotton waste or with PTFE tapeto seal the spaces between the threads. If themale thread is very long, the connection willbottom on the spigot, and to ensure that thejointing material applied to the thread is kepttight inside the joint a back nut is screwed onto the male end before the fitting and then itis screwed back tight against the fitting, trap-ping the joint material between itself and thefitting. If that isn’t complicated enough, theconnector can be straight or bent, i.e. the pipeand fitting are in line or the pipe and fittingare at right angles to each other, respectively.

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Fig. 12.7 Female iron to copper fitting connection.

Figure 12.7 shows a female iron to copperfitting connecting to the threaded spigot on atap. There is a back nut there ready to screwdown on the fitting, trapping jointing com-pound or tape and ensuring a watertight seal.

Figure 12.8 shows a male iron to copper fit-ting with a back nut ready to go through a thinwall piece of equipment such as a water cis-tern. Washers would be put in place to makea watertight seal.

Range of fittings

The range of fittings available is quite stagger-ing, and the text would be lengthened consid-

Fig. 12.8 Male iron to copper connection with a backnut.

Fig. 12.9 Selection of bends, elbows, straight cou-plings and Ts all with compression joints.

erably by including a detailed description ofeach one. They all come as straight couplingsor connections, bent, elbows, Ts, swept Ts,crosses, and all of these could have flanges forfixing to some kind of background. Even man-ufacturers have their unique extensive range,and yet it is sometimes necessary to use dif-ferent makes of fitting to make a neat job ofan installation. The best we can do here in thelimited space is provide some photographs ofgeneric types of fitting and refer the reader tomanufacturers’ catalogues and websites.

Figure 12.9 shows a selection of bends, el-bows, straight couplings and Ts, all with com-pression joints. From the top left and mov-ing clockwise: a straight coupling; an elbow;a male iron to copper connection; a femaleiron to copper connection; a T with branchreduced; a plain T.

Figure 12.10 shows a selection of straightand bent capillary couplings and Ts. At thebottom right is a straight tap connector andimmediately above it a blank end.

Figure 12.11 shows a selection of bent andstraight, male and female iron connections allwith compression joints.

Figure 12.12 shows a selection of push fitcouplings for hot and cold water pipe work.In the centre is a male iron to copper straight

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Fig. 12.10 Selection of straight and bent capillary cou-plings and Ts.

connector and beside it is a back nut. The fit-ting on the top right with the four branch cou-pling is called a manifold and might be usedin central heating installations where largerbore pipework supplies hot water to a centralpoint and then the four branches distributethis to smaller bore pipes connected to theemitters. Moving clockwise there are: a crosswith branches reduced; a tap connector; a Twith end reduced; a T with both ends reduced;a straight coupling and an elbow.

Figure 12.13 shows a selection of compres-sion couplings for disposal pipework. Fromthe left and clockwise there is an elbow, a

Fig. 12.11 Selection of bent and straight connections,all with compression joints.

Fig. 12.12 Selection of push fit couplings for hot andcold water pipework.

swept T, a straight coupling and an obtusebent coupling.

Figure 12.14 shows a selection of solventweld couplings for disposal pipework. Fromthe top left and working clockwise there is aswept bend, a female iron to waste pipe con-nection, pipe clips (not exclusive to solventwelded installations), a straight coupling, anelbow, an obtuse bent coupling and a 45◦ Tpiece.

Manufacturers generally print cataloguesand mini-handbooks of their fittings. The lat-ter are about A5 or A6 size and are ideal forthe student to obtain. Try your local plumbersmerchant for some literature or visit thesewebsites. Some will allow you to order at leastsome general literature:www.yorkshirecoppertube.comwww.imiyf.com

Fig. 12.13 Selection of compression couplings fordisposal pipework.

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Fig. 12.14 Selection of solvent weld couplings fordisposal pipework.

www.hepworthplumbing.co.ukwww.polypipe.com/bpwww peglerhattersley.com

Valves and cocks

The carrying of water to equipment and ap-pliances has to be capable of being shut offso that pipework and appliances etc. can berepaired or maintained. This is achieved byinserting valves or cocks at various points inthe pipework. The websites above will includeinformation on the manufacturers’ own ideasabout valves and cocks. There are a numberof different types of valve and cock knownby a variety of names, and some by the samename as a totally different product. Table 12.2attempts to sort out the confusion.

All of these valves, in their most basic form,are supplied with tails for copper. Essentiallythis means they join to the pipe at eitherend with a non-manipulative compressionjoint. Manufacturers such as IMI YorkshireFittings Ltd make them with capillary jointsas well as compression joints. Some of themanufacturers of plastic H&C pipe, such asHepworth and Polypipe, manufacture valveswith push fit ends with a grab ring. Draindown valves can be obtained where they arepart of another fitting, such as a bent coupling

Table 12.2 Valves and cocks, nomenclature.

Common name Correct name

Screw down valve, stop cock Screw down valveGate valve Gate valveBall valve Ball valveBall valve, ball cock, float

valveFloat valve

Plug cock, stop cock, stopand frost cock

Plug cock

Drain down valve Drain down valve

or elbow or even a T. Valves can also be ob-tained with flanges for fixing to some type ofbackground.

Depending on the manufacturer, brass andDZR versions of the valves are made, andthere can be one or both ends with male or fe-male threads. So one could have one end forcopper and one male or female end, or bothmale ends or both female ends. To the bestof the author’s knowledge no-one has made avalve with one male and one female end. Hav-ing valves with single male or female endsmeans that a connection can be made to an ap-pliance and then the pipe connected directlyto the valve.

Figure 12.15 shows schematics of the dis-tinct valve and cock types. Follow these sket-ches as you examine the photographs whichfollow.

Figure 12.16 shows a selection of valves,starting at the top right and moving clock-wise: a plastic plug cock with push fit endsfor copper or plastic pipe; a drain down valveon a (long) straight coupling; a drain downvalve male iron end; a ball valve operated byusing a screwdriver in the slot – the slot showsit to be open; a gas cock; a gate valve; a screwdown stop valve.

Figure 12.17 shows a plug or stop cock withcompression tails for copper. The position ofthe rectangular top shows it to be closed.

Figure 12.18 shows a pair of float valves,the upper one is a diaphragm float valve, thelower one a plunger float valve, known asa Portsmouth pattern valve. The floats havebeen taken off.

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Stuffinggland

Jumper

Washer

Flow

Compressiontails for pipe

Seat

Screw down valve -- partopen

Tapered plugground into

tapered holein body

Rectangularhead for 'key'

Note alternate shapepresented when cock is open

Plug cock -- shut

Spring & plainwashers

Nut


Ball valve -- closed

Plastic seals

Female ironto copper

connections

SS ball

Alignment of hole through ball and screwdriver slot to showvalve in open or closed position

Wheel headand nut

Stuffinggland

'Gate'


Gate valve -- part open

Float valve -- Portsmouthpattern -- open

Float valve -- diaphragmpattern -- shut

Nozzle NozzlePlungerWasher

Lever arm

Pivot point

Diaphragm

Plunger

Lever arm

Pivot point

Fig. 12.15 Schematics of the distinct valve and cock types.

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Fig. 12.16 Selection of valves.

Figure 12.19 shows an exploded version ofthe Portsmouth pattern valve. Across the bot-tom is the lever arm with the float and at theother end the hole for the pivot pin and the ear-like appendages which engage in a groove inthe plunger. Above, from left to right, a pairof back nuts and a washer to secure the valvein the tank or cistern; the male iron spigot withthe nut which attaches it to the valve body;the valve body with the nozzle and the holefor the pivot pin; below the body, the pivotpin – a brass split pin; the plunger which hasa soft washer in the end and a groove alongthe side for the ears on the lever arm; a cap tokeep the plunger in the body. What cannot be

Fig. 12.17 A plug cock with compression tails forcopper.

Fig. 12.18 A pair of float valves.

seen is the nozzle in the body against whichthe washer on the plunger presses to seal offthe water flow.

Services generally

Before we go on to look at appliances andequipment, we should look at the classifica-tion of services which are piped around a typi-cal house and the kind of pipe most commonlyused. Table 12.3 summarises that information.

Hot and cold water services

Cold water is brought into a building fromthe water main, which is situated outsidethe boundary of the property. The supply

Fig. 12.19 Exploded version of the Portsmouth pat-tern valve.

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Table 12.3 Service and pipe used.

Service Type of pipe

Hot and cold water supply Copper, polybutylene for refurbishment workCentral heating CopperWaste piping under 50 ABS, polypropylene, uPVCSoil pipe over 80 uPVCOverflows ABS, polypropylene, uPVCPipe sleeves∗ Any plastics pipe

*A pipe sleeve is a cutting of pipe built into a masonry wall through which a service pipe ispassed. It protects the service pipe and allows it to expand and contract or move lengthwise inthe sleeve without damage to itself or the wall. It always bridges cavities in walls and soprevents pipe contents from spilling into the cavity in the event of a leak. A pipe sleeve ismandatory for gas service pipes passing through a cavity or any other walls. There should neverbe any pipe joints secreted in a pipe sleeve.

authority arranges for the tapping into themain, fitting an underground valve with anaccess cover – a toby – made of cast iron sit-ting on a brick or precast concrete seat andleaving a length of water pipe just inside theproperty to which the plumber will connectthe supply pipework. Depending on the re-gion in which building takes place, the wa-ter authority may also provide a water meterso that the cost of water used is charged perlitre rather than being an unmeasured supplycharged for with the Council Tax. The waterauthority will make a charge for the work in-volved in a connection to their main supplypipe.

The plumber will use a blue polyethylenepipe to bring the water supply from the tailleft by the supply authority into the house.This is done via a duct under the foundationand up through the solum or concrete groundfloor of the house. This pipe ends with a valve,which can be used to shut off the water sup-ply to the house. If there is a storage tankin the roof space, a pipe is run to the tankfrom this first valve. This pipe is known asa rising main. Generally there is a branch offthis pipe to the cold tap in the kitchen whichsupplies fresh water for drinking and cook-ing. Ideally the branch should have a valveas close to the rising main as possible so thatthe branch pipework and the kitchen cold tapcan be isolated. If no tank is supplied for the

storage of water then pipework must be takento all cold water draw-off points in the house.These would include kitchen sink, bath(s),shower(s), WC(s), WHB(s), water heater(s),hot water storage cylinder(s), central heat-ing header tank, etc. If a tank is supplied,then pipework is run from the tank to all thecold water draw-off points in the house, listedabove. That pipework is known as the coldfeed.

The reason for having a storage tank in theroof space is quite simple. In the event of atemporary cut in the mains water supply, thetank will still supply water for flushing WC(s),hand washing, dish washing and for the hotwater system. Without a storage tank any de-vices to control the water flow into appliancesor equipment must be capable of dealing withwater at mains pressure.

Hot water supply can be arranged in twoways. A hot water cylinder can be installedand pipework put in to take hot water to all ap-pliances which require it. Various means canbe adopted to heat the water in the cylinder.Hot water is drawn off the cylinder via a pipefrom the top of the cylinder and cold wateris fed in near the bottom from the cold feed.A valve should be placed on the cold feed tothe cylinder. Shutting the valve stops hot wa-ter being delivered at any tap as there is nopressure of cold water to force it out of thecylinder. It is normal to run cold feed pipe

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Duct for incomingwater supply pipe

Sink

WC

WHB

Bath

HWcylinder

CW storage tank

Screened venton lid

Fig. 12.20 Identification of the appliances and equip-ment to be connected up.

through a tee piece at the bottom of the cylin-der, the branch being a male iron spigot (teeslike this are often referred to as boiler tees asthat is where they were most commonly fit-ted). This branch is connected to the cylinder,and the other end of the tee connects to a scourvalve (usually a plug cock) with a pipe go-ing to the outside of the building. Openingthe scour valve empties the hot water systemcompletely above the level of the scour valve.If the valve on the cold feed is left open, thecold water cistern is emptied.

The arrangement in Figure 12.20 will beused for schematics of various classes ofpipework and identifies the appliances andequipment to be connected up.

Figure 12.21 is a schematic of the cold wa-ter supply pipework. Note that screw downvalves are used where there is mains pres-sure against the valve when it is closed. Gatevalves are used on cold feeds since they havea straight through opening and pressure is notlost in any significant quantity.

Figure 12.22 is a schematic of the hot wa-ter supply pipework. Note that a gate valve isplaced on the hot feed to the sanitary appli-ances but there is never any valve put on theopen vent. If a valve was placed there, closed

sG G

G

B

BB15

15

15

22

22 22

22

22 22

22

DDs

s

Sc

S - screw down stop valveDD - drain down valveG - gate valveSc - scour valveB - ball valveconnection to tap orsimilar male thread

*

* outlet to HW cyl. higherthan other outlets

Fig. 12.21 Schematic of the cold water supplypipework.

G

BB

B

22

2222 15

15

15DD

Fig. 12.22 Schematic of the hot water supplypipework.

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and the cylinder heated, the system would ex-plode.

The schematics above show only the ap-pliances with the hot and cold water supplypiping. The basic drawing from Figure 12.20will be repeated showing individual pipe-work networks as they come up for dis-cussion, such as soil waste and ventilationpipework, waste pipework, central heatingpipework and overflow pipework.

Soil and ventilation stacks

Disposal of waste from the sanitary appli-ances shown in the previous schematics mustalso be considered. It is usually carried out inplastics pipe as previously explained. It is al-lowable, but not often done, to make the mainpipe from the WC an 83 pipe. More usual isthe use of 100 nominal bore pipe. The term‘nominal’ is applied here for although allpipes are measured outside diameter, ‘od’, theindividual manufacturers use different wallthicknesses and slightly different bores so thatit is not always possible to mix and match fit-ting and pipe from different sources – somewill, some won’t.

Figure 12.23 shows a schematic for the appli-ances already piped up for water supply. Note

Access

Access

Balloongrating

32

50 50100nom.

40

Fig. 12.23 Soil, waste and ventilation pipework.

the access on the main vertical pipe at the bot-tom at ground level and opposite the junctionwith the WC branch. Note also that the pipecontinues vertically up past the eaves levelwith just two bends. It is allowable to haveonly two bends above the last WC junction onthis part of the stack. The top of the pipe isopen and protected from debris and birds en-tering by being fitted with a grid or grating ofsome type. The open end is important for ven-tilating the drainage network and the sewernetwork. In old installations the drainage waskept sealed off from the sewers by a trap calleda disconnecting trap. The trap incorporateda fresh air inlet for the house drains. Sewerswere ventilated at their highest point and theVictorian drainage engineers frequently dis-guised these large ventilation stacks as flagpoles, factory chimneys, etc.

Overflows

Overflows must be fitted to any storage cis-terns which are fed through a float valve.Should the valve stick in the open position,not close properly because of dirt on the seal,or otherwise fail, the cistern would quicklyoverflow and the water could cause muchdamage to the fabric and decoration of thebuilding. Overflows allow any excess waterto be discharged outside the house, prevent-ing spillage in the house. The discharge pointshould be arranged so that it is in as con-spicuous a place as possible. Over a kitchenwindow or a front door are good places. Fig-ure 12.24 shows typical overflows required.Pipework is usually plastic, the larger boresbeing in the same material as the waste piping,the smaller pipe for the WC overflow being aone-off.

Water supply from the main

External services will not be dealt with herebut it should be noted that a supply fromthe water main in the street is arranged bythe supply company or water authority in the

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40 min.

20

Fig. 12.24 Overflow pipework.

area, and they will arrange to tap the mainand lay a blue plastic water pipe to the housesite boundary, where a toby is built into theground, fitted with a cast iron cover. At thebottom of the toby cover, the plumber willjoin a blue plastic pipe to the main with a‘stop cock’, in fact a plug cock. The rectan-gular head means that a special key is re-quired to open and close the plug cock. Oldinstallations used a special plug cock whichhad a hole in the side of the cock body anda three-way port in the plug such that whenthe straight through hole in the plug was inthe off position – against the mains pressure –the third port in the plug was opposite thehole in the body. This allowed the water in therising main to drain back down and out intothe earth surrounding the stop cock. What itmeant was that there was no water left in thepipe to freeze and burst the pipe. This is notsuch a problem with plastic pipework.

Figure 12.25 shows a cast iron toby cover inthe street.

Equipment

Equipment in a domestic situation normallyincludes only a hot water cylinder, a water

Fig. 12.25 Cast iron toby cover in the street.

storage cistern and a feed and expansiontank where open vented central heating hasbeen installed. The HW cylinder is usuallyplaced on the highest floor in the house, andthe cistern and tank in the roof space.

Assuming that there is a water storage cis-tern in the house, this is placed on a boardedarea supported on timber bearers which spanacross several ceiling ties in the roof structureso that the load is spread as far as feasible.The rising main is connected to a ball cock2

mounted on the side of the tank near the top ofthe rim. Cold feeds are taken out of the tanknear the bottom, one for the general supplyof cold water and one for the supply of coldwater to the hot water cylinder. General coldwater feeds are taken off the tank about 50from the bottom, but the feed to the hot wa-ter cylinder is taken off about 60–75 above thebottom. This means that the pressure forcinghot water out of the cylinder will stop beforethe supply of cold water to taps fails, whichshould obviate scalding when occupants are

2 It is easy to confuse the two valves – ball valves andball cocks – but the ball valve only appears in the runof pipework and is an actual ball, whereas the ball cockis a plunger or diaphragm valve actuated by a float,commonly in the shape of a ball. The correct name forthis latter valve is a float valve, and there are at leastfive patterns to choose from. Many people in the tradeuse the terms indiscriminately.

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using mixer taps on baths to supply a shower,or those rubber push-on attachments on bathor basin taps. Additional individual cold feedsare taken off the tank to supply cold water toshowers which rely on the hot water cylin-der for their hot water supply, and again theseshould be taken off the tank below any sepa-rate feed to a hot water cylinder.

Cold water storage cisterns

Cold water storage tanks or cisterns have beenmade from asbestos cement and galvanisedsteel sheet, both of which required paintingwith a black bituminous compound after theholes were cut and before fitting in place. Theyare now invariably made from plastics – poly-olefin or olefin copolymer.

They are available in rectilinear or cylindri-cal form. Plastic cisterns are very flexible whenempty, but so long as they have full supporton their base the weight of the water makestheir sides quite rigid. The sides could deformwhere the ball cock enters due to the leveraction of the ball and arm, but manufactur-ers supply a galvanised steel reinforcing platewith each tank to overcome this. Other con-nections pose no such problems. The benefitof installing a plastic cistern is that if it fails itcan be brought out of the roof easily througha relatively small ceiling hatch – and the newtank goes back into the roof just as easily.

Cisterns are made in a range of capacitiesfrom 18 to 500 litres, the water line on the twosmaller tanks being 100 below the rim andon all others 115 below the rim. To complywith the appropriate British Standard, the wa-ter line must be marked on the inside of thecistern. It is between the rim and the waterline that the hole for the ball cock and the holefor the overflow must be drilled, the overflowbeing nearer the water line than the ball cock.All cisterns must be installed with a lid to pre-vent the entry of dirt and vermin. The lid mustincorporate a ventilating cap with a screento keep out insects, and the overflow mustturn down inside the tank below the waterline.

Generally a capacity of around 250–300 litres is adequate for the average three-bedroom house; however, in areas where thesupply is not always reliable or there couldbe pressure drops etc. a larger capacity mightbe advantageous. If this cannot be accommo-dated in a confined roof space as a single cis-tern, two or more cisterns can be linked to-gether. This is done with normal water pipeand fittings joining one cistern to the next inseries. The rising main is connected to the ballcock in the first cistern and cold feeds are takenfrom the last tank in the series. Connected likethis there is a constant flow through all thecisterns whenever water is drawn off and nocistern goes stagnant. All tanks should havean overflow pipe connected to a common pipedischarging at eaves level.

Figure 12.26 shows a cold water storage cis-tern and linked cisterns.

Hot water storage cylinders

Hot water storage cylinders have been madefrom galvanised steel and from polypropy-lene but by far the most common material iscopper. HW cylinders are made in capacitiesfrom around 90 to 500 litres, and their shape

Cold feed to HW cylinderOther cold feeds

Silencer pipe

Float valve

Waterlevel

LidScreened air vent

Hole in silencer

pipe preventsback siphonage

Rising main in

Draw offs

22Linkpipe

22Linkpipe

O/flowO/flow O/flowFloat valve

Fig. 12.26 Cold water storage cistern and linkedcisterns.

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is an upright cylinder with a domed top andbottom, the bottom dome being internal andthe top being external.

Each cylinder has a number of solid brassconnecting points for pipework brazed to thecopper. These may be female iron or male iron,the former being the more common. Coppercylinders are manufactured in three grades,1, 2 and 3. The difference is the thickness ofmetal used and therefore the pressure exertedby the head of water and when the cylinderis heated. Grade 1 cylinders have the thick-est metal and their maximum working headof water is 25 m. Grade 2 have a maximumworking head of water of 15 m and Grade3’s maximum working head of water is 10 m.

HW cylinders are available in capacitiesfrom 65 to 210 litres, a capacity of around150 litres being suitable for a typical three-bedroom house. HW cylinders can be pur-chased with or without a layer of insulation,but if without they must be insulated wheninstalled with a proper insulating jacket. HWcylinders are manufactured in a number ofconfigurations depending on the source ofheating used for the water and bearing inmind that people will come in contact withthe heated water.

Pretty well all cylinders manufactured nowhave provision for a top entry immersionheater or circulator powered by electricity. Animmersion heater is a simple heating coil in-serted into the cylinder and so surrounded byall the water in the cylinder. A circulator has aheating coil as well but it is enclosed in a closefitting tube, open top and bottom. This heatsthe water very fast and brings the hot waterto the top of the cylinder without it mixingwith the rest of the cold water. Thus a circu-lator will give a faster initial quantity of hotwater than an immersion heater. It does notappear to be any faster or slower to heat theremaining water in the cylinder. Whether animmersion is installed or not is a matter ofchoice for the occupier or builder, but it is sel-dom the only way to heat the water.

The most simple way is to connect two pipesto the side of the cylinder, one near the bot-tom and one near the top. The other ends

are connected to a boiler in the same orienta-tion. The water in the boiler rises in the upperpipe into the cylinder pushing cooler waterdown the lower pipe into the boiler, where itis heated until the whole of the HW cylinderhas hot water – and then the water could startto boil.

Heated water expands, and this expansiontakes place up the open vent pipe fitted tothe top of the cylinder and into the feed andexpansion tank (usually) placed in the roofspace. Unless the heat to the boiler can be con-trolled, this is what will happen and it is notuncommon to see expansion pipes spurtinghot water and steam because the heat sourceis a solid fuel fire or stove.

A cylinder with this method of heating thewater is known as a direct cylinder. Note thatthere are no controls when a solid fuel fire isused for heating the water and the water onlytransfers from boiler to cylinder and back bygravity: the hot water rising, the cooled watersinking. In Figure 12.27, the boiler is a recti-linear ‘box’ of thick copper plate with four 22female threaded connections, two in each end.One takes the cold feed from the cistern in theroof, and the other, higher, connection takesheated water up to the top of the HW cylin-der. At the other end the upper connection isblanked off and the lower connection receivescooled water from the cylinder. The cold feedto the boiler is also the cold feed to the HWcylinder. The open vent pipe is the open ventand expansion pipe for the whole system.

Figure 12.27 shows a direct HW storagecylinder connected to a back boiler such asone would find in an open fireplace or closedstove. The pipe which carries hot water to theHW cylinder is termed the flow pipe or sim-ply the flow; the pipe bringing cooled waterback to the boiler is termed the return pipe orsimply the return.

A further development was the indirectcylinder, and this is made in two versions:single feed and double feed. Figure 12.28 is aphotograph of an indirect HW storage cylin-der with factory applied insulation. Note theimmersion heater fitted on the top, and thatthe two connections for the boiler circuit water

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Scourvalve

Back boiler builtinto open fireplace

Open ventpipe

Cold feed fromcistern

Flowpipe

Returnpipe

Fig. 12.27 A direct HW storage cylinder connected toa back boiler.

have protective plastic sleeves fitted over themale iron ends.

Taking the double entry indirect cylinderfirst is not the most obvious choice but it isthe easier one to understand. In this type thecylinder is supplied with water from the coldcistern, and the HW cylinder contains a coil

Fig. 12.28 An indirect HW storage cylinder with fac-tory applied insulation.

of copper pipe – a calorifier – the ends ofwhich are connected to a boiler, which is fedwith water separately from the HW cylinder.Thus the water in the cylinder does not mixwith the water in the boiler. This means thatthe boiler water can be treated with inhibitorswhich prevent corrosion and reduce the elec-trolysis due to mixed metals in the system, aswell as containing an anti-freeze, thus allow-ing the boiler, connected pipework and calori-fier to be left full of water in the winter time.

Figure 12.29 shows a schematic for a doublefeed, indirect, HW storage cylinder.

Now the single entry indirect cylinder: inthis type there is a calorifier but it is open to thewater in the cylinder and so, when the cylin-der is fed water for the first time, it fills the

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Calorifier coil

Open vent discharging into storage cistern or

out onto roofHot feed

From storagecistern

Flow and return

from boiler

Plug cock as

'scour valve'

To outside ofbuilding

Tapping forimmersion

heaterAutomatic

air ventor bleedscrew

Scour allows HW cylinder to be drained down and if gate valve

on feed is open it will drain the CW cistern as well. Feed

is the first of the feeds, the boiler feed is the second.All connections direct

to the cylinder are femalebosses; the calorifier has

male iron threaded projections.An automatic air vent on the

boiler circuit can only be installed if the CH system is open vented.

Gate valve

Fig. 12.29 A double feed, indirect, HW storagecylinder.

calorifier and the boiler and boiler pipeworkas well. When the boiler is fired up, the wa-ter in the boiler circuit will heat up and moveround the pipework and the calorifier, but be-cause of the way the entries in the calorifierare made it will not mix with the rest of thewater in the cylinder. Expansion of the boilercircuit water is taken up by the cushion of airmoving out of the calorifier into the cylinderand finally up the open vent pipe; contractiondraws water in from the cylinder.

Figure 12.30 shows a schematic for a singlefeed, indirect, HW storage cylinder. The mostcommon HW cylinder used in domestic workis the double entry indirect cylinder with acentral heating boiler fired by oil or gas andcontrolled by thermostats and time clock(s)to give an adequate supply of hot water at

Open vent discharging intostorage cistern or

out onto roofHot feed

From storagecistern

Flow and return

from boiler also feeds water into

boiler whencommissioning

the system

Plug cock as scour valve

To outside ofbuilding

Tapping forimmersion

heaterAutomaticair ventor bleedscrew

This HW cylinder can only be used with an open vented boiler

system and only where the boiler is below the level of

the bottom of the HW cylinder. Scour allows HW cylinder

to be drained down and if gate valve on feed is open it will drain the CW cistern as well.

Feed is the first of the feeds, the boiler feed is

the second.All connections direct to the

cylinder are female bosses; the calorifier has male iron threaded

projections.

Gate valve

air

level of HW cyl. water

Level ofprimary

wateri.e. boiler

circuitwater

Fig. 12.30 A single feed, indirect, HW storagecylinder.

around 65◦C. The aspect of control will be dis-cussed later in this chapter when we look atdomestic central heating systems.

Feed and expansion tanks

All that has been said about CW storage cis-terns applies to feed and expansion tanks.These are only installed when a boiler is fedseparately from the HW cylinder, either as adouble feed indirect HW cylinder or as partof a central heating system which also heatsdomestic hot water. They are generally ofmuch smaller capacity, 18 litres being suffi-cient for the normal domestic heating system.Only where larger boilers and more radiatorsfor larger houses are installed, would it be

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necessary to provide more expansion space.For that is what these tanks are all about.

They receive water via a ball cock from themains supply – a branch off the rising main.Initially this water fills the boiler circuit andthereafter acts only to top up the boiler cir-cuit losses from evaporation, radiator bleed-ing, etc. The water level marked on the cis-tern is ignored, the ball cock lever being set sothat water will enter only in sufficient quan-tity to cover the cold feed to the boiler. Thiscold feed pipe comes from near the bottomof the tank, and while supplying the boilerwith cold water also acts as an expansion pipefor the water in the boiler circuit. This expan-sion forces water back up the cold feed pipeinto the tank, which has to be large enoughto cope with the expansion and not allow wa-ter to escape out of the overflow. This wouldnot be a disaster if it happened but if thatboiler circuit is filled with inhibitors and anti-freeze, then when the boiler cools, fresh waterwill enter through the ball cock and the boilercircuit water is diluted. A typical, modern,three-bedroom house will have an expansionof around 6–8 litres when the boiler is fullyoperational. Allowing around 5 litres perma-nently in the cistern, there is a small safetymargin before the contents overflow.

Central heating

Equipment for central heating can vary enor-mously but, ignoring the fuel used, for mod-ern domestic heating the low pressure hot wa-ter system commonly used can be either afully vented or pressurised system. Both sys-tems will heat emitters of various kinds as wellas towel rails, and will also heat domestic hotwater on an indirect basis. So the terms fullyvented or pressurised apply only to the boilercircuit3.

It is not proposed to attempt a discussion onboilers and the merits of the various options

3 It is possible to have the domestic hot water supplysystem pressurised but this is rare in speculative hous-ing in the UK, although very common in continentalEurope and North America.

that manufacturers make available. However,the principal features are:

� A choice between pilot light ignition andelectric spark ignition (the first is giving wayto the second)

� A user adjustable thermostat to control thetemperature of the water delivered by theboiler

� A foolproof method of keeping any pres-surised system topped up with water andat the right pressure.

Naturally the manufacturers try to make theoverall internal design feature intrinsicallyfail safe. This is helped by the control boxwired up with the system which ensures thatthe pump is always running before the boilerfires, and it will only run if the motorisedvalves on the flow pipes are in the open po-sition. The boiler controls are all now usingmicrochips, and the program in them ensuresthat the fan on the flue always runs to purgethe fire box of all gases except fresh air; theover temperature switches are in the ‘on’ posi-tion; the pump is running sensed by a pressureswitch; there is correct water pressure sensedby another pressure switch; fuel is available(another sensor); and finally the fuel ignitesand a sensor detects a flame and the sys-tem heats the water. If no flame is sensed thesystem shuts down. In addition there shouldbe a pressure relief valve which when activeshuts the system down. If ignition is by pi-lot light, a flame out sensor is required onthat which shuts down the fuel supply to bothmain burner and pilot light.

Fuel can be natural gas from the mains sup-ply in the street; LPG (liquefied petroleumgas) from a storage tank permanently sitedadjacent to the building or from refillablecylinders; oil from a permanent storage tankadjacent to the building; or a solid fuel stoveburning small particle bituminous fuel andthus self-stoking. The last are vary rare butare the only solid fuel appliances which canbe controlled anywhere nearly as closely asthe other fuels. All liquid or gaseous fuels canbe shut off with fairly unsophisticated valves.

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*

M M

Emitter Emitter

EmitterEmitter

a a

aa

a

a

T

T

T

T T

Fuelin

a - air bleed ventT - thermostatic radiator valve (TRV)

M - motorised valveDD - drain down valve

P

P - pump

Towelrail

DD DD

DD

22

22

15 15 15 15

22

1515

22 15

22

15

Feed andexpansion cistern

Calorifiercoil

22

Alternative outletfor open vent

Overflow 20

v

v - screened vent on cistern

Fig. 12.31 An open vented central heating system –gas or liquid fuelled.

These cannot be made to work with small par-ticles of coal or coal derived fuel pellets.

Figure 12.31 shows an open vented centralheating system – gas or liquid fuelled. Thepipe which carries hot water to the emittersor to the HW cylinder is termed the flow pipeor simply the flow; the pipe bringing cooledwater back to the boiler is termed the returnpipe or simply the return.

The most common type of pressurised sys-tem is one where the central heating system ispressurised but the domestic hot water sup-ply is left open vented. This does away withthe need for an expansion cistern in the roofspace but still retains a cold water cistern.

Figure 12.32 is a schematic of a pressurisedcentral heating system – gas or liquid fuel-led. Figure 12.33 shows the pressure vesseland valve arrangements of a pressurised cen-tral heating system. Figure 12.34 shows theschematic of the pressure vessel and valve ar-rangements for a pressurised central heatingsystem.

Piping for central heating systems

Looking at the texts which concentrate onbuilding services, the reader is regaled withall sorts of pipe layouts – drop feed, rising

M M

Emitter Emitter

EmitterEmitter

a a

aa

a

a

T

T

T

T T

Fuelin

a - air bleed ventT - thermostatic radiator valve (TRV)M - motorised valveDD - drain down valve

P

P - pump

Towelrail

DD DD

22

22

15 15 15 15

22

1515

22 15

15

Feed andexpansion cistern

Calorifiercoil

22

Alternative outletfor open vent

v - screened vent on cistern

For arrangementin here, seenext figure

v

Fig. 12.32 A pressurised central heating system – gasor liquid fuelled.

feed, ladder layout, etc. – all related to feedinghot water through pipes to emitters by gravity.Here we will deal only with pumped systems.These have taken over the domestic market,giving the plumber almost complete freedomon where to site the boiler, run the pipeworkand site the emitters. Good plumbers, how-ever, observe some fairly basic rules:

� There must be an air vent at the top ofevery vertical run of pipework unless it

Fig. 12.33 The pressure vessel and valve arrange-ments of a pressurised central heating system.

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Pressure gauge

One-way or check valve

Ball valve, knob operated

Flexible hosewith femaleiron ends

Pressure relief valve with manual

over-ride

To boiler

Branch offrising main

Maleconnection

and cap

Nitrogen under pressure

Water at 1 to 2 bar pressure

Diaphragm depressed under water pressure

Pressure vessel

Discharge outside building

Note: The mains water pipe is used to fill the heatingsystem but cannot be permanently connected

to the system in case contamination of the watersupply takes place. Flexible hose is used for filling and

topping up but must be disconnected and the capsreplaced on the pipe ends. The one-way valve

prevents boiler water flowing back into mains ifpressure is less in the main

Ball valve,knob operated

Fig. 12.34 Schematic of the pressure vessel andvalve arrangements for a pressurised central heatingsystem.

ends with an emitter (with a built-in airbleed valve)

� There must be drain down valves on thelowest point(s) of a system

� There must be sufficient isolating valves toallow repair and/or maintenance of parts ofthe system without having to drain downthe complete system

� When draining down there must be no lowor dead legs which will not drain.

This still leaves quite a choice of pipe and pipelayouts, wide enough to cover nearly everysituation that the plumber is likely to comeup against.

For new build work the favoured installa-tion is small bore pipework. The runs of pipeto and from the emitters are carried out in15 pipe, usually copper. Where a pipe is feed-ing, say, a floor area, pipe from the boiler tosome convenient and central location mightbe 22, branching off to some of the emitters

in 15 and then reverting totally to 15. Locallyto the boiler there might only be 22 pipe orat the most some 28 from pump and boiler,branching into 22.

For installation in existing buildings, cop-per is still favoured but polybutylene barrierpipe finds favour when getting into awkwardspaces, feeding pipes through several holesin timbers or partitions, etc. It is, however, amore expensive option overall.

A further option for new build but certainlyfor work in old buildings is the use of micro-bore tubing. The boiler and pump are installedas normal, with 22 pipe taken to central pointson each floor (two points still in a single-storey house) where both pipes are connectedto manifolds. From the manifolds, flow andreturn pipes are run in 6, 8, or 10 mm fully an-nealed copper pipe to each individual emitter.Arrangements at the emitter can be the sameas that for small bore pipework, or a singlevalve with double entries can be fitted. Poly-butylene pipe can be used for the microboreportion and is well suited to installation inolder properties.

Underfloor heating is growing in popular-ity once again. In old systems installed in thelate 1950s to the early 1970s, electric cableswere installed in floor screeds on a fully em-bedded or fully withdrawable system. In thefully embedded system a single cable was laidin a zig-zag pattern over an insulated concretesub-floor and the screed was poured over it.The cable was monitored continuously duringthe pour so that early detection of any dam-age and effective repairs could be carried out.For the fully withdrawable system, a systemof ducts was formed in the screed using an in-flatable hose. Cables were drawn in after thescreed had set. Control was through a wallthermostat and a time clock, the power usu-ally being off-peak. The hot water version uses15 mm polybutylene pipes solidly embeddedin a floor screed over an insulated sub-floor,each room being controlled by a thermostat-ically operated valve. The pipe(s) from eachroom are brought to a manifold which in turnis connected to the boiler by a larger pipe, 22 or28 mm.

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This form of heating has a number of ad-vantages:

� The room attains an even level of heatingover its whole area.

� There are no hot or cool spots.� There is heat at foot level, unlike emitters

which circulate the air to the upper part ofthe room and the cool air sinks to the floorlevel.

� The screed stores heat, giving a long slowbuild up but a long slow cool down. Thisis particularly advantageous in lightweightbuildings where the thermal lag is generallyquite fast.

� It is claimed to be more efficient and there-fore more economic to run than an emitter-based system.

For suppliers of underfloor heating systemmaterials and controls visit the websites ofPolypipe and Wavin – see later section.

Emitters

Emitters is the preferred term for the individ-ual room heating devices normally associatedwith central heating.

What we normally refer to as radiators onlyradiate part of the heat which is given off.Much of the heat is given off by convection.One only has to look at the back of the radi-ator to see that many of them have shapedmetal baffles welded to the panels, whichheat up and in turn heat the air surround-ing them, which rises into the room drawingcool air from floor level. Double panel emit-ters are also now available with factory-fittedside and top covers which boost the convec-tion of warm air. Emitters are also availablewhich have the water panel enclosed in a cas-ing along with a fan. When heat is called for bya built-in or remote thermostat, the fan runsand the air is heated as it is blown across thepanel. The panels are heavily finned. Theremay or may not be a valve on the pipe intothe panel so that hot water is not circulatingwhen no heat is called for.

Fig. 12.35 A typical emitter.

They are common as kitchen heaters wherethey are designed to sit behind the kickingboard under fitted kitchen units. At the tem-peratures at which radiators operate there isnot a lot of radiant heat available. One hasto be within 600 of the panel to feel anythingin most installations, and where there are chil-dren or old people the water temperature fromthe boiler has to be kept low enough to avoidburning vulnerable people should they touchthe emitter surface. Figure 12.35 shows a typ-ical emitter and Figure 12.36 shows a typicalthermostatic radiator valve (TRV).

Appliances

The term ‘appliances’ covers things such assinks, wash hand basins, baths, showers and

Fig. 12.36 A thermostatic radiator value (TRV).

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WCs. We will not consider appliances used inpublic buildings or large institutions.

Appliances come in a wide range of mater-ials: metal, particularly brass, bronze andstainless steel; ceramics such as porcelain, vit-reous china and earthenware; glass; fibreglassreinforced resins; acrylic sheet; and so on. Nomatter what the material the methods of con-nection do not change greatly except for thetype of washers inserted to avoid damage tothe appliance material, or necessary to makea watertight connection on especially thin orthick materials. We will consider kitchen sinksin mineral-filled resin and stainless steel, WCsand WHBs in vitreous china, and baths inacrylic sheet or fibreglass reinforced resin. To-gether with the appliances we must includethings like taps, traps and wastes, flushingmechanisms and so on.

There seems little point in includingsketches or photographs of these appliancesas we all use them daily. What we don’t all dodaily is really look at them or appreciate howthey function – that is, until they break downand we have to call the plumber. I don’t expectreaders to go off and attempt their own repairsafter reading the text, but I do expect that theyshould look at the sanitary appliances theyhave in their own houses and those of theirfriends and really attempt to understand howthey function. Not all of them will be mod-ern appliances or up-to-date in the way theyare connected to supply or disposal pipework,but a lot will be learned.

The website to visit is www.thebathroominfo./relax.html

Kitchen sinks

Kitchen sinks in stainless steel and mineral-filled resins are made in a variety of configura-tions: twin bowls, single bowl, bowl and a halfand all with or without single or double drain-ing boards, holed for a pair of taps or holedfor a monobloc tap, etc. The permutations areendless when one adds sinks which can fit intocorners of worktops, those which can be fitted

with a waste disposer, etc. Look at a websitesuch as www.plumbingsupply.com/elkayand www.titanic-sinks.co.uk

WCs

Water closets have been made in a variety ofmaterials – earthenware, porcelain, stainlesssteel and vitreous china. The ceramics usedare generally referred to under the generaltitle of ware. For domestic purposes the mostcommon material is vitreous china, availablein white and a range of colours, the shadeof which depends on the manufacturer – onepink doesn’t necessarily match another pink.Of more relevance here are the two basic stylesof WC available and the three combinations offlushing cistern and WC available.

WCs in the UK are most commonly of thewash-down type without any assistance fromsyphonic action to produce a clear flush. Amore expensive option is to have a siphonbuilt into the flushing action and trap, a fea-ture which uses the flushing water passingthrough and out of the trap to create a nega-tive pressure immediately between the flush-ing water and the contents of the trap. Thisnegative pressure literally sucks the contentsof the trap over and into the disposal pipework. Siphonic WCs are generally quieter inoperation than the plain wash-down type.

Examination of a WC will show that it hasa trap built into it during manufacture andthat the outlet in the older models could ei-ther be parallel to the floor or at right anglesto the floor on which the WC was mounted –i.e. the traps were either of P or S configura-tion. Modern practice has almost completelyobviated the necessity to manufacture in twoconfigurations. Most are now P traps with theconnector to the disposal pipework being any-thing from straight through a succession ofangles up to 90◦. Figure 12.37 shows ware topipe connectors: on the left a long tailed bentconnection and on the right a short obtuse an-gled connection. Both have hard plastic pipewith soft (black) synthetic rubber seals pushed

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Fig. 12.37 Ware to pipe connectors.

into or onto the pipe. The seals have severalfins which seal to the ware and soil pipe.

Flushing cisterns have been made from avariety of materials – lead-lined wood, castiron, plastic, earthenware, porcelain, vitreouschina, enamelled steel, and so on. Plastic andvitreous china predominate in the modernmarket. All are now supplied with a lid, and asiphon is built in, operated by a lever, to sendthe flushing water through the WC. Their con-figuration with the WC is what sets each cis-tern apart. They can be:

� High level. The cistern is mounted on thewall above the WC at least 2.00 metres abovethe floor level. It is connected to the backof the WC flushing rim by a pipe of about40 bore. In old installations this pipe wouldhave been of lead or enamelled steel but isnow plastic. This type of cistern is not nowused in domestic installations.

� Low level. The cistern is mounted on thewall between 300 and 400 above the WCand is connected to the WC flushing rim bya plastic pipe of around 30 bore. Cisternsare either of plastic or, as in the illustration,vitreous china and of a colour to match theWC. Figure 12.38 shows a low level flushingcistern and WC.

� Close coupled. The WC has an extendedshelf at the back of the flushing rim with

Fig. 12.38 Low level flushing cistern and WC.

an aperture over which the flushing cisternis mounted with a soft rubber ring seal be-tween the WC and the cistern. The two areclamped together. The cistern is of vitreouschina in a colour to match the WC. Fig-ure 12.39 shows a close coupled flushingcistern and WC.

WHBs

Wash hand basins in a bewildering variety ofstyles are now made in vitreous china for do-mestic use and incorporate an overflow sys-tem in their construction. This is an opening inthe basin, high up at the back with a duct in thebody of the basin leading down to an annularspace in the hole provided for the connector tothe disposal pipework. The connector to the

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Fig. 12.39 Close coupled flushing cistern and WC.

disposal pipework comprises a waste, usu-ally of chromium plated brass with a flangeand grid showing in the WHB and a lengthof externally threaded pipe passing throughthe ware. Underneath the wash basin, jointingcompound and washers with a large back nutare used to seal the waste to the WHB. Slotsin the waste align with the annular space forthe overflow.

Baths

Baths were traditionally of enamelled cast ironbut are now made in enamelled pressed steelor plastics – acrylic or fibreglass reinforcedresin. They have an overflow but rather thanbeing built into the body of the bath, an open-ing is left and an overflow grid secured withwashers and a back nut, plus a flexible pipeconnecting to the bath trap, are installed. Thetrap on the bath is specially designed to fitinto the relatively shallow space available un-der modern baths, and it has a connection forthe overflow pipe.

Showers

Showers can be of a number of types. Showermixer taps can be fitted to a bath. Basicallythese taps deliver hot or cold water through asingle nozzle to the bath or when a plunger isoperated delivery is through a flexible hose toa shower head mounted either in a cradle overthe taps, like a telephone, or the shower headcan be placed in a holder on the wall above thebath. The water temperature is not controlledother than when the taps are turned off and on.It is important when these types of shower areinstalled that the cold feed to the HW cylinderfrom a storage system is above the cold feed tothe cold water system in general. This shouldprevent scalding.

The more expensive but more satisfactoryoption is a thermostatic shower valve whichhas separate hot and cold supply pipes, bothfrom a storage system which delivers waterthrough a thermostatically controlled valvewhich automatically compensates for slightpressure differences, as well giving a moreconstant stream of water at a more consis-tent temperature. There is usually a fail-safeshutdown in the event of the cold waterfailure.

Finally, there is the electrically powered ‘in-stant’ shower. This is a shower system withonly a cold water supply taken off the mainssupply pipe. The water is heated by a smallelectric ‘boiler’ and the output is thermostat-ically controlled and has fail-safe shut off ofpower and water in the event of system fail-ure or breakdown. Earlier showers sufferedfrom poor flow but as power available has in-creased beyond the initial 7kW so has the flow,a 9kW unit giving almost as good a shower asa conventional shower. Figure 12.40 shows aninstant shower.

Taps

Taps on baths and WHBs are normally ofchromium plated brass and can be config-ured in different ways. Monobloc taps are in

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Fig. 12.40 An instant shower.

fashion on WHBs, and bath taps can be sep-arate hot and cold or combined in one blockwith or without a shower fitting. Whicheverstyle of tap is fitted they all require to beconnected to the supply pipework, and thisis done with a tap connector – see earlierphotographs of connections for pipework toappliances and equipment (Figures 12.9 and12.11).

Waste disposal piping and systems

Disposal piping is best looked at from thepoint of view of the appliance with the needfor the largest bore pipe – the WC; 83 pipe can

40

82 or100 nom.

50

32

50

Durgovalve

Access

Access

Fig. 12.41 The single stack system with an additionalWC on an extended side branch.

be used for one WC plus other appliances,or, for more than one WC plus appliances,100 nominal bore piping should be used. Thislarge bore pipework (usually referred to assoil pipe) is configured in one of a numberof very specific ways. The method illustratedin Figure 12.41 is the single stack system withan additional WC on an extended side branch.The principle of the stack is to allow air to en-ter the top of the pipe as the WC is flushed,thus preventing the build-up of a siphon inthe pipe which would take any sealing wa-ter out of any appliance traps. Sewers are nolonger vented and now rely on the open endsof pipework to keep a flow of fresh air cominginto the sewer, which then rises and exits fromthe top of the soil stacks. So, when we have apipe receiving the discharge from a WC andrising up above the eaves level of the buildingit serves, the complete pipework is referred toas a soil and ventilation pipe – SVP. The SVPis usually done in uPVC.

Connected to the soil stack we have the dis-posal pipe from the other appliances – WHB,sinks, bath, shower, and so on. This disposalpipework is referred to as waste pipe. Bathsrequire 50 mm pipe, showers and sinks 40 mm

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Fig. 12.42 SVP pipe fittings – a pair of branches, oneat 137.5◦ and one at 92.5◦ with blank bosses.

pipe and WHB 32 mm pipe. This can be uPVC,ABS or polypropylene.

It has already been mentioned that cast ironand copper are no longer used for domesticinstallations purely on economic grounds.Figures 12.42 to 12.44 illustrate fittings usedon soil and ventilation stacks. Figure 12.42shows a pair of branches, one at 137.5◦ andone at 92.5◦ with blank bosses; Figure 12.43shows an access pipe; and finally Figure 12.44shows a bend and a pipe with blank bosses.

The fittings in the illustrations are all sol-vent weld fittings. The fitting, a bend, in Fig-ure 12.45 has a synthetic rubber seal and is apush fit onto the pipe.

Note that all these fittings have a socket at allends for pipe. The pipe supplied by the manu-facturers has no sockets and is merely cut to

Fig. 12.43 SVP pipe fittings – an access pipe.

Fig. 12.44 SVP pipe fittings – a bend and a pipe withblank bosses.

length and welded into place, or the ends arechamfered off to facilitate pushing into thesocket past the ring seal. Having no socketsmeans that there is less waste of pipe; themanufacturers make straight couplings soshort lengths of pipe can be made up to alonger length to suit circumstances, especiallywith the solvent welded type.

Traps

The traps for waste pipe are all suitable forsinks or WHBs and are shown in three config-urations, P, S and a straight through. One man-ufacturer shows in excess of 50 trap types in

Fig. 12.45 SVP pipe fittings – push fit bend with syn-thetic rubber seal.

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Fig. 12.46 Basic trap shapes.

their catalogue, and the majority of these canbe obtained to fit 32, 40 and 50 pipe work. Fig-ure 12.46 illustrates three basic traps: from thetop left and anticlockwise, a P trap, an S trapand a straight through trap.

WCs should be sited as close to the soil stackas possible, reducing the length of the branchpipework to a minimum. Should the branchexceed approximately 1200, then some meansof ventilating the branch has to be fitted. Anexpensive solution would be to erect anotherstack, but apart from the expense it might notbe possible to accommodate another stack.The solution is to carry the branch pipeworkpast the WC and turn it up the wall next to theWC. The top of this pipework has to be higherthan the top of any other appliance connectedto the branch – frequently a WHB in the samecompartment. Then cap the pipe off with anair admittance valve, sometimes referred toby its original trade name, Durgo valve. It isillustrated in Figure 12.47, which shows a gen-eral view as fitted to the top of the SVP, and theview into the bottom, which doesn’t show a lotbut merely indicates that there is more to thething than just a fancy dome! In fact there is afairly delicate valve mechanism which opensand closes in the flow of air in and out of theSVP.

Websites to visit for information on soil andwaste pipe work and fittings are:www.wavin.co.uk/main/homepagewww.Marleyplumbinganddrainage.com

Fig. 12.47 Durgo valve.

www.McAlpineplumbing.comwww.Polypipe.com

Before leaving this section we should look atthe maximum allowable lengths of the varioussizes of waste disposal pipework, and wherethey may be connected to a soil, waste andventilation pipe.

For waste disposal, the sizes of pipes anddepth of seal of the trap are given againsteach type of sanitary appliance in Table 12.4.Note that there is a maximum length of

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Table 12.4 Disposal pipe sizes, trap seals and maximum branch lengths.

Disposal pipe Trap – minimum Max. length ofSanitary appliance size (mm) seal (mm) pipe (m) Pipe fall (mm/m)

Bath 40 50 3 18 to 9050 4

Sink 40 75 3 18 to 9050 4

Wash hand basin 32 75 1.7 18 to 9040 3

WC 83 50 6 for a single 18 to 90100 WC

Bidet 40 75 4 18 to 90

pipe measured from the trap on the appli-ance to the soil, waste and ventilation pipe(SWVP).

Figure 12.48 illustrates other importantpoints regarding the junction with the SWVP.

WC branch

No connectionsin the shaded area

Min. 200 radius

Invert level

460No connections

in this area

Fromsanitary

appliance

Rodding cap

200

To drain

Fig. 12.48 Schematic of disposal pipe connections toan SVP in domestic premises.

Insulation

Insulation of the HW cylinder has alreadybeen mentioned, but what of the rest of theplumbing installation? There are two reasonsfor insulating the installation:

� To prevent the installation freezing up incold weather

� To reduce the loss of heat from storage ves-sels and pipework carrying hot water.

To prevent freezing up all pipework must belagged, i.e. covered with a layer of insulatingmaterial. In the not so distant past 50 mm widestrips of felted fibre on hessian were wrappedround the pipe and held in place with a bind-ing of copper wire. A better approach is theuse of preformed plastic insulation. This isformed as a tube slit along its length, and itopens up to clip over the pipe and then ad-hesive tape can be applied to keep it closedround the pipe. Some manufacturers make aninsulation with a zip-type closure – more ex-pensive but simple to apply and seal. The in-sulation is made in a variety of bores to suithot and cold water supply pipe and in an eco-nomic thickness to comply with the Regula-tions. As well as preventing pipework fromfreezing, the insulation will also reduce theheat loss from hot water pipework.

Cold water storage cisterns anywhere, butespecially in the roof space, are vulnerable tofreezing up and so must be insulated with aproper layer of material which covers top and

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sides. Where the cistern is low and near theceiling finish of the upper floor, it is easy tocarry that side insulation down to the ceiling.No roof insulation should be placed under thecistern, so any heat rising through the ceil-ing escapes into the space around the cisternand helps to reduce the chance of it freezing.Where the cistern is high off the ceiling, specialmeasures must be put in place to seal from theside insulation to the ceiling. It is vital to getany heat from the ceiling up round the cistern.A simple box structure of thick polystyrenesheet (say 50 thick) should be built on a suit-able timber frame. No gaps must be left in thislayer. Proprietary insulating sets are made byall cistern manufacturers but only allow insu-lation of the cistern.

Not insulation but a suitable substitute incentral heating boiler circuits is the introduc-tion of an anti-freeze into the radiators andpipework. A 20% solution can give protec-tion from freezing down to −21◦C. Note this isonly suitable for systems which have a doubleentry, indirect hot water cylinder.

Corrosion

There are two sources of corrosion which mustbe dealt with. The first is where the water it-self is aggressive and would leach zinc out ofbrass alloys used for fittings and valves. Theuse of DZR fittings is the answer in these cir-cumstances.

The second cause of corrosion is the useof dissimilar metals in the systems, wherebyelectrolysis occurs. The first step is the exclu-sion of any metals which are detrimental to thebulk of the installation, e.g. aluminium alloys,galvanised pipe and equipment and plain ironor steel. In commercial installations or wherethese sorts of clashes cannot be avoided, it iscommon to include a sacrificial anode – usu-ally magnesium or an alloy of aluminium.This is corroded rather than the rest of the in-stallation and literally dissolves away. At thatpoint it has to be replaced.

So for domestic work, the answer is ex-clusion of metals which would have a bad

reaction with the copper pipe and brass orDZR fittings. Plastic pipe takes no part inthis process. There is one part of the sys-tem which will have steel in it – the cen-tral heating system. In fact it might havestainless steel and mild steel in the radi-ators and some of the pipework. It mighteven have cast iron in the boiler. Fortu-nately there is an easy preventative measure –the use of inhibitors. These simply make thewater incapable of supporting electrolytic ac-tion provided the correct solution has been in-troduced into the heating circuit. Note this isonly suitable for systems which have a doubleentry, indirect hot water cylinder.

Air locking and water hammer

When water is run into a system for the firsttime it sometimes happens that air is trappedin legs of pipework which fall against theflow of the water. A competent plumber willrun the pipe in such a way that this will beavoided, i.e. the pipework is laid with properfalls to allow it to fill naturally, pushing allair in pipe and equipment ahead of it as itrises in the system. Incompetent plumbers be-come expert at clearing air locks. Mostly it isa question of identifying where the air lockhas occurred and undoing or at least loosen-ing off a coupling to allow the air to escapeand the water to fill up to that point. Tight-ening the coupling and continuing to feedwater should solve that problem. Occasion-ally a sudden build-up of pressure behind thewater in front of the air lock will push theair over, and once the water starts to flowpast that point in pipework it should thencontinue.

Air build-ups are more common in centralheating pipework, especially when first com-missioned. Usually running the pump willpush air past the lock points, but of courseit has to go somewhere and that is usually thehighest radiators in the system. It is vital toget air out of the radiators, for if there is airtrapped in them hot water will not circulateand there will be little or no heat. Radiators

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Float valve

Washer

Backnuts

Boiler T,female iron

branch

Cistern wall

Cistern lid

Stop endfitting

At l

east

200

Rising main

Fig. 12.49 Shock absorber loop on rising main.

Fig. 12.50 Plumber’s first fixings.

all have bleed screws, and all that is requiredis to open the screw a little and listen for theair hissing out. As soon as water appears, turnthe bleed screw down. Water has dissolved airin it, and when heated this air is given off astiny bubbles. These bubbles are driven roundpumped systems easily enough but come torest and build up pockets of air at the highpoints. Look at air vents on the HW cylin-der sketches (Figures 12.27–12.30). Air bleedscrews on radiators and those air vents at HWcylinders all need regular attention to keep thesystem free of air.

Water hammer is generally caused by theshock wave generated when a float valveshuts against a fairly high water pressure. Itis exacerbated by loose pipework, so the firstthing to be done is careful site work whereall supply pipework is securely fastened withproper pipe clips. It is important to note thatplastic pipe should never be fixed with metalpipe clips. Plastic expands and contracts morethan metal, and the sharp edges of the metalclips would quickly wear through the wall ofthe pipe.

One device which certainly works is to in-clude a shock absorber on the pipework im-mediately adjacent to a float valve. The onethat gives most trouble because it feeds wa-ter at main pressure is the CW cistern. Figure12.49 shows how the shock absorber is fitted.Water from the rising main will push up pastthe boiler T compressing the air in the bentpipe. It is the compressed air which acts asthe shock absorber.

Float valves on WC cisterns seldom give anytrouble if fed from the CW cistern. Mains fedfloat valves on WC cisterns are another matterbut it should be possible to fit a smaller versionof the device already described.

First fixings

Finally, the photograph in Figure 12.50 showspart of the plumber’s first fixing. From left toright the pipes to be seen are:

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� A 100 soil waste and vent pipe connected tothe drain going under the foundation andready to receive a branch for the WC.

� A 15 copper pipe, insulated where it will beunder the floor, as the cold feed to the WCcistern.

� A group of three pipes, the middle one be-ing a 40 mm plastic pipe for the wash handbasin waste. The cables attached to it are

earth bonding wires. On the left is the hotfeed from the hot water cylinder and on theright the cold feed to the WHB – notice itis teed off the same pipe feeding the WCcistern.

� On the right another waste pipe, 40 diam-eter, for a bath connection, and then just insight a copper pipe, which can only be hotor cold feed to the bath taps.

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13 Electrical Work

Power generation 266Wiring installation types 267Sub-mains and consumer control units 268Sub-circuits 270Work stages 272

Electrician’s roughing 272

Earth bonding 273Final fix 275

Testing and certification 275More on protective devices 275Wiring diagrams 276Accessories 277

Electrical work embraces a very wide rangeof techniques and materials designed to meetthe needs of a wide range of users, so this textwill be short, dealing only with basic domesticinstallations.

The book of rules for all electrical installa-tions in the UK is the set published by theIEE – Institution of Electrical Engineers. Itis currently in its 16th edition with amend-ments.

Power generation

To explain simple electrical work in contextwe need to set the scene by describing brieflyhow we get electricity to our homes, and sowe start at the power station. The power sta-tion houses generators, each of which is an ar-rangement of coils and magnets, some fixed,some revolving on a shaft. As coils and mag-nets pass, electricity is generated and takenaway by heavy cables to be distributed tothe consumers. The current generated can beshown graphically as a sine curve.

When a wire passes through a magneticfield, a current is induced in the wire. Make thewire into a compact coil, the magnet producean efficient field, and the current generatedcan be put to practical commercial use.

Figure 13.1 shows in diagrammatic form ascheme for power generation and sine curveof the power generated.

The ‘events’ marked A to F can be fol-lowed: Considering only coil R, at A the coilis halfway between the magnetic poles andno current is being generated. At B, coil R isnearer the north pole and so current is gen-erated and we see the sine wave starting toform – and rise. At C, coil R is opposite thenorth pole and current generation peaks. AtD, it is halfway between the poles and cur-rent generation is again zero. At E, the coil isopposite the south pole and generation peaksagain, but on the opposite side of the X axisand at F it is halfway between the poles andno generation takes place. The cycle is aboutto begin once more.

If the coil traces that cycle through the mag-netic field 50 times per second, then the sinewave will form 50 times per second, i.e. thecurrent is being generated at 50 hertz. Thecoils in Figure 13.1 are shown ‘star’ wound,i.e. one end of each coil is joined and this isconnected to the neutral wire in the distribu-tion chain. The other ends of the coils eachprovide current as they pass through the mag-netic field of the magnet: thus we can havethree lots of current being generated, eachlot being taken off and distributed down the

266

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Electrical Work 267

A B C D E F

N

S

N

S

N

S

N

S

N

S

N

S

R

R R

R

R

BY

B

Y

B

Y

B

Y B YR

B

Y

A B C

D E F

Rotating coils

Fixed magnet

Fig. 13.1 Diagram of sine curve and power genera-tion.

power lines to the consumer, who can haveone, two or all three lots of current delivered tothe premises. In practice it is only one or threelots which are distributed to the consumer.

The ‘lots’ are known as phases, which arecolour coded red, blue and yellow. In theory,because the three phases generated are in per-fect balance, the neutral cable should carry nocurrent at all. A further cable is included andthis is the earth cable, usually incorporated asthe armour steel wire wound round the out-side of the mains cable in underground sup-plies, or as a separate wire in overhead trans-mission lines.

Generation of the electricity in this waygives rise to a current which flows in one di-rection, as seen when the sine curve rises andfalls on one side of the X axis, and then re-verses as the curve rises and falls on the otherside. Thus we have alternating current, a.c., ineach phase.

Transmission down the supply authori-ties’ cables is at a much higher voltage thanthat received by the customer, starting off at33,000 volts and transformed down in stagesuntil it comes into the street where there arecustomers’ premises.

The voltage delivered at the customers’premises on each phase in the UK is 230 voltsat 50 hertz. By combining two phases in thepremises, the consumer can have 440 voltsand with three phases 660 volts. These largervoltages are much more economical for run-ning machinery and industrial processes buthave been occasionally used in domesticpremises for electrical heating. This is now fartoo expensive to operate at the general tariffbut may be the only option for some house-holders who can only get off-peak power forstorage heating.

We will only consider the use of a singlephase supply.

Wiring installation types

Classification of electrical work can be on thebasis of the methods and materials used toprovide various types of electrical circuit:

� Sheathed and insulated cable� Insulated cable in conduit, there being three

grades of conduit: slip jointed, threadedand heavy gauge threaded. Also, conduitis available as black enamelled or as hotdipped galvanised

� Mineral insulated cable� Armoured cable

And so on. . .

The most simple installation is the first,sheathed and insulated cable. The cores whichcarry the current are insulated, and two ormore are laid side by side with a bare coreas an earth wire, and covered in a tough flex-ible sheath. This type of cable uses PVC forboth insulation and sheathing, although it isstill possible to obtain vulcanised rubber insu-lation and sheathing, and heat resistant varia-tions of both insulation and sheathing for spe-cial situations.

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Sub-mains and consumercontrol units

Starting at the beginning of any installationthere is a heavy cable entering the propertyfrom the mains cable in the street. This ter-minates at a piece of equipment frequentlyknown as a cut-out or cable head. Thishard plastic box contains: one, two or threefuses, termed service fuses, a neutral termi-nal block and an earth terminal block. Oursingle phase supply will have one fuse, oneneutral terminal and one earth terminal block.Current practice is to house the cut-out to-gether with the supplier’s meter in a cupboardaccessible from the outside of the building.

Figure 13.2 is a photograph of a typical sin-gle phase cut-out and meter cupboard. Thereare many different patterns of cut-out andmeter. The photograph is fairly self explana-tory. The cupboard, a lidded box, is made ofglass fibre and has a piece of 25 particle boardfitted in the back onto which the equipment is

Fig. 13.2 Single phase cut-out and meter in cupboard.

screwed in place. The mains cable comes upa duct into the cupboard and is wired in tothe cut-out. The main fuse and connection ofneutral and earth wires can all be seen clearly.Note how both live and neutral wires go intothe meter and then from the meter go throughthe wall into the house. The duct through thewall of the house is not very clear.

Inside the house, and ideally on the otherside of the wall from the supplier’s cut-outand meter, there will be a consumer’s controlunit (CCU). A sleeve will be built through thewall to allow the easy passage of cables fromthe meter and earth terminal to the consumerunit. A cable should never be built solid intomasonry as any movement could then dam-age the insulation or even break the cores. Thecables from the cut-out to the CCU are knownas tails and are generally single core insulatedand sheathed cable. There is also an earth tailof insulated cable from the earth terminal inthe box to the consumer unit. The tails shouldbe kept as short as possible. Collectively thesecables and the CCU are known as the sub-mains circuits. In large installations these sub-mains circuits can be complex, involving lotsof switch gear, circuit breakers and cabling fre-quently in conduit and trunking1.

Figure 13.3 is a photograph of a small se-lection of cables used in domestic work. Thecables, starting from the top, are:

� 1.00 mm2 twin and earth core cable (ECC) –note the solid cores

� 2.5 mm2 twin and ECC – note the solid cores� 6.00 mm2 twin and ECC – note the twisted

cores� 6.00 mm2 insulated earth cable – note the

twisted core and the anti-clockwise twist� 10 mm2 sheathed and insulated cable as

used for tails to CCU; the seven strands ofthe core have been separated out for pur-poses of illustration.

1 Conduit is a metal tube used to give mechanical protec-tion to insulated-only cables. In this context trunkingis a rectilinear section of metal boxing with a detach-able lid also used for mechanical protection. It can carrymore cable than conduit and can be subdivided inside.Trunking is also available in plastic in a wide variety offorms and finishes.

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Electrical Work 269

Fig. 13.3 Cables.

The consumer unit has a number of devicesbuilt into it:

� A switch which will break the continuity ofboth the current carrying cable (line cableor simply line) and the neutral cable or sim-ply neutral. A switch of this type is called adouble pole switch. The earth wire is neverbroken by a switch or other device.

� Devices to protect the cable(s) in the indi-vidual circuits which start in the consumerunit, such as those for lighting, cooker,water heating, small power, etc. They areknown as sub-circuits. Each will have a ca-ble of a size appropriate to the function (andtherefore current expected) to be carriedout. Lighting circuits which carry only 1 or2 amperes (a measure of current) will not re-quire cable quite so large as, say, the cookercircuit, which might be serving a doubleoven and grill and draw up to 12–15 am-peres at any one time

� A neutral terminal block� An earth terminal block� A residual current circuit breaker (RCCB)

(also known by the now obsolete nameearth leakage circuit breaker (ELCB),

Earthing terminal

NeutralterminalBus

barMiniature circuit breakers

connected to bus bar

Green/yellow sleeving

Red insulation Black insulation

OFF ON ON ON

Sheathed cable

Connecting pointsfor tails

RCD

Double polemain

switch

Internal wiring by manufacturer

Spare way

on bus bar

Fig. 13.4 Schematic of consumer control unit.

current operated type). See notes on RCCBslater in this chapter. The device is a doublepole switch activated when there is a faultin the sub-circuits and generally in addi-tion to the double pole manually operatedmains switch. A schematic for an RCCB isshown later in this chapter.

Figure 13.4 is a schematic of a consumer con-trol unit (CCU). It shows only the workingparts. These would be mounted in an enclo-sure of either metal or plastic, the choice be-ing made on whether or not the CCU might besubject to mechanical damage, and to what ex-tent. The enclosure would cover all the termi-nals and exposed parts of the bus bar, and theinternal cabling and sub-circuit wires enter-ing the CCU. The lid on the CCU would covereverything except the mains switch. Note thatthe diagram shows all the sub-circuits pro-tected by the RCCB.

Figure 13.5 is a photograph of a CCU. It hasa plastic enclosure, and the flip-down lid cov-ers the mains switch. Note that only four outof the nine sub-circuits are protected by theRCCB. It is important to emphasise that thefuse or MCB is provided to protect the wiringof the sub-circuit from a current which woulddamage it by overheating the core(s). It is notthere primarily to protect anyone from elec-trical shock, although should a core come intocontact with metal work in the house, the cur-rent would immediately rise, the fuse wouldblow or the MCB would trip and shock tothe householder would be prevented, i.e. ifthe householder then went on to touch thatmetalwork no current would now be passingthrough it.

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Fig. 13.5 Consumer control unit.

Personal protection is provided by the inclu-sion of an RCCB or ELCB in the CCU. The de-vices used to protect sub-circuits are of threekinds:

� Rewirable or semi-enclosed fuses� HBC (high breaking capacity) cartridge

fuses� Miniature circuit breakers (MCBs).

New installations now only use MCBs.Figure 13.6 shows a photograph of rewir-

able fuses, and on the left is an MCB. The up-per face in the photograph is what the con-sumer sees when the cover is opened. Thescrew holes for the electrical connections are ateach end, and in the middle of the T-bar is theswitch, tripped when the MBC is overloadedand which is depressed to reconnect the sub-circuit. The device clips into the CCU enclo-sure on what is called a DIN rail, and the busbar is fixed into one end and the sub-circuitwires into the other end, tightening the screws

Fig. 13.6 Fuses and a miniature circuit breaker.

provided. On the right of the photograph area pair of the now obsolete but widely usedrewireable fuses. The upper one is of porce-lain and the lower of plastic. The terminals forfixing the fuse wire can be seen on the lowerfuse carrier.

Sub-circuits

Sheathed and insulated cable was shown inFigure 13.3, and two types were illustrated:

� Twin and earth core cable� Single core cable.

Twin and earth core cable has a core with redinsulation, a core with black insulation and abare earth core embedded in the sheath. Al-ternatively (and fairly rarely) both insulatedcores are coloured red and the cable is usedsolely for switch wiring. This then is the basiccable used to bring electricity out of the CCUto fitting(s) and appliances.

Three core and earth core cable has threecores with red, blue and yellow insulationand a bare earth core embedded in the sheath.In domestic work this cable has always beenused for two-way switching of lights on stairsor in hallways; however, two-way switch-ing of central lights in bedrooms is becom-ing more common. Using this cable it is pos-sible to have any number of switches in thecircuit controlling just one light. For exam-ple, in a bedroom a central light can be con-trolled by a switch at the door and two otherswitches, one either side of the bed position. Ina circuit with multiple switches only two aretwo-way switches, the rest being intermedi-ate switches.

With these cables it is possible to wire up allthe mains sub-circuits necessary for the mod-ern house. Circuits themselves can be eitherradial circuits or ring main circuits. Each cir-cuit is connected to the CCU, the red core tothe fuse protecting the circuit, the black coreto the neutral terminal and the bare earth wireto the earth terminal.

In ring main circuits the cable is led fromthe CCU round the fittings being served by

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Electrical Work 271

the ring main and then back into the CCU.Both red cores are connected to the MCB pro-tecting the circuit, both black cores are con-nected to the neutral terminal, and both earthcores to the earth terminal. When the sheath isstripped back to get cable into CCUs or otherfittings, the bare earth wire is fitted with ashort length of green/yellow striped sleeve toprevent it from coming into contact with anyparts carrying electricity.

Examples of radial circuits protected by aMCB would be:

� All the lights on one floor or half the floorarea of a single-storey house

� A cooker – hob and oven� An electric immersion heater or circulator� An outlet point for supplying power to the

central heating control system. This is notelectrically powered central heating

� A dedicated circuit for telephony and/orcomputing.

Examples of ring main circuits protected by asingle MCB would be:

� Small power supplying 13 ampere socketoutlets. If the floor area of the house is lessthan 100 m2, an unlimited number of socketoutlets can be attached to the circuit; how-ever, it is prudent to allow two ring mains inthe average house, especially with the everincreasing number of appliances which canbe plugged into these socket outlets. Onering main should serve the kitchen and anyutility room(s) in larger houses.

� Power supply to a series of electric heatingappliances such as storage heaters. Whetherone or more ring mains would be necessaryfor this has to be determined by the circum-stances but it does mean more ring mains,not heavier cable and larger fuses or MCBs.

Each of these circuits will expect to have a verydefinite amount of current supplied, and asthe amount of current passing in a cable de-termines how hot the cable becomes2, the size

2 Several factors affect the heating effect of a current pass-ing through a cable, e.g. how cables are disposed, in con-duit, bundled or bunched, clipped to surfaces, buriedin walls or floors, under insulation, etc.

of the cores in the cable is critical if the houseis not to burn down. Core size is given asthe cross-sectional area in square millimetres.Looking at the circuits mentioned above, wecan use the core sizes from Table 13.1 and con-nect them to protective devices with the notedcurrent ratings.

Cables with 1.00, 1.50 and 2.50 mm2 coreshave single solid cores. Cables over thesesizes have stranded cores of seven wires witha very slow anti-clockwise twist. The mostusual metal for the wires in the cores is cop-per, although copper covered aluminium hasbeen used when copper was expensive3. Cop-per can now compete economically with alu-minium.

Further notes on ring mains must mentionthat socket outlets can be disposed of in differ-ent ways on the ring main. The majority mustbe wired in in parallel, i.e. the cable cominginto the socket outlet and the cable going outare wired in red and red, black and black andearth and earth. These socket outlets may beeither one or two gang. In addition to this, anumber of further sockets may be wired tothese earlier sockets as spurs, i.e. a separatelength of cable – 2.5 mm2 – is connected tothe terminals of a socket on the ring main andrun out and wired to an outlying socket. Fur-thermore, it need not be socket outlets whichcan be wired to the ring main or as spurs.Cable outlet plates can be wired in as wellas, or totally in place of, socket outlets.

Cable outlet plates are used where there isan appliance – such as a space heater or waterheater – to which power must be supplied. Acable – usually a three-core flexible cord – iswired from the plate to the appliance, some-times hidden in conduit or ducting or simplyset in the voids of a framed structure, and onother occasions it comes out of a hole in theface of the plate and out to the appliance.

When attaching anything to a ring main, ei-ther through a plug top in a socket outlet or

3 The reason for not using plain aluminium was that itwould suffer from corrosion problems. However, evenafter covering it with copper it was found to suffer frac-tures when put under pressure at screw down termi-nals. It would literally harden and break off.

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Table 13.1 Circuits and cables.

Current carryingCore area capacity of cable Protective device

Circuit in mm2 enclosed in a wall rating (amperes)

Lighting 1.00 and 1.50 11 and 14 5 and 10Oven and hob* 6 or 10 or 16 32 or 43 to 52 or 57 30 or 45Immersion heater or

circulator2.50 18.5 15

Central heating 1.00 11 5Telephone/computing 1.00 11 5Small power ring main 2.50 18.5 30Electrical heating ring

main2.50 18.5 30

Supply to garage** 2.50 18.5 15Supply to garage† 10 43 to 52 45

∗Depending on the total rating of the appliances or whether it is only an oven, and a gas hob has been fitted.∗∗This would only allow the fitting of a light and a double socket outlet in the garage.†This would allow a lighting circuit with several lights and a ring main with several socket outlets to be installed.

by a cable outlet, each individual appliancemust be fused with a 3, 5 or 13 ampere car-tridge fuse. Plug tops all make provision forthese fuses. Cable outlet plates have a fuse car-rier built into them. Both socket outlet platesand cable outlet plates can have some optionalextras: switches which are in the main singlepole; neon indicators which glow when poweris passing to the appliance connected; andRCCBs.

Work stages

Electrician’s roughing

Like many trades, electrical work is carriedout in a number of stages. The first stage,electrical roughing, is the installation of sub-circuit cable. First of all, the house has to bereasonably watertight overhead; the work isthen carried out when the internal partitionshave been put up but before any plaster orplasterboard is applied to masonry or timberframing.

Where there are masonry walls, the cablehas to be let into a groove or raggle cut into thesurface of the masonry. The cable is protectedfrom mechanical damage by a short length of

conduit fixed into the raggle. Current prac-tice uses plastic conduit. It is not as resistantas steel conduit to nails being hammered in,but anyone with common sense does not ham-mer nails into a wall immediately above orbelow a switch or a socket outlet etc. So theplastic conduit is more about not embeddingthe cable solidly into the wall and also aboutbeing able to withdraw it if rewiring is to beundertaken.

Also, in masonry walls which are to be plas-tered, accessories such as switches and socketoutlets will require a rectangular box to whichthey are fixed – a back box. The box is letinto a recess cut into the masonry, with justthe thickness of the plaster protruding. Boxescan be simply ‘cemented’ into position butit is more usual to have them plugged andscrewed back to the masonry. The boxes aremade of galvanised pressed steel and haveknock outs for the insertion of the conduitand/or the passage of cable. The edges of thehole at the knock out can be quite sharp, andto protect any sheathed and insulated cablepassing through, a rubber grommet is fitted inthe hole. A knock out is formed in the metalof the box by pressing a die against the metalwith sufficient force to weaken the circumfer-ence of the hole but not remove the metal. The

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Electrical Work 273

removal is done by the electrician, and thesepieces of metal can be poked out with the endof a screwdriver.

In timber framed housing or in stud parti-tions, the cable is simply drawn through holesdrilled on the neutral axis of the framing tim-bers. Where the cable crosses or passes along-side roof timbers it is usual to fix it downwith small plastic clips which have a hard-ened steel nail already in place. Various sizesof clip are available to accommodate differ-ent sizes of sheathed and insulated cable, andthese are identified by referring to the core sizeof the cable, e.g. 1.00 mm2, 2.50 mm2 and soon. Cables passing down through stud parti-tions or frames in walls merely pass throughholes drilled in the timbers.

An important point is that the cable is notpulled tight or fixed down under any kind ofstress. To do so would stretch the insulationand the sheath, resulting in their becomingthinner and so less capable of protecting orinsulating the core. In extreme cases the coreitself would become stretched and thereforethinner and less able to carry the full currentfor which it was rated.

When the electrician finally connects upall the cores to appliances or accessories, heor she takes great care to ensure that thesheath of the cable is enclosed within the ap-pliance or accessory, and always uses anycable clamps to grip the sheath and insu-lated cores together. Insulated cores are neverleft outside appliances or accessories or backboxes and are never clamped on their own.So when using steel back boxes the sheathis always drawn inside, but there is gener-ally no clamping arrangement. Quite gener-ous lengths of cable are left at the places whereaccessories such as switches and socket will bemounted.

Modern practice in plasterboard clad wallsand partitions uses a box of plastic which isplaced in a cut-out in the plasterboard andwhich clips to the plasterboard. The boxes areknown as dry lining boxes. With the dry lin-ing boxes, the entries into the boxes have aflap which allows the cable to pushed throughand which springs back to grip the sheath, an

improvement on the steel back box with noth-ing to grip the cable.

Earth bonding

Earth bonding is an extremely important partof the installation and there are two parts to itwhich must be carried out.

The first part is the connection of the earthcore in the twin or three core cables to theearthing terminal in the CCU. Where cablesreach appliances or accessories, the earth coremust be secured to the earthing terminalssupplied as part of the appliance or acces-sory. This earth core in sheathed cables hasno insulation, and yet to reach the earth ter-minals it may have to be left quite long. Toprevent this length of bare wire from touch-ing the other terminals, a short length ofgreen/yellow plastic sleeving is pushed overthe wire. Any old length will not do; it has tobe capable of insulating the whole of the earthwire protruding from the sheath.

The second part comprises a core withgreen/yellow insulation only installed fromeach metallic appliance or piece of equipmentwhich is to be installed in the house, all theway back to the CCU position. Metallic ob-jects would include any metal sink, pipeworkto sanitary appliances, metal cisterns, radia-tors or other emitters, gas or other pipeworkand so on. Patent clips are used for thinmetal edges, and earthing clamps are used onpipework.

Accessories such as switches, socket outletplates, etc. are fixed in position to boxes withthreaded inserts for the screws. The boxes aremade in different styles to suit different situa-tions. Accessories are either surface mountedor flush mounted. For surface mounting, thebox to which they are fixed is itself mountedon the surface of the wall and is fully exposed.For flush mounting, the box is sunk into thewall so that only the thin plate on the top of theaccessory is visible. All these boxes are calledback boxes.

Back boxes are made from steel (galvanised,zinc plated or black enamelled) and plastic,

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and different shapes and styles have evolvedfor surface and sunk finishing, but in the mainall require to be fixed back to the wall, floor orceiling with screws and plugs.

One special box for mounting within a plas-terboarded wall has patent clips which al-low it to be fixed flush with the surface heldonly by the clips which tighten on the plaster-board as the accessory plate is screwed intoplace. This particular style of box is commonlyknown as a dry lining box.

All the boxes are available as one or twogang or circular, and the surface mountingones are also available in different depths. Sur-face mounting boxes in plastic are also some-times called pattress boxes.

Steel back boxes and plastic dry liningboxes, like the accessories which are fixed tothem, are either single or double (it is possi-ble to have three or four but that is rare) andthis is referred to as one gang or two gang. Tomake things more complicated it is possibleto have a two or three gang light switch – i.e.two or three switches – in a plate which onlyrequires a one gang back box. Figure 13.12should make this clear.

Figure 13.7 shows a steel back box with con-nector for plastic conduit. It is a one gang gal-vanised steel back box of 25 depth, and in oneof the knock outs a connector for plastic con-duit has been fitted – a male connector witha back nut. Alongside it is another conduitconnector, a female bushed connector with amale bush. In both cases the plastic conduitis solvent welded into the plain socket andthe cable drawn down once the solvent weld

Fig. 13.7 Steel back box and plastic conduit connec-tions.

Fig. 13.8 Plastic dry lining boxes and cable clips.

cement has dried out. Note that the solvent inthe cement would attack PVC insulation.

Figure 13.8 shows plastic dry lining boxes –one gang, two gang, and circular, and cableclips. All push through plasterboard or othersheet material with the side clips withdrawninside the body of the box. Once in position,the side clips are pushed out behind the plas-terboard and pulled up tight. There is usuallya ratcheting action built into the plastic extru-sions. Finally, when the accessory is fixed tothe back box, the fixing points for the screwsare located in the lugs holding the box tight tothe plasterboard so that tightening up thesescrews serves only to fix the box in positionmore securely. The rectangular boxes are usedfor accessories with rectangular plates whilethe circular box. . .

In the bottom right of Figure 13.8 are a fewcable clips of the type used to secure twin andthree core sheathed cable. The nails suppliedalready fitted are of plated, hardened steeland will drive into masonry as well as timberbackgrounds.

Figure 13.9 shows two water supply pipesto a wash basin, with earthing clamps fixedto them, and an earth bonding cable attachedand running off to the left – hopefully to anearthing point.

Figure 13.10 shows a patent clip with anearth wire attached, which can be screwedfirmly to the welded edge of something like

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Electrical Work 275

Fig. 13.9 Earthing clamps on pipework.

a radiator, a metal sink or similar piece ofequipment.

Final fix

Once the first rough is completed, the elec-trician must wait for wall, floor and ceilingfinishes to be applied right up to the paintingand decoration. When that is at least underway the electrician can return and fit the lightswitches, socket outlets, etc. to the back boxesor dry lining boxes, the ceiling roses for lights,etc. and make the electrical connections ineach. The CCU is also mounted in a cupboardwhere the tails come through the wall fromthe meter, and all the connections are madein that. All the earth cabling from around thehouse must also be connected to an earthingterminal and this connected to the earthingterminal on the supply company’s cut-out.

Fig. 13.10 Earthing clip on radiator.

Testing and certification

The installation must now be tested for conti-nuity, polarity, insulation and earth resistance.

Continuity means testing to see that all coresare correctly connected and do not impede theflow of current to the appropriate accessoriesor appliances.

Polarity means testing to see that the cable(red insulation) connected to the phase wireis connected to the correct terminals of socketoutlets and fixed appliances, and that it is thecable broken for switches in circuits with sep-arate switches such as lighting.

Insulation means testing to see that there isno leakage of current from a core to earth oranother core due to the insulation being cut orotherwise damaged.

Earth resistance means testing to see that theresistance to the flow of current in all earth-ing cores connected back to the earth terminalat the supply company’s earth terminal is asnear zero as possible. This includes earth coresincluded in twin and three core and earth ca-bles, as well as separate earth cores leadingfrom isolated metalwork.

Once this has been carried out satisfactorily,the electrician can sign a certificate that thishas been done and that the installation com-plies with the IEE Regulations.

Now the supply company can be asked tocome and connect the system to the mains.Their engineers no longer test installations,so all they have to do is open the cover onthe cut-out, mount the meter in the box, andusing the cable left in the tails make the con-nection from cut-out to meter to CCU. Puttingthe appropriate fuse in the cut-out and replac-ing and sealing the cover completes the wholeinstallation.

More on protective devices

A further few points about RCCBs, MCBs andfuses.

Figure 13.11 shows a schematic of a currentoperated RCCB. It is essentially quite a sim-ple idea. While current flows through both

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Earth

Line

Neutral

Trip coil

Main contact

Main contact

Magneticcore

Main coil

Main coil

Residual current

coil

Fig. 13.11 Schematic of a residual current circuitbreaker.

conductors or cores in the cabling, the cur-rent is in near perfect balance, but should afault develop which causes the current to leakfrom any conductor to earth, then the imbal-ance pushes up the current flowing throughthe little coil, which in turn trips the spring-loaded double pole switch in a matter of a tinyfraction of a second. RCCBs generally havea test button which can be used not just totest the device but also to shut down all thesub-circuits which follow, thus obviating theneed for a double pole switch. They should betested regularly.

It was stated earlier that the present designof RCCB has rendered the ELCB obsolete. TheELCB was originally designed to operate inone of two ways: by reacting to an imbal-ance in current or to an imbalance in voltagebetween two conductors. Either type is nowobsolete, and they should be replaced withRCCBs which only react to an imbalance incurrent.

MCBs are the preferred protective device fordomestic sub-circuits, and all modern CCUssold in the UK now make provision for theseto be fitted as standard They are built with awhole range of ratings – the amount of currentthey carry before tripping – from 5 to 45 am-peres. HBC or HRC (high rupturing capacity)

cartridge fuses, obtainable in ratings of 5, 10,15, 20, 30 and 45 amperes, are termed Type1 fuses and used in CCUs; and the 60, 80 and100 amperes, available as Type 2 or 3 fuses, areused by the supply company in the cut-outs.These fuses should not be confused with thefuses used in 13 ampere plug tops and whichare rated at 3, 5 or 13 amperes. These lattercannot be used in CCUs.

Last, and no longer installed in the UK,are rewirable or semi-enclosed fuses. Theremust be hundreds of millions if not billions ofthis type worldwide, but they are definitelyfrowned upon in the UK and EU now andshould be replaced with MCBs if at all possi-ble. They comprise a base fitted over the busbars in the CCU into which a fuse wire carrieris plugged, two pins making contact with thebus bars. The two pins have screw terminalsinto which a length of plain or tinned copperfuse wire is fitted, passing through a hole inthe carrier between the pins. The CCUs madewith rewirable fuses at the time HBC fusesand MCBs were being introduced were gen-erally of a pattern which would allow any ofthe three devices to be fitted. In some instancesonly the overall cover requires to be changed,making the conversion of an existing CCU toa more effective circuit protection unit muchmore feasible and less expensive than com-plete replacement.

There is no retrospective legislation to makeconsumers update their control units, butwhen premises are being rewired or largelyaltered, competent electrical engineers willalways insist that MCBs are fitted into anyCCUs.

Wiring diagrams

No text on electrical work seems to be com-plete without line drawings of how to wireup a light with a one-way switch, with two-way switches (there is seldom any mention ofintermediate switches), ring mains and so on.This seems a pretty futile exercise as the draw-ings themselves are never colour coded andthe insulation is colour coded so that correctpolarity of the wiring can be observed. Also,

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Electrical Work 277

the insulated cores in twin and three core cableare shown all over the place, while in realitythey are never split out of the sheath or are oth-erwise running together in conduit. The finalobjection must be that it might provide the in-experienced with just enough information tomake them think they can do their own wiringup with disastrous results. Besides which, theamateur will not have the equipment to testthe installation correctly and so will neverknow if it has dangerous faults or not.

Leave this type of work to those with propertraining employed by a reputable company ofelectrical engineers with the proper tools andtest equipment.

Accessories

Accessories are such things as switch plates,socket outlets etc. The photographs which fol-low are of accessories and small parts used ina typical installation.

Figure 13.12 shows switch plates and a ceil-ing lighting pendant. On the left are three

Fig. 13.12 Light switches.

switch plates, from the top a one gang, a twogang and a three gang, and to the right ofthe last is a one gang architrave switch. Thethree square switch plates all fix back to a onegang back box. Architrave switches have spe-cial back boxes in galvanised steel only.

The other item in Figure 13.12 is a ceilingpendant. It comprises a number of compo-nents. Top left is a dark circular plate whichis screwed back to the ceiling board direct orto a circular dry lining box. A curved termi-nal block can be seen on one edge, a lengthof twin core flexible cord is wired into twoof the three terminals, and to the right-handside there is a bright metal terminal, the earthterminal. The flexible cord passes through thewhite disk on the right. This is the cover forthe back plate, and together they form a ceil-ing rose. Finally, the flexible cord is connectedto an all insulated lamp holder. (Note, electri-cians don’t talk about a light bulb – the properterm is lamp).

Flexible cord is different from the cable usedto wire up the sub-circuits, and it is principallythe cores which make that difference. Insula-tion is PVC just like the sub-circuit cable, butit may be flat or round and there may or maynot be an earth core. Line and neutral coresare insulated brown and blue normally andthe earth is always green/yellow. The coresare formed by twisted copper wires, anythingfrom 19 strands upwards being used. The

Fig. 13.13 Socket outlets, plug top and cable outlet.

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Fig. 13.14 Electrician’s roughs – cable coming to-gether at the point where the CCU will be sited.

particular example here has 19 strands of cop-per, blue and brown insulation and a whitecircular sheath. The more strands, the moreflexible the cord. Note: cord is the correct term,although many tradesmen refer to it as flex.

Figure 13.13 shows socket outlets, plug topand cable outlet plate. On the left are a pair oftwo gang socket outlet plates, the upper onebeing a surface mounted plate with an enam-elled steel surface back box. This type is usedfor commercial and industrial work as well

as domestic garages etc. It is generally usedwith a conduit system, although conduit todrops is a viable alternative. The lower twogang socket outlet has a metal plate – brass –for mounting on a recessed back box. On theright at the top is a one gang plastic socketoutlet plate and below it is a plastic switchedfused cable outlet plate. The fuse carrier canbe seen with the little screw holding it in placeand the plate is switched but does not have aneon indicator lamp. At the bottom is a plugtop. These should not be referred to as sim-ply plugs. Plug tops have provision for fusingindividual appliances.

Finally, Figure 13.14 is a photograph of theelectrical cable in a single-storey house com-ing to a point in the floor where the CCU willbe sited. Cable can be seen clipped to the sidesof the ground floor joists, and some plumber’sroughing can also be seen – the insulatedpipework for central heating. None of the ca-bles is identified but the size of cables for spe-cific circuits can be readily identified on site,and if there are more than one or two, a metercan be used to determine what goes where.

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BY019-App-A BY019-Fleming-v6.cls September 14, 2004 20:5

APPENDIX A

Maps and Plans

Maps

Maps are the two dimensional representation ofthe three dimensional features of a landscape1.They are produced to large scales by OrdnanceSurvey: 1:50 000, 1:2500, 1:1250.

The name Ordnance Survey came about fol-lowing the introduction of longer range, accu-rate artillery. To be able to hit a target some milesdistant, the artilleryman must know where hisgun is sited as accurately as possible. He mustalso know where the target is, relative to thegun. Guns of all kinds are collectively knownas ordnance and so any cartography carried outto assist in their use became known as Ord-nance Survey. The first survey of the whole ofthe UK was carried out with this in mind fol-lowing the onset of the Napoleonic wars, and al-though the civilian population derive great ben-efit from these surveys, their military use hasbeen the driving force behind all such surveysworldwide.

1:50 000 is the familiar Landranger scale. Mapsto this scale are used by a variety of the publicand professionals but are familiar to climbers,walkers and ramblers. Each sheet covers an areaof approximately 1600 square kilometres, andhas a grid of kilometre squares. Levels are shownby contours at 10 m intervals and spot heightson prominent high ground.

Individual large buildings in open countryare distinguishable but not capable of being

1 Charts show underwater features, buoyage and lightsoff shore, land masses and prominent features on landmasses such as major hills, church spires and tall build-ings and including lights for navigation etc. and insteadof contours show depths of water. They are preparedby the Admiralty Hydrographic Department.

measured. Location is only possible to the near-est 100 metres by giving a six figure reference. Byprefixing that reference with the letter referencefor the square of 100 km side, a unique refer-ence within the UK is obtained. A typical gridreference is given in Figure A.1 with referenceto Grange Farm from OS Landranger sheet 66.Try it with a Landranger or map of similar scalewhich you have. The procedure is simple and isillustrated in Figure A.1.

Larger scale maps are of use to the builderor developer, utilities companies, etc; 1:2500and 1:1250 maps show much more detail butcover a very much smaller area. On the 1:1250and 1:2500 maps, all buildings are distinguish-able and their size can be ‘measured’ to somedegree. Levels are shown as ‘spot levels’ andbenchmarks are shown. Even at these largerscales there is insufficient accuracy to allow usto plan the positioning of new buildings andalso show all the detail of services and ancillaryworks.

Plans

A typical site plan is shown in Figure A.2. 1:500is the largest workable scale and plans of thistype (note change of terminology with changeof scale) have not been available from OS, butthis will change in the very near future. They aregenerally plotted out from a survey carried outby the building client’s surveyors. Traditionally,civil engineers or surveyors would ‘survey’ thearea and plot out the results to a scale of 1:500.A 1:1250 or 1:2500 map might be used as a basisfor such a plot.

279

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Read letters giving 100 000 metresided square reference: Locate first vertical line to left of location. Enter two numbers. Now estimate tenths:Locate first horizontal line below location.Enter two numbers Now estimate tenths:

Together these give a unique reference within the UK

NT22

4 86

6

NT 224 866

Fig. A.1 How to determine a grid reference from a1:50 000 map.

The survey was traditionally carried out usingchains2 or steel measuring tapes and a dumpylevel or tacheometer (see Appendix B). Now so-phisticated instruments are used which measurelevels and distance as well as relative angles be-tween straight lines joining all points. They em-ploy EPDM – electronic position and distancemeasurement. The data is recorded in magneticformat and fed into a computer which does theplotting.

Surveyors must be able to measure out landand buildings and plot out all the salient fea-tures to a suitable scale as a site plan. The elec-tronic distance measurement equipment is veryexpensive and is only warranted on the largestof surveys or where the survey has to tie in withwork previously done and/or which will be pro-cessed on a computer. It is possible to hire spe-cialists who do any size of survey – for a price –and who will use the latest equipment to cutdown on the staff overhead. Larger developersuse it where the initial ground survey is done,the 1:500 or other scale plans are plotted andthen the same equipment is used to lay down agrid of reference points on the actual site. Theseare used by the trades foremen to set the build-ings correctly on the site as well as the alignmentand levels of roads, sewers, etc. Illustrations areincluded in Appendix B, Levelling.

If the need for survey work is infrequent or thesize of the survey does not warrant the expenseof hiring specialists, it will pay to have the capa-bility of reverting to traditional methods. Thatmeans having the basic equipment to hand andkeeping the necessary skills honed.

Carrying out a survey allows us to ‘fix’ theabsolute positions of all existing features, levels,etc. To the drawing which we plot out from thesurvey we add the proposed works – buildings,roads, services, alterations to topography. There

2 There were two kinds of chain in use, the Gunter chainand the engineer’s chain. The Gunter chain was 66 ftlong (22 yd or one tenth of a furlong). The engineer’schain was 100 ft long.

may be so much to add that it is split up overa number of separate plans. Our site in FigureA.2, however, is not so large and is now shownas a proposed development in Figure A.3.

Scales of 1:200 and 1:100 are occasionally usedfor site plans where the 1:500 would not be ca-pable of showing the required depth of detailand would not allow more accurate transfer ofinformation to the physical site: for example, ablock plan of a large building to be altered orextended. The overall size and shape of eachbuilding which is shown on the site plan istaken from the figured dimensions shown on thebuilding plans. These dimensions should neverbe scaled off. The position of the building shouldbe marked with written dimensions on the siteplan.

Plans of buildings

When a building is planned, drawings of it areprepared to a variety of scales depending on thedetail required at each stage in the design. Plansdrawn to scales of 1:50 or 1:100 are prepared toshow foundations, accommodation, roof, etc., asshown in Figure A.4.

‘Sections’ are also prepared. These are viewsof a ‘slice’ across the building. Scales used are1:50, 1:20, 1:10, as shown in Figure A.5.

Finally, ‘details’ are prepared. These showhow materials and components fit together.Scales include 1:1, 1:5, 1:10 and 1:20, as shown inFigure A.6.

� Scales of 1:500, 1:1250 and 1:2500 are knownas engineer’s scales

� Scales of 1:1, 1:5, 1:10, 1:20, 1:50, 1:100 and 1:200are known as architectural scales.

Only use these standard scales.If the drawing paper or film cannot accom-

modate the full building use a smaller scale orbreak up the building into different parts with a‘key’ drawing to a smaller scale. Never use non-standard scales. These can be misinterpreted bysomeone, with disastrous results. For example, adesigner was employed in the Far East and hada team of Philippine draughtsman to assist him.They were American trained and when askedto prepare full size details of window and doorsections, drew them up 1:2 – half size, markedthe drawings full size and didn’t put a scale onthe drawings. 1:2 is a common American scale.The first that was known about all this was whenflimsy windows started to arrive on site!

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Appendix A 281

Benchmark (BM) 85.32

83.00

84.60

81.50

83.10 82.75

BT

80.43

79.20

79.60

81.00

0 5 10 15 20 2554321Scale 1:500

Fig. A.2 A 1:500 site plan prepared for a small area of approximately 0.903 hectares.

Foul water drainage

Surface water drainage

BM 85.32

5000

83.00

81.50

House 4

FFL83.00

House 3

FFL82.25

5500 80.43

5000

5680

79.20

House 2

FFL82.00

House 1

FFL81.00

81.0082.7583.10

84.60

Fig. A.3 A completed site plan with details of the placement of buildings and finished floor levels.

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Fig. A.4 Part of the accommodation plan of Old Grange Farm to a scale of 1:50.

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A.5

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Old

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1:20

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283

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284


BY019-AppB BY019-Fleming-v6.cls September 14, 2004 20:5

APPENDIX B

Levelling Using the Dumpy Level

A variety of surveying instruments can be usedon a building site to assist in the accurate settingout of a building on the ground and in setting thelevels at which various parts of the building areto be built. The most basic of these instruments isa level, or dumpy as this type of instrument is of-ten called. Many types of dumpy level have been‘invented’ but many old models are still and willcontinue to be in regular use. These all use theprinciple of setting a telescope so that the line ofsight is truly horizontal no matter which way thelevel is facing. A model popular with builders onsmaller sites is one which uses a telescope butthe line of sight is set horizontal automaticallyby a series of pivoted mirrors which are counter-weighted. The whole instrument is contained ina small box which mounts on a tripod. It is notas accurate as the older style dumpies.

A more modern version uses a revolving laserlight projected in a truly horizontal plane so thatinstead of viewing a measuring ‘staff’, the laserilluminates a staff as it comes round. This is moreaccurate than the little automatic box but not asaccurate as the full size optical instrument. Ourexplanation will cover the use of a basic opti-cal dumpy level1. Once the principles have beengrasped, the use of slightly different instrumentsis only a matter of familiarisation with the par-ticular product being used.

Levelling

Levelling is a technique used by surveyors andothers in the construction industry to deter-

1 For some general information on surveying instru-ments look on the Internet and visit such sites as Leicaat www.leica-geosystems.com

mine the height of land surface or objects in alandscape relative to a known fixed height. Theknown fixed height may be a permanent markor a temporary mark. Such marks are knownas benchmarks (BMs) and can be incisions onthe masonry face of a building or pillar for per-manent benchmarks, or simply a block of con-crete, the corner of a manhole cover or even a pegdriven into the ground. The whole point abouta benchmark is that it has a known fixed height.This might be a true Ordnance Survey (see Ap-pendix A) related level or an arbitrary value. Ar-bitrary values are frequently used where thereis no need to relate to an Ordnance Survey (OS)value or where bringing in an OS level wouldbe impractical. Arbitrary values are usually setat 100.00 metres. All other levels on the job arerelative to this.

Figure B.1 is a photo montage of some bench-marks taken around the UK. It shows (from topto bottom): an OS triangulation (trig) point witha benchmark on its side; a detail of the bench-mark on that trig point; the top of the trig pointshowing the fixing arrangements for the theodo-lite2 used by the surveyors during the surveywork; a benchmark set up by the then War De-partment as part of a coastal gun battery; and fi-nally a typical benchmark inscribed on the wallof a building. The actual levels of the bench-marks are found by consulting a map with anappropriate scale, e.g. all maps will give the level

2 A theodolite is a more sophisticated instrument basedon a telescope which swings in a horizontal plane and avertical plane. The arc of the swing for any reading canbe read off a pair of scales in degrees. In addition, theinstrument will allow the surveyor to measure distancefrom the instrument to the staff. This technique is calledtacheometry. There are now modern instruments whichautomate much of this type of work.

285




(a) (b)

(c)

(d)

(e)

of the OS trig point but only 1:1250 or 1:2500will give the level of the mark inscribed on thebuilding.

From the relative heights we can calculate andshow as spot heights or contours the ‘level’ of theland on which we propose to build. The tech-nique uses a dumpy level and an analogue mea-suring device or staff, shown in Figure B.2, andthe usual notebooks, pencils, etc. Note that thestaff is of sectional telescopic construction, hav-ing five sections and extending to a height of 5 m.

Basic dumpy level

The basic dumpy level comprises:

� A telescope with a sighting device or stadiawires

� A bubble level on the telescope� A horizontal mounting which will allow the

telescope to rotate through 360◦� A tripod on which to mount the instrument in

a stable state.

The sketch in Figure B.3 shows the basic oper-ational parts of only one variety of dumpy levelbut will serve to illustrate what they are about.Starting at the centre top and working downthe sketch from left to right: the accurate bub-ble level is used to finally set the telescope trulyhorizontal just prior to each reading taken onthe staff; sometimes there is a mirror fitted overit or there is a small eyepiece which through a

Fig. B.1 Some benchmarks and an OS triangulationpoint. (a) is a photograph of an OS triangulation(trig) point, a tapering concrete pillar of square cross-section. The face of the pillar bears a benchmark whichis cast in bronze and then cast into the concrete. Thecasting incorporates a recess into which a bearer for alevelling staff can be placed. (b) is an enlarged viewof the benchmark. The hole under the benchmark is adrainage hole for all the recesses in the top of the trigpoint. (c) is a detail of the top of the pillar showingthe arrangement for securing instruments or sightingtargets. Trig points are only found on the tops of highhills and mountains where they were used in the basesurvey work. (d) is a typical OS related benchmarkcarved into a house wall. No indication of the level isgiven. This has to be obtained by reference to a 1:1250or 1:2500 map of the area. (e) is a benchmark erectedby the military next to a World War II shore batteryoverlooking Scapa Flow in Orkney.



Appendix B 287

Fig. B.2 Typical dumpy level and staff.

complex optical system allows the surveyor toview both ends of the bubble, and when these arein alignment, the instrument is truly horizontal.The object lens is the large light-gathering lensfacing the staff; the larger it is the more light therewill be and the further out the surveyor will beable to use the instrument. The trunnions eitherside of the telescope allow it to pivot up anddown on its optical axis in bearings set in themounting for the instrument. The focusing eye-

Focusingeyepiece

Accurate bubble level

Objectlens

Trunnion Levelling screw

Transverselockingscrew

Index markBearing

Scale of degrees

Bubble levelCoarse levelling

screws

Threaded portion to screw onto tripod

Fig. B.3 Schematic view of a dumpy level.

piece allows the surveyor to focus on the sight-ing and reading marks inside the telescope – seestadia wires below. The levelling screw is usedto bring the telescope from nearly level to trulylevel as indicated by the large bubble level. Thebearing shown is the pivot on which the whole ofthe telescope and its mounting revolves in a hor-izontal plane. The index mark allows the scalebelow to be read. The transverse locking screwclamps the bearing so that the telescope is point-ing in a ‘fixed’ direction. The scale of degrees onthe lower fixed part of the instrument mount-ing is not much used in general levelling but canbe useful in fixing the horizontal angle of vari-ous sights or readings relative to a fixed point inthe survey. The coarse levelling screws are usedto set the mounting to an approximate level asindicated by the bubble level between them –what is not shown is that there are three coarselevelling screws. Finally, the bottom plate of themounting has a large threaded hole for securingthe instrument to the top of the tripod.

The instrument works on the principle that ifthe telescope is set so that the line of sight ishorizontal, then measurements can be taken be-low (and above) that line of sight or collima-tion. These measurements can be used to de-termine relative height. If a known fixed heightis included as one of the measurements then allothers can be calculated relative to it, i.e. a bench-mark either OS or temporary.

Using the dumpy level

Figure B.4 illustrates the dumpy level in use.To begin, the dumpy level has to be set up onits tripod so that the telescope’s line of sight istruly horizontal. The tripod has to be set on firmground and the spikes on each leg pushed firmlyinto the ground or otherwise stabilised so thatthey don’t slip. The table on which the dumpy

StaffStaff

Staff

Line of sight orcollimation

Dumpylevel

Benchmark75.62

Tripod

A

B

Fig. B.4 Schematic of dumpy level in use.




Table B.1 Noting the readings in a level book

BS IS FS Collimation Level Notes

1.45 77.07 75.62 Benchmark2.30 74.77 A

2.96 74.11 B

level is mounted must be set fairly level – withinthe limits of the small bubble level(s) mountedon the table. Finally, the telescope itself is eitherlevelled for any angle of viewing (360◦ in a hori-zontal plane) or brought level as each reading istaken; it all depends on the type of dumpy levelbeing used. The dumpy illustrated in the sketchis of the latter type.

Measurements with the instrument are takenby placing the staff on the selected point andsighting on it with the telescope. The telescopeis focused on the staff and measurements on thebenchmark and points A and B are taken in turn.The first reading on the BM is termed a backsight. All others except the last are intermedi-ate sights. The last is a fore sight. A is there-fore an intermediate sight and B a foresight. It isnot by chance that the readings are termed backand fore sights, remembering that the work ofthe OS was first carried out for men who firedguns.

Table B.1 shows how the readings are writtendown in the level book and how the levels areworked out.

Readings takenon this line

Fig. B.5 Common form of stadia wire.

Stadia wires

A stadia wire is a device placed inside telescopesused for aiming, such as telescopic gun sights,periscopes in submarines, and here in a dumpylevel. They are also used in other surveying in-struments such as theodolites.

When first looking through the telescope totake readings, the surveyor focuses the eyepieceof the telescope on the stadia wires. Once this isdone it generally needs no further attention, thestadia wires remaining in focus no matter howthe focus on the staff is changed. Various stylesof stadia wires are made but the one in Figure B.5is a fairly common pattern.

The complete telescope is in turn focused onto the staff and the reading taken as shown inFigure B.6. Note that the staff appears upsidedown. This is the most common scenario, thereason being that the additional lenses requiredto turn the image right way up would reduce

Readingtakenhere

Fig. B.6 How the staff appears to the surveyor whenviewed through the telescope.



Appendix B 289

Peg to be at82.50

BM 83.30

Fig. B.7 Setting to a ‘level’.

Fig. B.8 An EPDM instrument on its tripod and inclose-up.

Fig. B.9 The peg and the nail under the EPDMinstrument.

the amount of light reaching the surveyor’s eye.This is not a problem for work on a building sitewhere distances would be relatively short, but inthe wider surveying field could be quite critical.

Patterns of markings on staffs vary.

Setting to a ‘level’

The instrument can also be used to assist in thesetting of objects to a given height in the land-scape. Refer to Figure B.7. The process com-mences with setting the telescope level and

Fig. B.10 The target.




sighting on the staff over a benchmark, thus ob-taining the line of collimation. If the reading atthe BM is 2.33, then the collimation is 85.63. Toobtain the level on top of the peg, the readingthere must be 85.63 − 82.50 = 3.13. The peg isdriven in until the reading is obtained, and thepeg is then at the correct height.

This technique is frequently used on buildingand construction sites to set the levels of foun-dations, floors, beams, column bases etc.

On the subject of modern instruments, Fig-ure B.8 shows an EPDM instrument on its tri-pod with its associated electronic gadgetry andpower pack. It is set up over that most basic in-dicator of position, a wooden peg with a nail

in it (Figure B.9). In Figure B.10 is the target onwhich the telescope of the instrument is alignedand which also houses its own set of electronicgadgetry, the two sets communicating with eachother.

The complete set was being used by a squadof bricklayers under the supervision of the siteengineer to set the position of the substructuresbeing built. Only the peg with the nail had beenset up – again with the EPDM equipment – bythe engineer. There were no profiles on the sitefor these buildings. Position and levels were alltransferred by the equipment. The engineer didadmit to checking the setting out with a goodsteel tape!

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BY019-App-C BY019-Fleming-v6.cls September 17, 2004 10:56

APPENDIX C

Timber, Stress Grading, Jointing,Floor Boarding

This is not meant to be a comprehensive treatiseon timber, stress grading or timber joints andjointing. Several textbooks have been written onthe subject already. What we are trying to givein this appendix is an overview of the more ba-sic features of timber as applied to the limitedconstruction forms and types in this text; a briefoverview of stress grading – what it is and whereit is used; and an explanation of the more com-mon joints and jointing methods used in moderndomestic construction.

The structure of timber will not be dealt with inthis text. The reader is referred to any good ma-terials textbook or one dealing only with timbertechnology.

Sawn and regularised timber forcarpentry work

In general timber can be divided into two cate-gories:

� Softwoods, obtained from resinous, conebearing evergreen trees (gymnosperms)which have needle-shaped leaves – firs,pines, spruces, hemlock, etc. The only decid-uous tree from which commercial softwoodis obtained is larch.

� Hardwoods are obtained from deciduous,broad leaved trees (angiosperms) – oak, ash,beech, mahogany, teak, etc.

In the UK the bulk of timber used in constructionis softwood and is mainly imported either fromRussia and Scandinavia – the European soft-woods – or from the USA and Canada. A small,but ever increasing, amount of home grown tim-ber is used. This discussion will concentrate onthe use of softwood.

Timber is obtained principally from the trunksof trees. A log is a tree trunk with the branchesremoved. Logs are rounded in cross-section – thetimbers we require are generally rectilinear.

The heavy line in Figure C.1 is the outlineof the log and the thinner, straight lines indi-cate saw cuts. The log in Figure C.1 has beenslab sawn. Note that the first and last slabs haveonly one flat face. Slabs have a rough edge calledwany edge or wane. Cutting up the log is knownas conversion.

Besides slab sawing, there are other ways ofconverting timber (Figure C.2). The upper il-lustration shows a typical log from a Europeansource and would be of 300–450 mm overall dia-meter. The lower illustration is a North Ameri-can sourced log and would be typically around700–1200 mm diameter.

Naturally, pieces of rectilinear timber have ac-quired names over the many years of use:

� A baulk is the largest square sided timberwhich can be obtained from a log.

� Planks are from 50 to 150 thick, 275 to 450 wideand 2.40 m and upwards in length.

� Deals are 50 to 100 thick and 225 wide.� Battens are 50 to 100 thick and 125 to 175 wide.� Boards are of any width and length but not

exceeding 50 thick.� Scantlings are resawn timbers 100 × 100,

100 × 75, 100 × 50, 75 × 75, 75 × 50, etc.

The third common way to cut softwood logs isquarter sawing (Figure C.3). Other cuts are usedon hardwood logs to enhance the figure or grainof the timbers. To get full use of all availablesections, quarter sawing can be combined withslab sawing.

A wide range of cross-sectional sizes are avail-able from sawmills. Those from BS 4471 are listed

291

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Fig. C.1 Log, slab sawn.

in the precis of that BS in Appendix L. Theshortest length available is 1.80 metres and stocklengths rise in increments of 300 mm. Lengthsare therefore: 1.80, 2.10, 2.40, 2.70, 3.00, 3.30, 3.60,3.90, 4.20, 4.50, 4.80, 5.10, 5.40, 5.70, 6.00, 6.30,6.60, 6.90 and 7.20 m.

Lengths and cross-sections available obvi-ously depend on the availability of suitable treesfor conversion. As the larger trees are felledor become the subject of conservation, longerlengths and wider and thicker timbers becomescarce and more expensive. Timbers over 6.00metres long are more expensive, as are tim-bers with one side over 200 mm wide. Combine

Converted into deals

Converted into boards

First or prime quality

Second or merchantable

quality

Fig. C.2 Logs, alternative sawings.

Fig. C.3 Log, quarter sawn.

excessive length with excessive width and evenmore expense is incurred. The precis of BS 4471in Appendix L gives some indication of the cur-rent situation regarding stocking at mills, resaw-ing, availability and relative economics of thedifferent lengths and sections.

Much of the timber imported now comes in acomplete variety of sizes as given in the BritishStandard but some will inevitably be resawn inthe UK from larger boards or deals shipped in.Resawing is done with either circular saws orband saws. Band saws have thinner blades andthus waste less timber.

Timbers produced and used directly from thesaw are described as sawn. BS 4471 lays downtolerances for finished dimensions of sawn tim-bers:

� −1 to +3 mm on timber thicknesses andwidths not exceeding 100 mm

� −2 to +6 mm on timber thicknesses andwidths exceeding 100 m

� On length −0 and unlimited excess length.

There are two further possibilities:

� Sawn timbers can be regularised, i.e. broughtto a common overall size. This can be achievedby accurate sawing to closer tolerances or bymachining.

� Sawn timbers can be planed, dressed or wrot,i.e. brought to a common overall size andgiven a smooth finish all round. This can onlybe achieved by machining.

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Appendix C 293

Sawn timber Direction of feed

Direction of feed

Front machine table

Difference in level of upper machine table front and back sets

amount of timber cut away

Machine table, in onepiece, rises and falls

accurately

The table is considered to be in one piece but both halves are adjustable. The back half is

set to the same level as the knives in the cutter, the front half below that level to determine the amount of cut

Back machine table

Fig. C.4 Schematic of planing and thicknessing ma-chine.

In machining, sawn timbers are passed througha machine with from one to six sets of rotat-ing cylinders, each cylinder having two, threeor four sharp blades and each cylinder presentedto a separate face of the timber. The timber runsalong a flat surface, a table, or in large machinesa horizontal roller bed, to be presented to thecylinder(s). In large machines timber is fed bypowered rollers and in small machines is pushedthrough by the operator.

Figure C.4 shows a schematic of a combinedplaning and thicknessing machine, generally re-ferred to as a thicknesser, with a four-cuttercylinder. The individual cutters are termed‘knives’. In the figure a four-cutter cylinderis shown. On planing machines and thick-nessing machines with one and two tablesrespectively this is the norm, and a single four-knife cylinder thicknesser is the most commonmachine of this type found in small mills andjoinery workshops. To obtain a wrot or dressedsurface on a piece of wood the timber is passedover the top of the cylinder using the split table,which in turn determines how much is beingmachined off, i.e. the difference in level betweenthe front and back parts of the table. The fronthalf is lower than the top of the cutter knives; theback part is the same level as the cutter knives.

To obtain a piece of timber of known and ac-curate thickness it must first be wrot on one faceand then passed between the cylinder and thelower table, the gap between them being set toa predetermined distance.

In large mills it is more usual to have a foursider machine as well as a thicknesser. The foursider as the name suggest cuts all four faces ofa piece of timber at one pass. Each cutter cylin-der is driven by its own electric motor. However,these faces need not just be plain surfaces. Theycan be any shape which can be ground onto theknives in the appropriate cutter block. A win-dow section could have a rebate and roundedgroove cut on one face for the glass; groovescut on a second face for a pressure break anda draught excluder/weather seal; a splay and arounded arris on a third face; and a splay, re-bate and groove cut on the fourth face. For eachface the knives could be ground to cut differ-ent parts of the timber, e.g. on the third facethe splay could be set on two of the knives andthe rounded arris on the third (and the fourthknife set for the splay), it taking less effort to cutthe rounded arris than the splay.

Setting up these machines is a highly skilledjob. It is very expensive to grind the knives andset them up, including test runs and the wastethat involves. So mills tend to stick to stock sec-tions which they run and keep in stock, or forvery large orders they will grind and set up cut-ters, but large orders only are economical. Evenif a client was willing to pay the premium, millsare not easily persuaded to take on short runs ofspecial shapes as it disturbs the general runningof the organisation.

In Figure C.5 the sketch shows a schematic lay-out of a ‘four sider’ with six cylinder cutters anda roller bed with power feed rollers at the startand end of the machine. Having two cylinders toserve the upper surface of any timber presentedmeans that that surface can have deeper and/ormore complex shapes made, the high load thatthese shapes put on the machine being spreadover two separate motors. There is one cylinderfor the underside and one for each side, plus abeading cutter on the underside. Although it isshown on the underside in our schematic, thesecylinders are frequently capable of being set upto cut on any of the surfaces. They are usefulfor cutting small quirk beads, small splays andchamfers, small cavettos or roundings or smallgrooves. These are frequently to a similar sizeno matter what else is being done to the timber,so the little cutters can be reused time and againand they save a lot of money setting up speciallarge cutters on the full size cylinders for a singlejob or run.

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Top cutter heads

Vertical side headsBottom cutter head

Two in-feed rollers

Roller bed rather thana solid table

Rollerbed

Two out-feed rollers

Timber machined top and bottom

Feed directionSawn timber

Timber machinedall round

Adjustable lateralalignment

Beadingcutter

Adjustable lateralalignment

Fig. C.5 Schematic of four sider machine.

Regularised timbers are sawn or machined togive a close tolerance on width and thickness.BS 4471 requires a tolerance of −0 +2 mm. Themaximum permitted reductions off sawn sizesof timber for regularising are 3 mm off any di-mension up to 150 mm, and 5 mm off any di-mension over 150 mm.

Wrot or dressed timbers are machined tocloser tolerances but also to a finer surface finish.BS 4471 requires tolerances of −0 +1 mm. Theactual amount of reduction from the basic sawnsizes depends on two factors:

� The application for which the dressed timberwill be used

� The basic size ranges.

The precis of BS 4471 gives details.Regularised timbers are used for the manufac-

ture of structural carpentry work or componentssuch as trusses for roofs or beams for floors –anywhere the timber is ‘engineered’ and regularsizes are required for jointing. Engineered com-ponents include trussed rafters where the tim-bers are joined at the nodes by pressing toothedsteel plates into opposing faces of the node. Theteeth on the plates are relatively short so the tim-bers must be of a good, even thickness, otherwisethey will prevent the teeth from penetrating fullyon one side of the joint.

In carpentry work, wrot or dressed timberwould be used, for example, to finish round theedges of a roof structure where the timbers wereexposed, or anywhere the timber has to have agood appearance either for painting, staining orvarnishing, for example, internal finishing tim-bers such as skirtings, architraves, door frames,etc.

North American timber

Timber obtained from North America willnormally follow the CLS or ALS sizing rules(Canadian Lumber Standard or AmericanLumber Standard). Surfaced softwood will beavailable 38 mm thick and in the followingwidths: 63, 89, 114, 140, 184, 235 and 285 mm.All arrises are rounded, not exceeding 3 mmradius.

Measuring moisture content

Timber changes shape as its moisture con-tent varies. It is, therefore, important to mea-sure timbers at a constant moisture content(MC).

BS 4471 stipulates that all measurements aretaken, and given, at an MC of 20%. For an MClower or higher to a maximum of 30%, the sizewill be adjusted by 1% for every 5% differencein MC.

Moisture content andseasoning of timbers

An important feature of timber is its ability toabsorb moisture and dry out, cyclically. This af-fects its use in a number of ways:

� Absorbing moisture or drying out affects thesize and shape of the timbers

� Logs when first cut have a high MC in the formof sap

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Appendix C 295

� There are optimal MCs for specific end uses oftimber

� Too high an MC renders the timber liable toinsect or fungal attack.

MC should be held below 20% to avoid fungalor insect attack. Some insects will attack timbersat far lower MCs than 20%.

MC is measured as the percentage mass of wa-ter against the oven dry mass of timber. MC canbe measured using electronic instruments or bytaking samples and weighing them ‘wet’ andthen oven dry, calculating mass of water andthus MC.

MCs for various end uses are:

� Ground floor joists – 18%� Upper floor joists and roof timbers – 15%� Framing and sheathing – 16% (timber frame

construction of walls)� Windows and external doors – 17%� General internal joinery finishings – 8–12%

depending on heating system.

Movement of timber due to varying moisturecontent can cause problems unless this move-ment takes place before the timber is fixed inplace and, in the case of dressed or speciallyshaped timber, before it has been machined. Tim-ber components such as doors and windows ormade up joinery should have a much reducedMC before manufacture.

Figure C.6 shows the distortion of timberwhen cut from various parts of the log and in

Slab sawn plank cups andbows when dried out

A radial cut gives

leastdistortion

Quarter cut timber turned

to a cylinder becomes oval after drying out

Diamonding of quarter

sawn timbers

Fig. C.6 Distortion of timber sawn from various partsof a log.

different ways. Starting at the top and workingclockwise:

� The slab sawn plank will cup and bow as itdries out

� The radial cut section gives least distortion� The quarter cut timber turned into a cylinder

becomes oval after drying out� The quarter cut timber left rectilinear ‘dia-

monds’ after drying out.

Reducing the MC to acceptable levels is calledseasoning. Unless the sap and moisture is re-moved from timbers, they will warp and shrinkbetween the time of building into position andnatural drying taking place, with the followingresults:

� Joints will open up� Joints will be weakened� Timbers will split� Timbers will shrink – usually unevenly� Timbers will warp� Various types of shakes will develop� Timbers are weakened� Timbers are extremely susceptible to fungal

and insect attack before seasoning.

Figure C.7 illustrates some natural defects intimber.

Seasoning can be done by natural seasoning:

� By stacking smaller section or thin timbers inthe open air with spacers between each sectionand a waterproof roof over the stack

� By immersing logs or baulks in water for a fewweeks before stacking.

Seasoning can be done artificially by subjectingthe stack of timber to a warm air stream in akiln. Steam is sometimes employed in the initialpart of the process or the whole process for smallbatch work. The kiln can be a simple box or a bar-rel for small timbers. At the other extreme largepurpose-built buildings are used, with complexcontrols for temperature of air and steam andfinal MC of the timber.

Natural seasoning is a long slow process andis expensive as one has a resource – the timber –which costs money to grow and procure, tied upin a pile somewhere for anything from 18 monthsto two years. Kilning is also costly – the capitalcost of the kiln, its maintenance and the energycost to run it. However, the timber is seasonedin a matter of a few days or weeks rather than

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1

2

3

1 An example of a cup shake where the annularrings of the timber separate. This can beexaggerated during seasoning.

2 A star or heart shake where the fibres of thetimber separate along the medullary rays. This can be exaggerated during seasoning.

3 Wany edge caused by cutting the balk or board too near the outside face of the log.

Fig. C.7 Natural defects in timber.

years, and with sufficient throughput the pro-cess is very economical.

Before using naturally seasoned timber, itmust be brought indoors to allow the MC to ad-just down to the ambient MC before it can beworked. Kilning can produce timber with justthe right MC for the job in hand. Naturally sea-soned timber can be subject to insect and fungalattack. Sawmill yards can be a prime source offungal spores and insects.

Inexpert seasoning can produce a number offaults:

� Timbers can be warped, twisted, sprung,bowed or split

� Too high a temperature in a kiln can ‘caseharden’ the timber

� Steam seasoning can alter the colour of thetimber so that it is no use for joinery timbers.

Figure C.8 illustrates some seasoning faults.

Surface check or splitBow

Spring

End split

Fig. C.8 Defects caused by seasoning.

Timber should not be fixed in a position withinthe structure of a building unless it is protectedfrom excess moisture. This is generally done bybuilding in DPCs and by providing ventilation.The MC of structural timber can then be keptbelow 20%.

Timber can be wetted for prolonged periodswhen leaks occur in roof coverings, overflowinggutters or leaking pipework. To give protectionduring such accidents it is common practice to‘treat’ timber with chemicals which are fungi-cides and/or insecticides.

Preservative and other treatmentsfor timber

BS 1282, Classification of wood preservatives andtheir methods of application, lists three classifica-tions of preservative:

� TO – Tar-oil types� OS – Organic solvent types� WB – Water borne types.

Tar-oil type is better known as creosote, of whichthere are two kinds recognised by the BSI:

� Coal tar creosote for pressure impregnation� Coal tar creosote for brush application.

Creosote is not favoured for internal use evenin building structures as it has a strong odour,contains phenols, bleeds out through paint etc.finishes and can be harmful to people and ani-mals long after it has supposedly dried out.

Organic solvent types include solutions ofchlorinated naphthalenes and other chlori-nated hydrocarbons, copper naphthenate, zinc

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Appendix C 297

naphthenate, pentachlorophenol and tri-butyl-tin oxide. Of these:

� All can be applied by pressure or double vac-uum impregnation (see below)

� Most are suitable for paint finishing� Some are clear and all can be tinted either for

decorative effect or to indicate extent of treat-ment, depth of penetration, etc.

Water borne types include solutions ofcopper-chrome, copper/chrome/arsenic,fluor/chrome/arsenic/dinitrophenol, coppersulphate, sodium fluoride, sodium pentachloro-phenate, zinc chloride and organic mercurialderivatives.

� These are suitable for painting over as they donot bleed

� All but a few are non-corrosive� Those made of two or more chemicals will not

leach out as they ‘lock’ chemically to the woodtissue.

Application of preservatives can be by:

� Brush� Dipping� Immersion� Pressure treatment� Vacuum treatment� Combination of pressure and vacuum.

Brushing and dipping are self-explanatory. Im-mersion is similar to dipping except that the tim-ber is left to ‘soak’ for a period of time. Noneof these treatments allows penetration of thepreservative into the timber for any apprecia-ble depth and are therefore unsuitable on theirown. Immersion or dipping can be used on cutends of timbers which are treated before cuttingto length. Brushing such ends is frequently spec-ified but is a poor treatment.

Pressure treatment involves immersing thetimber in preservative inside a closed vessel andapplying pressure to hopefully force the fluidinto the timber. This is only partially effective asFigure C.9 shows.

Vacuum treatment involves a sealed cylinderinto which timber is placed; the air is evacuatedand preservative introduced. The fluid hope-fully occupies the voids left by the extraction ofthe air from the cells of the timber. Because it isnot commercially viable to create a perfect vac-uum and because the vacuum causes the fluidto vaporise, the cells are not filled with fluid. So

Air in jar is pressurisedJar inverted and

pushed into liquid

Liquid

No matter how far down or how hard the jar ispushed into the liquid, the air trapped will never

be displaced. The cells in timber are in asimilar state. As the outer cells fill, the inner cells

containing air are merely pressurised.

Fig. C.9 Demonstration of air entrapment in cells.

pressure treatment and vacuum treatment arenot capable of giving complete penetration ofthe timber cells by the preservative. Figure C.10shows a pressure cylinder treatment.

There is another bad side effect. A lot of preser-vative is left in the timber and must be allowed todry off. It is dangerous to handle timber wet withpreservative. Only the water or the solvent driesoff, leaving excessive and expensive quantitiesof chemical behind. These can still be dangerous.

The double vacuum or vac-vac process avoidsmuch of that trouble:

� Timber is placed in a sealed cylinder� The air is evacuated as far as is feasible� Preservative is introduced� Pressure is applied� Preservative is drawn off.

Pressurecylinder

Air evacuated

Single timber

This partgets treated

Centre part getsno treatment due

to vacuum not being completeExcess fluid

drawn off

This part getsvapour

and air mixtureso only partly

treated

Preservativefluid in

Fig. C.10 Schematic of preservative treatment pres-sure cylinder.

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A vacuum is created and the excess preserva-tive from the timber is drawn off for reuse. Thetimber is recovered from the cylinder. The tim-ber will have just the right amount of preserva-tive chemical deposited in the cells. Because ofthe pressure applied, the preservative is drivendeeper into the timber. It will never penetrate100% in very large section timbers but is veryeffective in the range used for general carpen-try and joinery. There is no wasted material. Thetimber is almost dry when taken out of the cylin-der.

The double vacuum process is licensed by asmall number of preservative manufacturers toa large number of UK sawmills. Some tradenames are Tanalith and Celcure. There are oth-ers. Tanalised timber has a greenish tint. Celcurecan be tinted or clear. The latter is preferable forexposed timber which has been wrot or dressedand will be varnished.

Fire resistance

Besides treatment against rot or insect attack,timber can be treated to improve its resistance tofire. Timber has a natural resistance to fire whichmay seem a contradiction in terms when it isused as fuel by the bulk of the world’s popu-lation. This natural resistance is due the buildup of char, the carbon on the surface of timberexposed to heat or flame.

When considering fire in buildings it is wellunderstood that it is impossible to build a struc-ture which cannot possibly be damaged by fire.Buildings or parts of buildings are constructedto resist fire in three ways for set periods of time.The periods of time vary according to the type ofbuilding, its use and number of occupants. Thebest that can be done with fire prevention is:

� To build in such a way that the building willremain standing long enough for occupantsto escape and to allow fire fighters to enter tocontrol or extinguish the fire

� To prevent one part of the building radiatingheat onto another part, thus setting the latteralight or harming occupants while they escape

� To prevent the passage of smoke, flame andhot gases through a part of the structure, thussetting another part alight or harming occu-pants while they escape.

Floor supported by solid timber beam

Dotted line showsoriginal beamoutline

Grey areaindicates layer

of char

Undamaged timber leftin core of sufficientsize to support floor

Fig. C.11 Char on a solid timber beam.

Despite being a flammable material, timber hasa high natural resistance to attack by fire. To fullyutilise this resistance, exposed timbers used in astructure should be oversized by the amount ofchar a fire would produce in a given period oftime for which resistance is required – see Fig-ure C.11. That way there will be sufficient un-damaged material in the centre of the timber(s)to support the building.

Treatments for improving fire resistance wereoriginally based on strong solutions of zincchloride. Solutions of boracic acid salts are nowpreferred:

� They can be much more dilute� They are more effective� They are more economical.

How to join timbers in carpentryand joinery work

This section deals with the joining of natural tim-bers in carpentry and joinery work. Many of thejoints can be made in both sawn and wrot tim-bers. All of them can be made using hand toolsbut obviously are more easily and accurately ac-complished with machines, both stationary andportable. With the development of more pow-erful and sophisticated portable power tools,many of these joints can now be made on siteor even in-situ. How they are made is less im-portant to the student at present, but knowledgeof the variety of joints and their use in specificcircumstances is vital.

All joints depend on cutting the timber intoa variety of shapes or forming holes of various

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Appendix C 299

shapes – ‘operations’. These ‘operations’ all havenames and to fully appreciate jointing and otherwork on timber these operations must first beunderstood. Neither the list of operations northat of joints is exhaustive but it is fairly repre-sentative of what is found in modern domesticconstruction.

Joints have been developed over the centuriesto exploit the natural strengths and obviate thenatural weaknesses of timber. Timber is gener-ally cut so that the grain runs parallel to the longside of any plank or board. Loads applied at rightangles to the grain can be resisted most easily.Loads applied parallel to the grain cause shearor splitting. Splitting is the most common modeof failure in joints and is usually due to improperuse or production of the joint.

Of course there is an unceasing quest tomake things ‘easier’, and pressed steel fittingshave been developed over the past 40 yearsto allow complex joints to be made simplyby cutting the ends of timber square and ap-plying the appropriately shaped metal fittingbetween them. The fittings are normally hotdipped galvanised1 or sherardised2 and arepre-holed for small diameter nails. These fit-tings will be described later in association withFigure C.46.

Nails should be galvanised wire nails and theirlength should be at least 40 mm – sometimes lessif the timber is thinner. All holes in the fittingshould have a nail, otherwise the fitting will notdevelop its full strength. Forcing very large dia-meter nails through the fitting will end up withthe timbers splitting,

Before looking at operations a brief explana-tion of the two most common power tools usedin joinery work is necessary – the circular sawand the router.

1 Hot dip galvanising is done to protect steel from corro-sion, principally rusting. It means thoroughly cleaningthe steel and immersing it in a bath of molten zinc. Thelayer of zinc, if of adequate thickness, is of a crystallinestructure which if scratched will self-heal – the crystalsgrow towards each other and seal the surface damage.There is a British Standard for galvanising fittings usedin construction.

2 Sherardising is another method of coating steel withzinc metal. Again the steel is thoroughly cleaned butthe zinc is spread over the surface in the form of a finepowder. Steel and powder are heated to a temperaturefor a period during which the zinc is absorbed into thesurface of the steel.

The circular saw

Figure C.12 shows a 250 mm portable saw. Sawscan be fixed, in which case the timber is passedover or under the saw, or they can be portableand are passed over the timber. In either case thesaw is fitted with an adjustable guide or ‘fence’to maintain the width of cut. The blade retractsinto the ‘table’ or sole plate to adjust the depth ofcut. The sole plate can usually be tilted to allowbevelled cuts to be made.

Portable saw blades are generally no morethan 300 mm in diameter. Fixed saw blades canbe up to 2.50 metres in diameter, although mostare around 300–450.

The circular saw illustrated in Figure C.13 isan overarm cross-cut saw. Timber is placed onthe table against the fence and the saw is pulledacross, making a cut through or to any depthin the timber. Multiple passes at less than thetimber thickness can make a wide slot acrossthe timber (see trenching in a later section). Theblade and motor can be swivelled, and if setwith the blade parallel to the fence, timber canbe passed under the blade to cut it lengthwaysto any depth. A router can be substituted forthe blade and its motor, and routing carried outacross or along timbers.

MotorHandle with

integral switch

Saw bladewith guard

Sole plate

Saw pushed into wood

Depthof cut

Sole plate

Saw blade and guard

Widthof cut

Fence

Fig. C.12 Portable circular saw – schematic.

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Arm swivels and locks from side to side

Arm

Handle withintegral switch

Horizontalpivot point

MotorVertical

pivot point

Fence

Saw bladeand guard

FenceTable

Base

ArmHandle towind armup and down

Horizontal pivot pointTelescopic

column

MotorSaw bladeand guard

Base

Table

Handle with integral switchVertical

pivot point

Saw etc. mounted on carriage running in

track on arm

Saw moves in thisdirection to cut

Fence

Fig. C.13 Overarm, cross-cut circular saw –schematic.

The router

The router, illustrated in Figure C.14, was origi-nally conceived as a portable tool (passing over

Emu

Plunge returnspring on one

pillar only

Motorin body

(1300 watts)

Cutter

Colletchuck

Spindlelock

Adjustableplunge stopwith scale

Knob to wind plunge stop up and down

Plunge lock (open)

Handle

Locking knobfor plunge stop

Plunge pillar

Revolving turret withthree stops

Fence

2 x trammel bars

Circular base plateTrammel barlocking screw

Adjustableplungereturnstop

Plungepillar

Return stopnut with

quick release

Handle

ON/OFFswitch(off)

Fig. C.14 Plunge router – schematic.

Shank grippedby collet chuck

Cutter portionfrom 6--12 mmdia. & 20 -- 50

long

Tungsten carbidecutters brazed

to shank

A few of the wide variety ofcutter shapes available

Fig. C.15 Selection of router cutters.

timber) but it can be adapted to fit into fixedmounts (the timber is passed over or under thetool). A wide variety of cutter shapes is avail-able and some are shown in Figure C.15. Themost common is the straight fluted cutter whichcan cut rebates, grooves and shape timber andboard by following a template.

Depth of cut is variable on the router. A fencecan be fitted on trammel bars to vary width ofcut. The fence can be replaced with a ‘point’ toallow circular cutting. The trammel bars can bealmost any practical length.

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Appendix C 301

Rebate

Firstsawcut

Second saw cut

Fig. C.16 A rebate.

Operations

Rebating or rabbeting (Figure C.16)

Cutting out a rectilinear cross-sectioned slice oftimber from an edge of a plank or board canbe done with two passes of a power saw or asingle pass of a router, depending on the size ofrebate and the capacity of the machine. It is mostfrequently done on wrot timber.

Grooving (Figure C.17)

Cutting a rectilinear cross-sectioned piece of tim-ber from the body of a plank or board and

Fig. C.17 A groove.

Fig. C.18 A trenching.

parallel to the grain can be done by takingclosely spaced, repeated cuts with a power sawor passes of a router. It is most frequently doneon wrot timber.

Trenching (Figure C.18)

Trenching is the same operation as grooving butat an angle to the grain – usually 90◦. It is doneon sawn or wrot timber with a portable saw orrouter against a timber clamped to the board, orwith a cross-cut saw or router on a cross-cut sawarm.

Tonguing (Figure C.19)

Cutting away a rectilinear cross-sectioned sliceof timber from the corners of one edge of a timbercan be done with two passes of a saw or routerbut most frequently and more accurately is donein a stationary machine with multiple cutters ina single ‘head’. It is mostly done on wrot timber.

Notching or halving (Figure C.20)

This involves cutting away a portion of the cross-sectional area of a timber, i.e. across the grain.

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Cutter

profile

Tongue

machined

on timber

Fig. C.19 A tongue.

Notching

Halving

Fig. C.20 Notching or halving.

Direction of grainSides of morticeusually parallel

Blind mortice

Through mortice

Fig. C.21 A mortice.

Anything less than half the thickness or widthof the timber is a notching; cutting away half thetimber is a halving; cutting more than half thetimber is never done. Notching or halving aredone with multiple passes of a power saw orrouter, or multiple saw cuts and hand chiseling,on wrot and sawn timbers.

Morticing (Figure C.21)

Morticing is cutting a rectangular-shaped holein the face or edge of a timber at right anglesto the grain. A through mortice extends the fullthickness of the timber, i.e. a hole is visible onopposite faces. A blind mortice is a hole whichstops short of the opposite face. If a tenon is fittedinto the mortice, the short sides are tapered sothat the expanded tenon is jammed into placewhen wedges are driven into place.

Morticing is most common in wrot timberand is done using a purpose-built machine oris drilled and chiselled out by hand.

Tenoning (Figure C.22)

Tenoning is reducing the cross-section of the endof a piece of timber, usually to fit into a match-ing mortice. The reduction can be made on one,two, three or four faces of the timber. Variants

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Appendix C 303

Tenon

Fig. C.22 A tenon.

are shown in the later illustrations on morticeand tenoned joints. It is most common on wrottimber and is done using multiple passes of afixed saw or router, or a purpose-built machine.

Splaying (Figure C.23)

Splaying is cutting away the corner of a pieceof timber along the grain, usually at an angle of45◦. It is done using single or multiple passes ofa power planer or router, or a single pass of apower saw. The cutters used for tonguing andgrooving are modified. Splaying is frequentlyused to disguise a joint, and on both sawn andwrot timber.

Moulding (Figure C.24)

Moulding involves cutting a shape into the regu-lar edge of a timber, usually parallel to the grainbut it can be done across the grain, most usuallyto match the moulding parallel to the grain. It isdone for decorative effect but can be combinedwith a jointing technique to disguise a joint. Nor-mally done on wrot timber, a router is used ineither mode of operation with suitable cutter(s).

Dovetailing (Figure C.25)

This operation involves cutting one timber witha splayed projection or ‘pin’ and the other with a

Joint hidden by two 'splays'

Splay

Fig. C.23 A splay.

Fig. C.24 A moulding.

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Dovetailpin

Dovetailslot

Fig. C.25 A dovetail.

matching splayed slot or groove. There are manyvariants from single dovetails to multiple dove-tails, used in furniture and cabinet making. Itused to be cut by hand but is now done usinga special jig and a router. It is most common inwrot timbers.

Joints

Now that we have considered the tools and op-erations, we can look at how these are combinedto give simple joints. The list is not exhaustive,many more joints being permutations of severaltechniques used in several simple forms of joint.

Butt joint (Figure C.26)

This is not really a good joint on its own. Thetimbers are cut square. The end grain of one isplaced against the face of another and nails orscrews are driven through into the end grain ofthe first piece. Gluing is a waste of time and thejoint relies for its strength entirely on the nailsor occasionally screws. Improved nails give abetter grip in the end grain. This joint is satisfac-tory for rough framing and panels covered withboarding.

Nails should be the greater of 100 mm or 4 x t long to a max. of 150 mm

t

Fig. C.26 A butt joint.

Half checked or halved joint(Figure C.27)

The timbers can be joined to give:

� A single continuous length� A corner joint� An intersection

To form a continuous length

A corner

An intersection

Fig. C.27 A half checked or halved joint.

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Appendix C 305

The V-shaped fingers are cut in both pieces

of timber simultaneously by a single revolving cutter ensuring they

match exactly

Fig. C.28 A finger joint (1).

In carpentry work the timbers are nailed but injoinery work will be pinned and glued or gluedand screwed from the hidden face.

Finger joint (1) (Figure C.28)

This variant of the finger joint is cut by a specialmachine and can be used to join any regularisedor dressed timbers, even structural timber. Onlyglue is used in the joint, generally a waterproofresin-based type cured by RF (radio frequency)radiation. The joint is frequently stronger thanthe timber itself.

Finger joint (2) (Figure C.29)

This variant is used to join the corners of framessuch as door and window frames. Generallyonly glue is used in the joint, a waterproof glueallowed to set naturally or perhaps at a slightlyraised temperature. A wooden or metal (star)dowel can be driven through the centre of thejoint. The joint is frequently used as a corner jointfor framed material such as window sashes andlight doors.

Plain housing (Figure C.30)

This is frequently used to join the uprights orjambs of a frame to the upper or lower horizontalportion of the frame. It is also used to join studsto the head and sole plates of timber framed

Upright orstile

Horizontal rail

Fig. C.29 A finger joint (2).

construction. It is most frequently used for sim-ple door frames. No glue is used, the pieces be-ing simply nailed together. Two nails are a min-imum.

Shouldered housing (Figure C.31)

This is used to join wider timbers or sheet mater-ials at right angles. The extra width allows theedge of the joint to show a plain ‘butt’ joint while

Upright offrame

Horizontal upper member of frame

Fig. C.30 A plain housing.

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TrenchingStopped

end

Fig. C.31 A shouldered housing.

the housed portion adds the necessary strengthand rigidity. It can be glued or glued and nailedbut is often just nailed.

Plain morticed and tenoned joint(Figure C.32)

The illustration shows the most simple of thesejoints with the tenon being made with an equalreduction in cross-section all round. Note the useof wedges to expand the tenon in the mortice andso wedge it into place. Also note that the pressurefrom this expansion is exerted along the grain ofthe timber with the mortice. This joint is some-times glued; no nails or screws.

Shouldered morticed and tenoned joint(Figure C.33)

One of the problems with a plain m&t joint,shown in Figure C.32, is that on narrow timbersthe tenon is not wide enough to prevent twist-ing. By adding a shoulder to the joint we are ineffect ‘housing’ one timber to the other for theirfull width but need only a small portion of thewidth for the mortice.

Grain

Wedges

Fig. C.32 Plain morticed and tenoned joint.

Barefaced morticed and tenoned joint(Figure C.34)

This variation of the m&t joint is used for thebottom and mid rails of bound lining doors –see Chapter 8 on Doors. The illustration showsthe bottom rail with the tenon cut, the stile with

Wedges

Shoulder

Grain

Fig. C.33 Shouldered morticed and tenoned joint.

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Appendix C 307

Grain

Gra

inSawcut

forwedge

Sawcut for wedge

Lower shoulder

Upper shoulder

Rebate for edgeof boarding

Mortice

Housing for lowershoulder

Housing for uppershoulder

Fig. C.34 Barefaced morticed and tenoned joint.

a mortice and the space left for the thickness ofthe boards covering the door.

Blind morticed and tenoned joint withfoxtail wedges (Figures C.35 and C.36)

This variation is used to join the rails to the stilesof a door in high quality work. The morticesare ‘blind’, i.e. they do not show on the exposededges of the doors stiles. Note that the tenon is

Grain

Sawcutsfor

wedges

Fig. C.35 Blind morticed and tenoned joint with fox-tail wedges (1).

Gra

in

Grain

Tenon is shorter than depth of

mortice. Wedges butt againstthe bottom

of themortice as the joint

is cramped together

Gra

in

Fig. C.36 Blind morticed and tenoned joint with fox-tail wedges (2).

now made up of four separate pieces all workedon the end of the rail. This is described as asplit, double tenon. Cramping the stiles close to-gether on opposite sides of the door forces thewedges into the saw cuts in the tenons. Wedgesare dipped in glue just prior to cramping up.

Tusk tenoned joint (Figure C.37)

This is used to join beams and joists when fram-ing up large openings in timber floors etc. Thewedge draws the joint tightly closed while themain part of the tenon provides a bearing inthe larger part of the mortice.

Shoulderpreventstwisting

Wedge pullsjoint

together

Tusktenon

Outline of timber andmortice

Vertical bearing surface

Grain

Slot in tusk longer sothat wedge can pulljoint fully shut

Fig. C.37 Tusk tenoned joint.

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Wooden dowel

Note that hole in tenon is not in alignment with hole in mortice

Fig. C.38 Draw boring morticed and tenoned joints.

Draw boring morticed and tenonedjoints (Figure C.38)

M&t joints are always wedged but to keep thejoint firmly closed the timber frame must be re-strained with sash cramps until the wedges aredriven up tight. On frames which are subject toa lot of vibration etc. the wedges might not bestrong enough to maintain the joint firmly andso a timber dowel is often passed through bothtimbers to coincide with the centre of the tenon.The holes are bored such that in passing throughthe hole in the tenon, the dowel pulls the tenonmore firmly into the mortice.

Toe joint (Figure C.39)

This joint is frequently used for struts when incompression and for diagonal bracing on sim-ple ledged and braced doors. Any compressionforces on the strut are transferred to the maintimber through the flat angled face in the match-ing angled notch.

Scarfed joint (Figure C.40)

The simple scarfs shown here join two carpentrytimbers together as a continuous length. Notethe difference between a splayed and a plainscarf. Scarfs are only suitable for timbers in ten-sion. The joints are nailed together; glue is notused.

Brace

Rail

To prevent sidewaysmovement, the brace issometimes notched over

the rail

Fig. C.39 Toe joint.

Joint skew nailedboth sides

Splayed scarf

Plain scarf

Fig. C.40 Scarfed joint.

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Appendix C 309

Fig. C.41 Scarf joints – variations (1).

Scarf joint variations (1) (Figure C.41)

This variation shows the joints made with a‘joggle’ which if properly fitted should preventthe timbers pulling apart. The scarfs shownare termed tabled scarfs and splayed indentedscarfs. Note that the joints are bolted togetherwith coach bolts, nuts and washers.

Scarf joint variations (2) (Figure C.42)

The joints here have either ‘keys’ or pairs ofwedges inserted into square mortices cut acrossthe diagonal cut of the joint. The ‘keys’ aresimply square section pieces of hardwood setinto the mortices which prevent the joint frompulling apart. Wedges inserted in the morticesfirst of all pull the joint together and go on to pre-vent pulling apart. Once the wedges are tight-ened, the joint is bolted together.

Single and double notching(Figure C.43)

The upper timber is bearing down on the lowertimber and the notchings provide resistance tolateral movement of one or both timbers. Thisjoint is used for joists over beams etc. and isnailed but not glued.

Folding wedges

Keys

Fig. C.42 Scarf joints – variations (2).

Single notching

Double notching

Fig. C.43 Single and double notchings.

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Fig. C.44 Cogging.

Cogging (Figure C.44)

This is similar to double notching and is usedfor jointing rafters over purlins, and similarpurposes.

Dovetailing (Figure C.45)

Dovetailing is more common in cabinet workand the illustration is of a dovetailed joint for

Drawer front

Drawer sides

GrainGrain

Note that the grain of the sides runs

into the pins of the dovetail

Fig. C.45 Dovetailing.

Fig. C.46 Sheet metal connectors.

a drawer side to a drawer front. The dovetailshown is a blind dovetail, i.e. it does not showon the drawer face.

Using a sheet metal connector(Figure C.46)

The connectors illustrated here can be used to‘bolster up’ a plain butt joint or can be used tojoin two timbers crossing one another at rightangles. Note that they are stamped and foldedfrom one piece of sheet metal.

Stress grading

The Building Regulations now require all timberwhich is used for structural purposes to be stressgraded, so it is important that what is meant bythe term is understood even if the intricacies ofthe technique etc. are not.

Timber being a natural material can vary inany of its qualities, even within the same plankssawn from the same log or even from plank toplank. Compare this with steel products such asbeams columns and joists. These are all man-ufactured to known sizes and shapes from amaterial whose properties are closely controlledand thus entirely predictable. We can confi-dently assume that when we specify Grade43 steel, the ultimate fibre stress of that steel

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Appendix C 311

will be the same as that of every other piece ofGrade 43 steel produced. So it is that a struc-tural engineer can calculate exactly what sizesof steel to use when building anything from ablock of flats to a chemical engineering plant.Prior to stress grading, it was difficult to de-sign timber structures without providing muchgreater cross-sectional sizes, just to be sure thatthe structure would remain standing. The tim-ber engineer had to be very conservative withthe stresses he assumed the timber would bear.

The first attempts to overcome this problemlooked at the defects which could weaken tim-ber:

� Knots. Particularly the size and condition ofthe knots – small and tightly fixed in the timberwere not a great problem but large and looseor even missing were serious defects. Also, theposition of the knot in the timber cross-sectionwas important. Even a small tight knot at theedge or on the arris of the timber would ‘pop’if overstressed and so weaken the timber.

� Shakes and splits. These gaps running withthe grain of the timber mean that the fibresare no longer in contact with each other. Thisobviously weakens the timber but if they oc-cur in the length this can mitigate their effects.

� Wane reduces the overall cross-section andthus weakens the timber.

� Insect and fungal attack would preclude theuse of a piece of timber because although itmight appear sound on the outside, with justa few pinholes or discolouring, no-one can seeinside the plank or board. Insects can almost

take the inside of plank right out and hardlyleave a mark on the surface. Similarly fun-gal attack can spread inside a timber, but themore serious effect is that the timber would besubject to further attack if the MC was to riseabove 20%.

Stress grading using machines is now the norm,with visual checks for certain classes or gradesand for certain purposes. The machines operateon one of two principles:

� Apply known load to a length of timber acrossa pair of rollers and measure the amountby which the timber deflects between thoserollers

� Apply a load to deflect the timber by a setamount and record the load required.

Machines of the first type are used in the UKand those of the second type are used in NorthAmerica.

Figure C.47 shows a schematic of a typicalstress grading machine, which would have thetimber power fed into it. Once the timber is inposition the load is applied, the readings takenand the fibre stress of the timber is calculated,whereupon the machine stamps the grade onthe timber. So simple when put this way, but thecurrent standards recognise 15 discrete grades.In any length of timber therefore it is the lowestgrade which that length can sustain which hasto be stamped upon it.

There are at least three companies offer-ing these machines in the UK market. All are

Computer

Deflectionsensor

Fixed positionrollers

Load cell

Load roller

Stamp

Power feed rollers

Fixed positionrollers

Power feed rollers

Fixed position rollers aknown distance apart

Fig. C.47 Stress grading machine – schematic.

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different, all have advantages and disadvan-tages and only five of the machines are fully ac-cepted as compliant with BS 4978.

The idea of permanently recording a grade onthe timber is very important. It assures the de-sign engineer that the correct timber has beenused and assures the building contractor, theclient and building control officer that the tim-ber is what has been specified for the work.Figure C.48 shows two typical stamps for UKgraded timber. It should be noted that to complywith BS 4978, the mark stamped on the timbershould include the grade and the species.

Building Regulations now include load/span tables for structural timbers such as joists,roof timbers and structural partitions, so that tocomply with these regulations the designer hasonly to match the expected load with the pro-posed span and select the appropriate timbersize at a given grade.

Fig. C.48 Stress grading marks.

Floor boarding

Traditional floor boarding is still available, thekind which is in long lengths and narrow widthswith a tongue and a groove on opposite longedges. Figure C.49 shows a traditional flooringboard machined from solid timber. Note in thetop part of the figure that, as well as the tongueand groove, the board has a series of V shapedgrooves on the underside. These ease the ten-sions in the board when drying out and help toreduce the tendency to cup and bow.

The thicknesses currently available vary from16 to around 32 and can be supplied in almostany length. One cannot be dogmatic about avail-ability, for the mills will tend to keep only a lim-ited range of thicknesses and lengths generallycalled for in their area. One mill approached onthe subject only kept two thicknesses in stock,19 and 27, the first for new work and the secondfor refurbishment in old buildings in the area.Face widths available were 90 and 115 for the 19boards and 140 for the 27 thick board. Lengthsstocked were 4.20, 4.50 and 4.80 m.

Manufactured board materials are now widelyused for flooring over joists or battens andalso for providing floating floors over insula-tion where the sheets are glued together. Ply-wood was one of the first materials to be given a

Fig. C.49 Traditional flooring board machined fromsolid timber.

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Appendix C 313

tongued and grooved joint on all four edges, andthe standard size sheets (2440 × 1220) were thefirst to receive this treatment. These were of USmanufacture and were too expensive to competewith traditional floor boards in the UK. Theystill reign supreme in North America. It wasn’tuntil the introduction of home produced parti-cle boards3 that a product which could competewith floor boards was brought onto the market.This is now manufactured in a stock size of 2400long × 600 wide and the most common typeused has a tongue machined on two adjacentfaces and a groove on the other adjacent faces.Thicknesses available are 19 and 22.

The material is made by taking debarked logsand shredding the timber into particles, whichare bound with a resin and hydraulically pressedinto a continuous sheet of material which is fi-nally cut into suitable sizes as the penultimateact of the production line. The final act is themachining of the tongues and grooves. The lay-ing down of the particles is no random act. Theexternal faces of the sheet are made up of finerparticles which press down to give a hard sur-face, while the core is of coarse particles giving alighter centre to the sheet. The sheet can also betreated to render it moisture resistant (MR). Thisdoes not mean to say that the sheet can be im-mersed in water and not suffer damage – ratherit is not damaged by superficial wetting as longas solid water is cleared from the surface rea-sonably quickly. Prolonged wetting can result inpermanent swelling of the material, as we havealready recounted.

Figure C.50 is a photograph of a corner of asheet of flooring grade particle board, showingthe upper surface.

Fixing down to joists or battens is done withimproved nails, perhaps ring shank nails, thenorm being four nails across the width of 600for every joist. Alternatively the sheet can bescrewed down, again four screws for every joistin the 600 width. Screwing of course would onlybe done using power screwdrivers and espe-cially those which had an automatic feed for col-lated screws. Power nailers are used by largehouse builders/developers’ workmen, and asfor the screwdrivers, collated nails are used. Allthat the workman has to do is press the nose

3 Particle board is the preferred term for board made fromwood chippings or particles and bound with a resin andpressed into sheets of varying sizes.

Fig. C.50 Corner of a sheet of flooring grade particleboard.

of the tool against the floor sheeting and thisunlocks the safety mechanism. Pulling a triggerthen drives either nail or screw through the sheetinto the joist below – less than a second per nail orscrew. The collated nails are of an improved pat-tern, usually square twisted shanks as they aretougher than the ring shank nails, which tend tobe brittle. Figure C.51 shows a fixing with fournails in width across a joist or batten.

Nails, screws and other fasteners are discussedand illustrated in Appendix G.

The t&g all round facility means that theboarding can be laid across joists without havingthe ends fall on a joist, and thus reduces waste. Itis much faster to lay and thus has almost oustedfloor boards completely from the domestic mar-ket. Figure C.52 shows how it is laid, minimisingwaste, and with all the internal partitions builtoff its surface.

When laying these boards, the carpenter al-ways starts at the LH (left-hand) corner, placesthe tongue of the sheet to the room side and fixesit down. On plan A, the carpenters have placedthe first full sheet, FS, in the bottom LH cornerof the building. They then cut a sheet so that the

Fig. C.51 Four nails in width across a joist or batten.

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FS C1

C1

C2FS

FS

FS

FS

FS

C1

C1

C2

C2 scribed FS scribed

Plan A

Plan C

Plan B

Fig. C.52 Laying t&g all round particle board.

LH end of the sheet fits into the space C1. It has agroove on the LH end which fits over the tongueon the RH end of the first FS. On plan B, the bitleft from cutting C1 from a full sheet is placedon the LH side of the room (C2). The cut end is tothe wall and the grooved end is ready to receivethe next sheet, which is marked as a full sheetbut might be a fraction less. If so, the narrowstrip cut-off is discarded. On plan C, the layingcontinues with an FS laid on the LH end of thefloor boards already laid, and a cut C1 is put innext, cut end to the wall on the right. C2 fromthat cutting is then cut down in width – scribed –but only the tongued edge should be removedas we need the groove to fit over the edge of theprevious tongues. Finally, a full sheet is scribedto the wall and laid with the grooved edge overthe previous tongues. Quite simple and logicalonce you work it out.

Tongue and groove boards have been the sub-ject of innovation for as long as the joint hasbeen around. Figure C.53 shows just a few of theforms adopted at one time or another, as well

Upper surface

Traditional t&g joint

A

B

C

Pavadillos joint

t&g joint on particle boardfloor grade boarding

Secretnail

Multiple t&g on thickboards

Fig. C.53 Tongue and groove forms and secret nailingof flooring boards.

as showing how secret nailing of tongued andgrooved floor boards is carried out. The nailsmentioned are illustrated in Appendix G.

The top illustration in Figure C.53 shows aslightly exaggerated form of the modern tongueon the left-hand side of the sketch; the tonguededge shows that there is more board above thetongue than there is below it. This is quite delib-erate. Above the tongue is the wearing surfaceof the board, and should it be worn down atall, there is more timber above the tongue thanbelow it, where no wear takes place. Also, onthat side of the sketch a nail is shown hammeredhome in the angle of the tongue projection. Thereis no hammer made nor carpenter alive who canhammer a nail down into that position with thehammer alone. The nail is hammered close tothat position and then a punch – a nail set – isplaced over the head and the punch struck todrive the nail down so that the head is totallyinto the wood. Having A larger than B allowsthe nail some grip in the board and somewherefor the head to lodge. On the right-hand side

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Appendix C 315

the groove is shown with the underside piececut back from the upper piece. This allows theboard joint to close on the upper face withouttoo much effort should dirt accumulate underthe board joint over a joist.

Further information

The Timber Designers’ Manual by Ozelton &Baird, now in its third edition and revisedby Carl Ozelton, opens with a comprehensivechapter on timber, structural fasteners and

structural glues. It is published by BlackwellPublishing.

BRE (Building Research Establishment) andTRADA (Timber Research and DevelopmentAssociation) are good sources of information ontimber and stress grading. Some of their infor-mation is available in short leaflet format and isfree.

A good website to visit is the Timber Trade Fed-eration’s site at http://www.ttf.co.uk, which haslots of good information and lots of good links –to BRE and TRADA especially.

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BY019-App-D BY019-Fleming-v6.cls September 14, 2004 20:6

APPENDIX D

Plain and Reinforced In-situ Concrete

Concrete has been mentioned in the text so of-ten that there must be an early pause in ourstudy to consider some of the more impor-tant features of the material before we pro-ceed further. The study of concrete is vast.Books have been written about some of thefeatures of the material, and there is no sin-gle comprehensive book – the subject is just solarge.

So, how does the student go about finding outabout the material and how to use it? Thankfully,it is possible to function quite well in the indus-try with a simple, basic knowledge appropriateto the type of work with which one is associated.If all you do is build one- or two-storey housingthen you hardly need the knowledge requiredto build a multi-storey, prestressed, precast con-crete car park.

We will proceed at a simple level appropriateto the type of construction we deal with in thetext as a whole.

Concrete

Concrete is a plastic mixture of aggregates, OPCand water. Occasionally additives are includedwhich have much the same effect as those putinto mortar. The plastic mixture is poured intomoulds or directly into the ground and allowedto set. Reinforcement of steel rod or bar can befixed in place before pouring in the plastic mix-ture. Such concrete would be termed reinforcedas opposed to plain concrete. The concrete canbe placed in its plastic condition in the positionit is meant to occupy in the building – in-situconcrete. If cast in moulds away from its finalposition, it is precast concrete.

Moulds

When concrete is placed in moulds, these arereferred to as formwork or shuttering. Form-work is frequently made of timber as well asplywoods, various fibreboards and other tim-ber products. Steel sheet, both plain and perfo-rated, together with angles and other sectionsare also used. Formwork is frequently made tobe used to cast the same shape over and overagain. This makes it economical to make in rela-tively expensive materials and maybe introducecertain adjustable features so that different sizesof concrete products can be cast.

Concrete aggregates

Aggregates can be:� Coarse – retained on a 6 mm sieve� Fine – passing through a 6 mm sieve.

Except for the manufacture of no-fines con-crete (see later in this appendix), aggrega-tes are graded. Grading ensures that, whenmixed, all voids between the particles are filledand all particles are evenly coated with cement.Aggregate size is given as the largest mesh sizeused for grading, e.g. 25 mm aggregate wouldhave all particles passing a 25 mm sieve.

The sketches in Figure D.1 show a theoreticalview of a graded coarse aggregate and a singlesize coarse aggregate. Note how the graded ag-gregate has the smaller particles filling the voidsbetween the large particles.

Sources of aggregate are:

� Naturally occurring gravels from pit or river� Crushed stone

316

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Appendix D 317

Single size aggregate

Graded aggregate

Fig. D.1 Grading of aggregate.

� Crushed brick� Crushed concrete� Furnace ash or clinker, pulverised fuel ash

(PFA)� Crushed slags – a residue left when smelting

metals from ores� Lytag – expanded clay pellets now frequently

seen topping plant pots because of their abilityto hold large quantities of water.

� Vermiculite and perlite; both natural miner-als closely related to mica which have beensoaked in water and subjected to a suddenrise in temperature which causes the leavesto expand or ‘exfoliate’.

Crushed or irregularly shaped aggregates canbed down more tightly together than roundedaggregates. The latter generally require a higherproportion of fine aggregate to fill up the gaps.It is possible to purchase aggregates which fulfilthe function of coarse and fine combined. Theyare known as all-in aggregates and are sized bythe largest particle ‘down’, e.g. 40 mm down.Voids in coarse aggregates represent a propor-tion from 20% to 45% by volume.

Fine aggregate, also known as concretingsand, must also be graded and has a wider rangeof particle sizes than building sand. Due to a rel-atively cheap and plentiful supply, concretingsand is almost entirely sourced from pit or riversources.

Selection of aggregate is critical when thestrength of the final concrete is important. Ag-gregates which contain soft or friable materialcannot be used in high strength concretes but

might be all right in concrete used as a filling. Wewill see examples of this later in this appendix.

Water

As with mortars, water is required to react withthe OPC to obtain a set. Water is also importantin two important properties of concrete:

� In wet concrete, a high proportion of water al-lows easy placing of concrete in confined areasand round heavy reinforcement, but too muchwater weakens concrete and may also lead towater voids being formed where compactionof the concrete is inadequate.

� A low proportion of water is desirable wherehigher strengths are required.

Water/cement ratio

� Unlike mortars, the proportion of water usedin concrete mixes is carefully controlled:� Enough to allow easy placement� Enough to give a ‘set’� Just enough to avoid water pockets in the

concrete.

It is given in litres per 50 kg of OPC.When using low water/cement ratios a num-

ber of techniques can be adopted to ease place-ment:

� Use of vibrators� Addition of air entraining chemicals to the

mix – for every 1% by volume of air a 6% lossin strength is incurred

� Addition of water retaining or wetting agents� Use of a smaller coarse aggregate.

Traditionally, concrete mixes for general build-ing work were specified by volume. A 1:2:4 mixcontained one part by volume of OPC, two partsfine aggregate and four parts coarse aggregate. A1:2:4 mix was generally conceded to be a strongconcrete, a 1:3:6 mix a medium strength mix.

Concrete proportions

For large works designed by an engineer, mixesused to be specified by mass, the aggregate being

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proportioned against 100 kg of OPC. Modernpractice provides two options for specification:

� Designed mixes� Prescribed mixes.

With designed mixes the purchaser is responsi-ble for specifying the performance; the provideris responsible for selecting the mix proportions,aggregate size etc. to meet that performance.Performance is generally a minimum compres-sive strength in newtons/mm2. For example, apurchaser might require a concrete which hasa compressive strength at 28 days of 35 new-tons and will be poured into column and beammoulds which have a high proportion of rein-forcement.

The addition of a timescale takes account of thefact that setting OPC takes place over a periodof time. Although the initial set of the OPC mayoccur in two or three hours, the concrete will notgain any appreciable strength for several days.The period of 28 days is generally recognised tobe the minimum time for the concrete to gain itsworking strength.

There are two kinds of prescribed mix, boththe subject of BS 5328:

� Ordinary – A list of grades of concrete is givenin the British Standard, each with a full speci-fication.

� Special – Special requirements not coveredabove. A full specification would have to begiven in any specification issued to the con-tractor.

Reinforcement

Two types of steel reinforcement are available:� As bar, which may be round, square twisted

or deformed in cross-section� As fabric manufactured in sheets or strips and

made up from bar which is welded at all thejunctions.

The steel may be mild steel or high yield steel,hot rolled or cold rolled, and may also be hotdipped galvanised. Figure D.2 illustrates:

� Round bar, also known as rebar (shortenedform of reinforcement bar)

� Square twisted� Deformed – many cross-sectional shapes are

available.

Round bar

Square twisted

Deformed

Fig. D.2 Reinforcement bar shapes.

Reinforcement can be of bar or mesh. Both aremade of steel. Bar reinforcement is covered bytwo British Standards:

� BS 4449 Hot rolled steel bars� BS 4461 Cold rolled steel bars.

Steels used are:

� Mild steel 250 grade (tensile strength is 250 N/mm2)

� 450/425 high yield steel (tensile strength is450–425 N/mm2).

Mild steel is only ever rolled into round sec-tion bar. High yield steels are available in round,round deformed and square twisted bars.

Mesh reinforcement is the subject of BS 4483.Fabric reinforcement is manufactured to complywith BS 4483. Fabrics are made in two types:

� Square mesh� Rectangular mesh.

They are formed by welding smaller section re-bar at intersections. Both types can be of mildsteel rebar or high yield steel rebar. Only roundor square twisted rebar is used. Rebar meshesare shown in Figure D.3.

Meshes are available as square and rectan-gular hole shapes, made by welding reinforce-ment bars at right angles to each other. Varyingwire sizes are used to make stronger or weakermeshes. They are available as sheets or strips.

P1: KTX


Appendix D 319

Equal

Equal

Short

Long

Fig. D.3 Reinforcement meshes.

Reinforcement must be firmly ‘fixed’ in theconcrete. It must not move. Concrete, when set-ting, shrinks and so grips the bar. The grip canbe measured by casting a bar into a block of con-crete and placing in a testing machine whichpushes against the protruding end of the bar.When the grip gives out, the load applied isrecorded by the machine. A prepared sample fortesting is shown in Figure D.4, and the differencebetween holed and bent ends on rebar is shownin Figure D.5.

Ways to improve the ‘grip’ are:

� Bending or hooking the ends of the rebar� Using deformed or square twisted bar� Using both the above.

No-fines concrete mentioned above was a spe-cialist material used extensively for local author-ity housing from around 1942 until the early1970s. The technique was originally from Ger-many and was used there for the first time inthe 1920s, being picked up and the subject ofexperimental housing during World War II bythe then Scottish Special Housing Association(SSHA). Following several years of testing vari-ous techniques for casting the material in-situ, atechnique was evolved where two-storey housewalls were cast – for the complete house – inone fell swoop. The shuttering was erected ona prepared substructure and the whole of the

Load applied bytesting machine

Rebar

Con

cret

e bl

ock

Rebar goes right throughblock of concrete

Fig. D.4 Testing for grip of rebar in concrete.

wall was cast in place with all the door and win-dow openings, local reinforcement over and un-der the openings, vent holes, pipe holes, etc. Afew days later the shuttering was stripped and aprefabricated trussed roof was hoisted into placeand made watertight. Whole terraces of houseswere frequently cast in lots of four or five housesat a time. The system was used by the DirectLabour Organisation (DLO) of the SSHA andone other contractor, Wimpy.

There are thousands of no-fines houses in Scot-land, from single-storey bungalows to ten-storey

P1: KTX



Bend on ends of rebar

Hook on ends of rebar

Fig. D.5 Bent and hooked ends of rebar.

blocks of flats. Both the SSHA and Wimpy devel-oped their own detailed techniques and housedesigns, but there was general collaboration onthe techniques and the mixes etc.

The mix was composed of one part OPC to 8or 10 parts aggregate, plus a measured amountof water. The aggregate had to be a natural riverwashed aggregate of one size, as uniform as pos-sible and with a good crushing strength appro-priate to its use in the final wall. The mixing wascritical, as what was required was an even coat-ing of cement paste over the whole of the aggre-gate.

Weigh batchers were used for the mixing. Aweigh batcher is a concrete mixer with someimportant attachments. The first is a mechani-cal shovel to load aggregate and OPC into themixer drum. OPC was loaded by the bag, largebatchers using several bags at a time. There wasa water tank fed by the mains over the drumand it would deliver a measured amount of wa-ter at the appropriate time in the mixing cycle.The amount of aggregate being loaded was con-stantly weighed and shown on a dial just abovethe drum. The correct mass was shown on thedial using an adjustable needle of a contrastingcolour.

The whole operation had to be capable of out-putting an accurate and massive amount of con-crete in as short a time as possible. The wet mixwas poured out into a skip hoisted by craneto the top of the shutter, which incorporated awalkway for workmen who emptied the skipinto the shuttering until full. Careful vibration

was used to ease the concrete around door andwindow frames etc. The wall thickness was usu-ally 10 in (254 mm) for up to four-storey work,with slightly thicker lower storeys in high riseblocks and perhaps stronger mixes.

The concrete itself was almost the anti-thesis of conventional dense concrete. We talkedthere about getting an aggregate graded andcombining coarse and fine aggregates to fill allthe voids, so giving us an extremely strongproduct – well in excess of the compressivestrength of the bricks we would normally ex-pect to build with. Well, no-fines was not likethat. Its crushing strength was much lower butthen it seldom got down to anything like thatof the bricks with which the industry is all toofamiliar. No-fines used only a well roundedaggregate and coated that with cement pasteso that when the whole mixture was emp-tied into the shuttering, the individual stonesstuck to their neighbours but there was a lot ofempty space as well. This empty space had twoadvantages:

� It reduced the overall mass of the walland therefore the load on the foundations.Ordinary strip foundations were the normon all but the most difficult sites, and thewalls were built in common brick, the earlierhouses using selected common bricks,weather jointed where the wall was exposed.

� Any rain penetrating the outer covering onthe no-fines would drain down the wall andrun out of the bottom into the cavity of thesubstructure wall.

Reinforcing steel does not generally rust whenembedded in dense concrete because of the al-kaline environment it is in. However, rebar inno-fines concrete is badly exposed and it wasthe accepted practice to coat it with a rubberlatex/OPC mixture before fixing it in the shut-tering. Modern practice would use hot dippedgalvanised steel.

The outer covering of the wall was a two-coat wet or dry dash render, the first orstraightening coat being OPC and sand andthe top or rendering coat being a cement/lime/sand mix. The top coat either incorporateswhinstone chippings in it before application (awet dash) or was dashed with coloured chip-pings after the coat was applied and still notset. White OPC and silver sand were used inbackgrounds for light coloured chippings such

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Appendix D 321

Concrete Mix A

3 course corbel10 hot bitumen DPM

Hardcore of broken brick orstone blinded with ashes,

tamped wet

300 min

150

550

Upfill with materialfrom sub-soilexcavations

ConcreteMix D filling

cavity

Weep

!00 strip of expanded metal lathbuilt across cavity

Gravel poured into cavityover lath strip

250

20 wet or dry dash renderin two coats

Wall strapping of 41 x 20Tanalised S/w

12.7 and 40 plasterboard andfoam sandwich fixed to

strapping 95 x 15 S/w skirting

100 fibreglass insulationbetween joists

20 MR particle grade flooring

100 x 38 Tanalised S/w wallplate

Polypropylenemesh to support

fibreglass

No-fines concretecast in-situ

Underfloor vents have been omitted for clarity. They would be built in with

a clay lliner

Fig. D.6 No-fines concrete – typical substructure detail.

as Skye Marble, which gave a dazzling whitesurface. The inside of the wall was finished withplasterboard, either nailed to treated woodenstrapping or to vertical stripes of ‘nailable’ plas-ter applied to the no-fines. Fixing to the no-fineswas simple. Cut nails1 could be driven throughany timber and into the concrete. When the pointof a nail hit a hard stone it simply bent round thestone. Once fully in, it was almost impossible toget them back out.

Figure D.6 is a detail of a no-fines substructure.There are a few points to note about it. It is not ex-actly as the substructures were built back in the1970s. The Building Regulations have moved onsince then, the most significant change being theneed to provide much better insulation in thewalls and the ground floors. Note the gravel ina section of the cavity. This was placed there toprevent the wet no-fines concrete from actually

1 Cut nails are discussed in the main text and in Ap-pendix G.

filling the cavity. That two course depth was re-quired to act as a drain for the whole two-storeywall height. Note also the double layer of DPCover the inner leaf of brickwork, the upper layerturned up the inside face of the no-fines wall be-tween it and the layer of insulation. This and thewall strapping holding the insulation clear of thewall contributes to keeping any water runninginto the cavity below and out of the weeps.

We will not pursue no-fines concrete any fur-ther in this text. It seems to be a forgotten tech-nique now and there is no current literature gen-erally available. One text, which the author hadin his possession at one time, was The No-finesStory written by Ronald MacIntosh, who was,for many years, Manager of the SSHA’s DLO. Itmight be possible to find a copy in some of ourNational Libraries or obtain it on inter-libraryloan.

A good website to visit belongs to theBritish Cement Association at http://www.bca.org.uk


BY019-AppE BY019-Fleming-v6.cls September 14, 2004 20:7

APPENDIX E

Mortar and Fine Concrete Screedslaid over Concrete Sub-floorsor Structures

A screed may be described as a layer of mortar ora small aggregate, fine concrete laid in a smoothlayer over a concrete subfloor (in-situ or formedwith precast components) to give a smooth finishin itself or as a substrate for a sheet or tile finishsuch as vinyl, linoleum, thermoplastic material,mastic asphalt, carpet, clay or ceramic tile.

There are three ways to lay a screed:

� Monolithic� Bonded� Unbonded.

The following factors are interrelated and choiceof one affects choice of another:

� Method of laying� Thickness� Mortar or concrete materials� Mortar or concrete mix� Self-finish� Applied finish.

There is a choice of three basic materials:

� Cement/sand mix no stronger than 1:3� Cement/granite dust/sand mix usually 1:1:2

and usually self-finished� Cement/whinstone dust/sand mix usually

1:1:2 and usually self-finished.

Granite dust and whinstone dust are ‘manu-factured’ aggregates, being quarried graniteor whinstone (black basalt) passed through acrushing machine and graded through a set ofsieves to give a fine aggregate, i.e. up to 6 mm.The mix containing granite dust is known asgranolithic and is widely used as a self-finishedlayer for hard wearing paving and flooring.

Screeds using whinstone dust are also hardwearing but not as hard as granolithic.

Table E.1 summarises thicknesses of screedsby material and method of laying.

Laying monolithic screeds

Monolithic screeds are cast onto the concretebase within three hours of laying the concretebase. The concrete in the base is still what istermed ‘green’ and the fresh cementitious ma-terial laid on it mixes at the point of contact, en-suring a secure bond.

Laying bonded screeds (1)

Bonded screeds are cast on the concrete basemore than three hours after the base has beenlaid, frequently having been left overnight. Thesteps in laying it are:

(1) Keep base damp with a spray or by coveringwith damp hessian until about to lay screed

(2) Clean surface of base and remove laitance(3) Spread thin neat cement grout and lay screed

within 20 minutes of laying grout.

Some terminology

Laitance: When beds of concrete are laid thewet material is spread out and compacted tothe required thickness. Working the material inthis way brings the mixing water to the surface,and this water contains cement. When the waterevaporates, a layer of cement powder is left onthe surface. This layer sets but does not adhereproperly to the surface and must therefore beremoved before a screeding layer is added

322



Appendix E 323

Table E.1 Thickness of screed by method of laying and type of material.

Method of laying Sand/cement screed Granolithic or whinstone/cement screed

Mono 12–15 maximum 10–25 maximumBonded 40 minimum unless in small areas – 25 40 minimum unless in stairs – 15–25Unbonded Minimum 50 – unheated

Minimum 60 – heated50–75

Thin neat cement grout. Grout is basically amixture of cement powder in water. A bond-ing agent may be added to the water. Neat refersto the need to have only cement powder in thegrout without any fillers or extenders such asPFA. Aggregates are never included as it wouldthen be a mortar. Thin grout has a ‘creamy’ con-sistency

Laying bonded screeds (2)

The alternative method for laying bondedscreeds utilises a bonding agent instead of a ce-ment grout. The screed is laid more than threehours after the base has been laid:

1. Remove laitance from the base2. Spread bonding agent over surface of

base3. Lay screed within drying time of the bonding

agent.

Note that the base should be covered with wethessian between laying and screeding to preventdrying out, but there is no need to keep it ‘wet’.

Bonding agents

Bonding agents are generally based on poly-vinyl acetate – PVA, or ethylenevinyl acetate –EVA. Proprietary names include FEBONDand TRETOBOND – look at the literaturefrom the manufacturer or visit their websites:www.feb.co.uk

It is usual to add some bonding agent to themixing water used for the screed. These sameagents are useful in plastering and rendering on‘difficult’ surfaces.

Screeds containing PVA based bonding agentsare not suitable for use in areas which arecontinuously wetted or damp, such as showerareas, swimming pools, dairies, etc. where EVA

would be better. Rates of application shouldfollow the manufacturers’ recommendations,which vary depending on the precise formula-tion of the agent.

Laying unbonded screeds

Brush loose material from surface of base. Themortar or fine concrete layer is laid direct ontothe dry, set concrete base. No attempt is made tobond both layers together. The screed can alsobe physically isolated from the base to preventa mechanical key by first laying a separatinglayer – bituminous felt, building paper or a layerof insulation.

Unbonded screeds can fail in principally twoways:

� They can ‘curl’ – just like a dried out sand-wich – even when quite thick, and then theraised areas break up under the imposed floorload.

� Unless reinforced, they can crack badly.

To prevent cracking, or at least control it, a sim-ple layer of galvanised chicken wire was oftenincluded, but fine wire welded meshes are nowavailable which are more effective. Even whenreinforced, the screed can still ‘curl’.

Large areas should be laid in bays. Bay sizesshould be confined to areas of approximately50 m2. Unbonded screeds should be separatedfrom surrounding masonry walls by a com-pressible strip, and bays should be divided bycompressible strip. Suitable material for com-pressible strip includes bitumen impregnated fi-bre board (Flexell), cork strip, rigid foamed plas-tic, bitumen impregnated soft foam plastic, etc.

A silicone or polysulphide-based mastic issometimes ‘gunned’ over the compressible stripto prevent occasional water, cleaning materials,etc. penetrating the joint.




Heated screeds

Reference to heated screeds was made inTable E.1. Electrical cables can be embeddedin the screed or can be drawn into ducts castinto the screed, or plastic or metal pipes canbe cast into the screed and fed with hot water.Microbore plastic pipes can also be drawn intoducts cast in the screed. Try the Polypipe websitefor further information on underfloor heating

using plastics pipework buried in a screed –http://www.polypipe.co.uk

Any of the above will heat the floor and soheat the room above. Insulation under andaround the edges of the screed is essential. Theoutput of cables or pipes should not exceed150 W/m2, which is the upper limit for humancomfort and will generally meet the needs fordomestic heating.


BY019-AppF BY019-Fleming-v6.cls September 14, 2004 20:9

APPENDIX F

Shoring, Strutting and Waling

This refers to what is commonly known in theindustry as timbering – the provision of supportto the sides of excavation works for whateverpurpose. This is done for two basic reasons:

� To protect the workmen down in the hole fromcollapse of the ground into the excavation.This happens more frequently than the layperson can imagine but thankfully not alwayswith fatal results.

� To obviate the necessity of re-excavation fol-lowing a collapse. This can not only meandoing it all again but could also mean the exca-vation is oversize and could mean additionalconcrete to fill up a foundation or just con-crete to infill what would not have been re-quired in the first place. Either way it costsmoney for which the contractor will not bereimbursed.

The exact technique for timbering varies accord-ing to the type of soil and depth of excavationas well as the type of excavation. At present thetext is concerned only with relatively shallow ex-cavations for trenches into which concrete stripfoundations will be poured or in which pipesand other services will be laid. Any trench fillwork is usually done in a naturally cohesive soilwhich might not require timbering at all, or onlya minimal amount.

The material traditionally used for this workis timber and breaks down into three main typesof sizes/kinds of sawn timbers:

� Poling boards are relatively short but fairlywide planks which are placed verticallyagainst the side of the excavated hole. Typi-cal sizes are 200 × 38 to 250 × 50. They are putin, in pairs, and each is propped one againstthe other with a stout timber called strutting.

� Strutting is usually square section timbersfrom 75 × 75 to 100 × 100 wedged between thepoling boards, sometimes simply cut as a tightfit and more unusually fitted with foldingwedges for adjustment. Telescopic, tubular,metal props are frequently used rather thantimber struts, the base plates being drilled fornails to hold the struts in position.

� Walings are broad and relatively thin andquite long timbers, from 200 × 38 to 250 ×50, which are used to support poling boardswhere these are placed closely together andwhich would result in the use of too manystruts. Struts are therefore spaced out alongthe lengths of the walings. If the excavationis relatively shallow there might only be onewaling per face, otherwise two per face areput in.

A quick look at Figure F.1 will explain all of theabove much more simply than a further thou-sand words.

Finally, we have to be able to deal withthe fluid-like conditions of running sand andsilt. The technique used is either close sheet-ing or sheet piling. Close sheeting is the tech-nique of placing the poling boards close to-gether edge to edge, braced with walings atleast top and bottom and strutting the walings.It is suitable for dryish conditions where thereis not a lot of ground water. Timber sheet pilingor steel sheet piling has to be used where theground water is high – and plentiful. This is nec-essary to reduce the pumping effort to keep theexcavation dry and workable. Figure F.2 illus-trates these techniques, the views being of oneface of the excavation.

What is important to realise is that virtuallyevery excavation of any depth and especially in

325




Poling board with singlestrutting

Poling boards, singlewaling and strutting

Poling boards with doublewalings and strutting.Poling spacing varies

Poling boards buttingwith three walings

and strutting

Strut

Ground level

Bottom of excavation

Fig. F.1 Shoring strutting and waling using timbers.

friable or ‘fluid’ soils requires shoring, struttingand waling to some degree or other, and all otherwork in the bottom of that excavation has to becarried out around the timbers or other materialinserted.

Once the work in the excavation is complete,the timbering has to be taken down and re-moved. This operation is called striking. Whereexcavation of trenches, particularly for extendedlengths of pipe or cable, is required, the frequent

Sheet metal endcladding

Ground level

Expected bottomof excavation

Timber piling

Section through sheetsteel piling.

Note the interlock onthe edges.

Sheet piling is driven into the ground before theexcavation takes place -- the whole point of the exercise

being to excavate in ground which is otherwise too wet ortoo unstable. The sheeting must penetrate below the

point at which excavation is expected to finish. Once thesheeting surrounds the area to be excavated, digging

takes place, with pumping where necessary, and finisheswhen a layer of concrete is placed over the bottom, thus

sealing off the hole from the surrounding ground andwater. Steel piling can be left in place; timber is best

withdrawn.

Walings and struts put in as hole is dug

Fig. F.2 Timber and steel sheet piling for earthwork support.



Appendix F 327

erection and striking of timbering becomes notonly tedious but time consuming and expen-sive. In an effort to reduce the time and cost fac-tors, specialist contractors frequently make useof purpose-made all-metal support. Figure F.3 isa schematic of how this works.

The use of this equipment is fairly self explana-tory – which does not mean to say there arenot do’s and don’ts associated with its use. It isgenerally used for pipeline or cable laying workwhere there is a need to excavate a trench con-tinuously, digging shoring, laying striking anddigging again to progress the pipe or cable lay-ing swiftly. Note that although using steel sheetfor the sides, it is not a watertight shoring andground or surface water would have to be dealtwith by pumping. For our purpose in this text,just remember that such equipment is available.

Hydraulic rams

600--1000

Hydraulic rams

Edges ‘sharp to penetrate

ground as excavation is

done

Lifting rings

Double steelskin panel

sides

Fig. F.3 Steel trench support.


BY019-AppG BY019-Fleming-v6.cls September 14, 2004 20:10

APPENDIX G

Nails, Screws, Boltsand Proprietary Fixings

This appendix describes a small selection fromthe vast numbers of fixings and fastenings avail-able to the building industry. Figures through-out the appendix attempt to illustrate the rangedescribed, with photographs and sketches.

Nails are generally of two types:

� Wire nails (Figure G.1)� Cut nails (Figure G.2).

These are available – although not in all sizes – ina variety of materials and forms. The basic ma-terial is steel for both types. For wire nails thatsteel can be round or oval in section. The formergives the name round wire nails to the most gen-eral of carpentry and joinery fastenings. Theyhave a small flattened head, a sharp point andare available in lengths from 20 to 150. Anythinglarger than 150 which resembles a nail is morecorrectly termed a spike. Closely related are thenails made from oval wire – oval brads. Theyhave a bullet head, a sharp point and are avail-able in lengths from 12 to 65. Of course, just toconfuse things, brads are also made in roundwire in the same size range. Brads are used forfinishing work such as the fixing of skirtings,architraves, window sill boards, etc. Very smallgauge or short versions of the round brads arecommonly known as panel pins, glazing pins(for fixing timber glazing beads – not the glass1)hardboard nails, etc.

The larger sizes of wire nails and brads areavailable in special finishes – hot dipped gal-vanised, sherardised and plated with zinc. Brads

1 The small steel pins used to fix the glass in place whenglazing in putty are called glazing sprigs and were orig-inally formed by cutting sharp slivers from sheet steel.They had no head so that they could be tapped intoplace close to the glass with the edge of a firmer chisel.

can also be obtained in brass in the smallersizes. Galvanised wire nails are used in situa-tions where rusting would be a problem, such asexposed timbers in roofs, tile battens and sark-ing boards, and in external works such as fencingand screens, garden huts and garages.

A galvanised wire nail with an enlarged head,termed a clout nail, is used in a variety of lengthsfrom 12 to 65. The shorter lengths from 12–25 arefrequently referred to as felt nails owing to theiruse in fixing down bituminous felt roof cover-ings and similar sheet materials. Lengths of 50to 65 are used to fix down slates and roof tilesand were referred to as slate nails or tile nails,but they are all simply clout nails.

It is for roofing that other materials for wirenails become common, the most common beingaluminium and copper. These metals are gen-erally quite soft, but for these nails a speciallyhardened version is used.

Cut nails are quite literally cut from steel sheet.Before the advent of cheap wire nails they werethe first of the mechanically formed nails. Priorto that, all nails were hand forged, not in fact-ories but as a cottage industry where individualsmiths working from their own homes wouldcontract to make nails for a larger company tosell on. Steam driven machinery allowed the useof large cropping machines which would slice asliver of steel off the end or edge of a mild steelplate in a rough nail form. Some nails requiredto be stamped to form their head.

Cut nails are available hot dipped galvanisedand in stainless steel to special order. They madea resurgence for a long period in the 1950s to1970s when they were used in large quantitiesto fix timbers to no-fines concrete, the shank ofthe nail being soft enough to bend round the ag-gregate in the concrete, making them impossible

328



Appendix G 329

Fig. G.1 Nails. Top left, starting with 150 and 125 wirenails down to 25 wire nails; to the bottom of that group,a few ‘improved nails’, and to the left of that are twogroups of oval and round brads. On the right, a fewaluminium slate nails and galvanised felt nails, thentwo improved nails called drive screws. The latter areused to fix down corrugated metal sheet to timber.Below them, a range of wire nails which have all beengalvanised.

to pull out. So strong was the grip that the tim-ber being fastened would have to be split andthe nail broken off if things went wrong. Fromthe 1960s to the present day they have foundready use whenever aac concrete blocks havebeen used. Provided the carpenter doesn’t hitthe nail too often – in other words, only enoughto get the head down to the surface of the tim-ber – the cut nail will grip well in that concrete.As with the wire nails, the coating with hot zinc

Fig. G.2 Cut nails. At the top are a 125 and two 150stainless steel cut nails; bottom right, plain steel cutnails and, bottom left, some flooring brads.

is there to protect from excessive dampness, butthe use of stainless steel sheet for their produc-tion was occasioned by a specialist treatment toadd thermal insulation to the outside face of no-fines concrete walls. The head of the nail wasspecially formed to give a T-section, and theywere then known in the trade as T-nails.

There is hardly a trade or industry which doesnot use nails of some kind but round wire, cutnails and brads are the mainstays of the buildingindustry.

The introduction of the wire nail brought nailswith sharp points to the industry, and with thatcame the problem that when nailing throughthin finishing timbers these would often – andstill do – split. This is because the sharp pointpushes its way past the timber fibres, pushingthem aside and so inducing a split. The old cutnail had a square, blunt end which actually cutthe timber fibres and forced the cut ends to curlback; if the nail was driven with the long side ofits section parallel to the grain, there was littlechance of splitting. This is why we see joiners,when nailing up thin timbers with wire nails,blunting the point of the nail and then driving itthrough.

Occasionally copper boat nails (Figure G.3) areused for fixing timber cladding, especially cedarcladding. These nails are made from square sec-tion material with a small countersunk headstamped at one end and a point cut at the other.The point about using them is their high resis-tance to atmospheric attack, especially in coastalregions.

Fig. G.3 Copper boat nails 35, 50 and 100 long. Notethe square shank and what is termed a rose head. Usedfor fixing weather boarding etc. in exposed, aggressivesituations – near the sea etc.




Fig. G.4 Improved nails – on the right the long nail hasa square twisted shank and on the left are two groupsof nails with ring barb shanks; the smaller nails are ofplain steel and the larger are of phosphor bronze.

Copper tacks also feature in plumbing workwhere lead sheet has to be secured along an edge.They are used in lengths from 12–25.

Improved nails are a feature of everyday workon today’s building sites. These nails give abetter grip in the timber and are used for fix-ing wall ties in timber frame construction, sheetfloor boards, plasterboard, even structural tim-bers and pressed steel framing plates, anchorsand hangers. The annular ring shank nail is oneof the most popular; a twisted square shank nailis also widely used, plated versions of both beingused for plasterboard work. Several improvednails are illustrated in Figure G.4.

Driving lots of nails with a hammer is a tiringand time consuming business and when time ismoney the use of power nailers can become anattractive option. They can be powered by com-pressed air or electricity, both at 110 and 230 voltsa.c. Compressed air is most economical when acentral compressor and pressure tank is set up,as in a factory situation. Electricity, particularly110 volts, comes into its own on building sites.These nailers use collated nails, i.e. the nails areheld in a plastic strip or gathered close togetherand bound with an adhesive tape (Figure G.5).

Screw nails are widely used; indeed they havesupplanted ordinary nails in a lot of areas ofstructural timberwork. There is a huge range ofthread forms available, from the traditional ta-pered thread on the first two-thirds of the length

Fig. G.5 Collated nails for an electrically powered orpneumatic nailer. Both sets of nails are 75 mm long;one set is sherardised and each has a ring barb shank,and the other set is zinc plated and they have squaretwisted shanks.

to the multi-start forms on parallel shanks fromtip to head. Figure G.6 shows a variety in a rangeof sizes.

Top left are three screw heads – from leftto right a slotted head, a Phillips head and aPozidriv head.

To the right, a group of 3 bj (black japanned),rh (round head) slotted screws; then 3 brass rhslotted screws. On the extreme right, a 3 inch anda 4 inch long screw, csk (countersunk) slottedhead, and on their left a pair of brass csk slot-ted screws and, further left, 3 zp (zinc plated)steel screw nails with csk slotted heads. Thesesix distinct sizes of screw nail are of a traditionalpattern, with approximately two-thirds of theirlength having a tapered thread and with a rela-tively blunt point. The shank above the threadis always parallel and the full diameter of thescrew.

Fig. G.6 A selection of screw nails for timber



Appendix G 331

The remaining screws are of more modern de-sign. They have either csk or pan head shapes –easily distinguishable – and have either Phillipsor Pozidriv recesses for a screwdriver. Up toaround 40 long, the thread is formed for the com-plete length of the shank. Over 40 long the pro-portion of thread to plain shank is the same as thetraditional pattern, the real difference being thaton many this plain portion of shank is smaller indiameter than the given size of the screw nail.

The threaded portion is parallel and thereare actually two threads running one inside theother – twin start threads. Note how sharp thepoints are in comparison with the traditionalscrew nails on the right.

While it is perfectly possible to drive tradi-tional screw nails with a power screwdriver,these tools were never meant to be used thatway, and the plain flat blade required frequently‘cams out’ of the slot and may score or other-wise damage the surrounding woodwork. Themodern screw nail with a Phillips or Pozidrivhead (less likely to cam out), the twin start threadand the sharp point are designed for fast powerdriving. This can be done one at a time or thescrews can be mounted in a plastic strip or beltand fed automatically into a power screwdriverdriven either electrically (battery or mains) orwith compressed air. We have already seen ex-amples of nails which are fixed this way, and theterm collated can also be applied to screw nails.

Each screw nail is supposed to perform a par-ticular function better than any other. Screwnails have very definite features which set themapart from other fastenings:

� They have a partly or fully threaded shankwith a sharp point

� The point may be self-drilling� The head is formed with a plain slot, a cru-

ciform recess – Phillips or Pozidriv, a squarerecess (a very new idea), a Torx recess, or amulti-splined recess; the last two are virtuallyconfined to the motor industry.

Besides the arrangements for driving the screws,the heads are obtainable in several differentshapes – countersunk (csk), raised countersunk,round, pan and cheese.

Screw nails which the building industry mightuse are available in lengths from 10 to 100 and indiameters from 2.00 to 6.00. Diameter of screwnails used to be measured as a gauge, the smaller

the number the smaller the diameter. For ex-ample, 8 gauge or No. 8 screws were obtain-able in lengths from around 15 to 75 long. Theshorter screws in that range were also obtainablein smaller diameters and the larger screws inlarger diameters. 8 gauge was the equivalent of4.5 millimetres. With metrication the same situ-ation applies.

Screw nails are available in a wide range of fin-ishes and materials, plain steel being availableand also plated with chromium, brass, nickeland zinc. Black enamel is also used but onlyto finish round-headed screws. Hot dipped gal-vanised was available but was used mainly bythe boat building industry. Other base materi-als used are brass, which may also be chromiumplated, stainless steel, bronzes and aluminium,which may be left plain or anodised to matchsatin anodised ironmongery.

Screw nails are not available in every size, basematerial and finish permutation; only the moreusual combinations are made, and not all arestocked by every supplier.

Drilling holes

Driving screws into softwood can often beachieved with nothing more in the way ofpreparatory work than making a small holewith a bradawl, especially if a power tool isused. However, with larger and thicker screwsit would be helpful to drill a clearance hole inthe first timber and then a pilot hole in the sec-ond timber. This is always done when fasteninghardwoods. If the screw nail has a countersunkhead, the equivalent hole – the countersink –must be made in the timber. The countersink canbe drilled with accuracy by using a countersinkblock. This only works if the drill is in a verticaldrill stand.

A countersink block is a steel or iron block witha selection of countersunk holes and throughholes drilled in it. Take the screw which is tobe used for the fastening and fit it to one of theholes and countersinks in the block. Mark thathole with chalk. Put the countersink drill bit inthe drill chuck. Place the timber on the drill standwork table; place the countersink block on thetimber; pull the countersink bit down until it fitsneatly into the selected shape in the block andset the depth stop on the drill at that point. Nowreverse the position of the timber and the block,




Fig. G.7 Cutters/bits for, left to right: a counter-sink; countersink and clearance hole; and countersink,clearance hole and pilot hole.

and countersink the timber. You will now findthat the screw head fits neatly and as flush or asrecessed as you set it in the block.

Drill bits are available which combine a drill tomake a clearance hole in the timber with a coun-tersink recess and even a pilot hole. Figure G.7shows such drill bits, from left to right: a sim-ple bit to cut a countersink shape in a previouslydrilled hole; a drill bit for a clearance hole witha countersink and counterbore attachment; anda pilot, clearance and countersink and counter-bore bit all in one. Note that the portion of thebits which drills the countersink portion of thehole slides onto the hole drilling portion and islocked in place with a grub screw. This allows fordifferent lengths of screw/thicknesses of timber.

Drilling the start of the hole through a timberas large as the diameter of the head allows thescrew head to be sunk well below the surfaceof the timber. Forming this deep hole is calledcounterboring and the hole is a counterbore.It can be used to allow a shorter screw lengthto reach the second timber, but its real use isto allow the joiner to pellet over the head ofthe screw, thus hiding it from view. The pelletused is cut from a scrap of the same timber, fas-tened using a plug cutter. Plug cutters come ina range of standard diameters so that one canbe selected to make a plug for the hole drilledor vice versa. The plugs are slightly tapered andare tapped into the hole with a little glue usinga hammer. When the glue has set, the bit of theplug protruding can be left proud as a featureor more usually it is flushed off with the restof the timbers. Plugs cut from the same timberand with decent grain matching and alignmentcan pretty well disappear – on other occasions

Fig. G.8 Plug cutter and plugs through various stagesof pelletting over.

a contrasting timber can be used to decorativeeffect.

Figure G.8 shows a plug cutter, how the plugsare cut and broken out from a block of timber,a plug set and glued into a counterbore, and aplug planed down flush with the timber.

Coach screw

One screw which is used for heavy work is thecoach screw (Figure G.9). The shank is taperedand the bottom two-thirds have a tapered threadwith a relatively sharp point, but the head iseither square or occasionally hexagonal, both

Fig. G.9 Bolts and coach screws. Top left, four coachscrews, galvanised and black iron; below them, fivecoach bolts; on the right, a selection of hex bolts and asingle nyloc nut.



Appendix G 333

to allow the use of a spanner. It is used to joinheavy timbers together, particularly where onlyone side of the timbers can be accessed. It is alsoused under the same circumstances to join metalsections to timber.

Coach screws are made from steel and can beobtained either black enamelled or hot dippedgalvanised. Care must be taken when tighteninglest the metal thread strip out the timber. This iseasily done if long-handled spanners are used ora spanner is given more leverage by slipping apipe over the handle. Avoid these things if pos-sible.

Bolts

Bolts (Figure G.9) are used for heavy work incarpentry and for light work in finishings, par-ticularly fixing knobs and handles and sundryitems of ironmongery.

Starting with the heavy stuff first, these boltscome as two types, coach bolts or hex bolts(short for hexagonal, which is descriptive of thehead shape). They come in a range of lengthsfrom about 25 upwards, the longest the authorhas used being hex bolts in excess of 350.

The coach bolt has a head shape which wasoften described as cup/square or pan/square. Itwas primarily designed for work with timber.The head starts with a large diameter (comparedto shank diameter) flattened dome and then ashort length of square section shank followedby a round shank, which has a length near theend which has a thread. The nut supplied can beeither square or hexagonal shape; washers usedshould be large diameter.

All timbers to be joined should be bored thebare shank diameter. The bolt is hammeredthrough the hole to a point where the squaredpart of the shank penetrates the timber. Care hasto be taken with the hammering, and it is oftenbetter if the bolt is pressed home with a cramp orsimilar device to avoid splitting the timbers. Fi-nally, a large diameter washer and a nut are runonto the threaded portion of the shank and tight-ened. The squared portion of the shank preventsthe bolt from turning in the hole while the nut istightened. Overtightening is always a fault withthese bolts and they should only be tightenedup to the point where the large diameter panhead or the large washer is being drawn into thetimber. As soon as timber fibres start to break

at either end stop tightening up. These bolts canbe used to bolt metal sections to timber but thecup/square head should always be on the tim-ber side.

Hex bolts have a hexagonal head, a roundshank part of which is threaded, washers anda hexagonal nut. Washers should be large di-ameter, and in certain circumstances 50 squaresteel plate washers are used. Holes through tim-bers should be bare shank diameter or 1–2 morewhere multiple timbers are being bolted. Largesizes are used in stand-alone mode, and smallerdiameters and shorter lengths can be found inproprietary fastenings.

They can be used for all timber joints ormetal/timber joints. They are used in prefer-ence to coach bolts for certain structural workrequiring the use of Bulldog connectors (FigureG.10) or shear plate connectors. These connec-tors were developed to allow the designed join-ing of timbers in trusses – lattices of timber madeup of several layers which are bolted at the nodeswith the connectors between every timber. Bothconnectors were made double-sided – timber totimber – or single-sided, which allowed timberto metal joints.

The truss had to be made in a jig, and it wasnot uncommon in the 1950s to see a rough shel-ter built on a site and under it the carpenterscutting out and laying timbers in a jig, boringholes through them all and then fitting connec-tors in place over a high tensile steel nut and boltwith several large, well greased washers fitted. Along-handled ring spanner was used to tightenup, the teeth on the Bulldog connector or the ringon the shear connector biting into the timbers. Ifthere were too many layers, the bolt had to beput into only a few at a time, and then moretimbers and connectors added until the whole

Fig. G.10 Bulldog timber connector.




joint was properly bedded down. Once that wasdone, the high tensile bolt was taken out and re-placed with an ordinary hex bolt, washers eachend and hex nut tightened up, but only to startthe washers pulling into the timbers.

Once the truss had been in place in a dry build-ing for a few months these nuts were often foundto be quite loose, as the timbers shrank whendrying out, and old specifications stated that thecontractor had to retighten after a certain pe-riod. These connectors were most often used tobuild roof trusses and a series of designs wasdone and promoted to the industry by TRADA,the Timber Research and Development Associ-ation, High Wycombe. Look at their website onhttp://www.asktrada.co.uk

The smaller bolts (diameters less than 6) canhave hex heads or the same variety of heads asscrew nails and can also find themselves incor-porated in proprietary fastenings.

All threads on bolts now manufactured havemetric threads, and the sizes of bolts are given,for example, as M10 × 65 – a metric threaded 10diameter bolt, 65 long. The length is measuredfrom the end of the threaded part to the under-side of the head.

For all ordinary timberwork the bolts need beno better than mild steel and they are generallycalled black bolts since they come as forged,covered in the black scale of oxide caused byheating. Bolts the reader may be familiar with aresomewhat polished and steely in colour. Theseare bright steel bolts and are used in the metal-work industries, the automotive industry, etc.Threads invariably run for the complete lengthof the shank. They are more commonly knownas machine screws. They are rarely used in car-pentry work but find their way into joinery inthe small bolts used there for ironmongery. Onlywhere fastenings have to be ultra strong wouldhigh tensile bolts be used.

Nuts on the other hand can be plain or self-locking. To ensure that plain nuts do not comeloose due to vibration, a star washer can beplaced under them before tightening. The teethon the star washer grip into the nut and under-lying material, preventing it from turning back.Self-locking nuts have a hard fibre or nylon ringset into the top of the nut, and when the nut istightened down to that collar the thread on thebolt has to press out a corresponding shape inthe fibre or nylon. This prevents the nut fromshaking loose.

Special fasteners

Special fasteners have been invented and con-tinue to be invented – every one a better mouse-trap! They all purport to fulfil some special func-tion in some special background material towhich we wish to fasten timber or fittings or fix-tures in every trade.

First, the special nail – most basic of all is themasonry nail (Figure G.11). This nail has a pre-cision ground point, or so the literature tells us,and it is certainly smooth and curved and quitesharp. The nail is made of hardened steel with abullet head and can be driven straight throughwood into a brick or concrete block backgroundwith ease. Softer concretes present no problembut some very dense concretes can prove dif-ficult. It is an effective means of fixing but notreally suitable for finishing joinery work. Thenails can be driven direct with an ordinary clawhammer.

The nails are also available with a steel washerjammed on the shank near the point. These nailsare meant to be driven with the aid of a spe-cial hand tool which holds the nail, protects theoperative’s hand and allows him to use a muchheavier hammer with fewer blows to drive thenail home. The washer has two functions: toalign the nail in the tool and to prevent thebullet head from disappearing into the timber.This is even more important in the ‘gun’ usedto drive these nails (Figure G.12). It is literally agun, the explosive charge being a blank cartridgeand the projectile the washered nail. Both nail

Cut nail

Stardowel

Plainmasonry

nail

Masonry nailwith plastic

or steelwasher

Manual driving

tool

Nail

Plastichandle

Head of piston

struck with 2 lb

hammerShield to protect

operative's hand Piston

Returnsprings

Magnetic tip on piston holdsnail in place

Fig. G.11 Some nails for masonry and the hand toolfor masonry nails.



Appendix G 335

Fig. G.12 Hilti gun, cartridges and nails.

and cartridge are loaded into the gun, and thenose of the barrel is depressed against the tim-ber. This releases the safety catch and the triggercan be pulled. The cartridge fires and the nailis driven into the timber and the background.A range of cartridges is available which allowsbackgrounds of different types and densities tobe penetrated. It is possible to nail timbers to theflanges of steel joist and beams. A Hilti gun, car-tridges and nails features in the photograph inFigure G.12.

Fixing into masonry backgrounds has alwaysbeen an area which has attracted much inven-tion – ever since the first Rawlplugs came on thescene just over 80 years ago. The first wall plugwas a brass strip pressed into shape and markedwith a thread, which was folded, pushed into apre-formed hole and the screw threaded into it.The name is so well known that even plugs notmade by the company and ‘invented’ by othersare known by the name. The company went onto give us Rawlbolts and a host of other deviceswhose designs have been imitated, tweaked, im-proved upon, etc.

At the most basic level there is the wall plugmade from an offcut of timber and driven intoa hole in the masonry. Nail(s) or screw nailscan be used to fasten to this. Many plugs aresimply roughly rounded in section to fit a sim-ilar hole but others can be split and shaped tofit into the perpend of brickwork or stonework(Figure G.13). These can be given a twist whenthey are cut and so wedge into the joint muchmore tightly. In Scotland the latter are not justplugs, they are dooks.

Wedge shapeSplits

Mortar in perpendcut out

MarkCut on this line

Dooks or plugs driven into wall at approximately 900 centres. Chalk line stretched over plugs on the line

required and 'snapped' to mark top of plugs. Cut off plugs at mark. Nail timbers to plugs

Fig. G.13 A plug or dook shaped to wedge into abrickwork perpend.

So, fixing with screws to masonry is perfectlypossible (Figure G.14):

� With a wood screw and a soft material in-serted into a pre-drilled hole; soft materials in-clude preformed fixings in plastic, bituminouscoated fibre and ‘woven’ metal wires such assilicone bronze. Other soft materials includeamorphous mixtures of silica fibre and OPCsupplied dry and applied as a plastic mixturewith water.

� With a large coarse threaded screw (No. 12 andlarger) directly into aac block.

� With a hardened steel screw directly intolightweight concretes and softer grades ofbricks or a precisely drilled hole in denser con-cretes (Figure G.15).

The preformed soft material fastenings werefirst introduced by the original Rawlplug Co,now Artex-Rawlplug, and were simple tubes of

Timber

Wood screw

Clearancehole

Pilot holein plug for

woodscrew

Lip preventsplug beingpushed into

hole

Fibreplug

Slits in sideallow plug toexpand aswood screwis driven in

Hole madeby bradawl

Fibre/cementmixture

Clearancehole

Timber

Wood screw

Fibre wall plugPlastic

wall plug

Fibre/cementcompound mixed withwater and packed intoirregularly shaped hole

Fig. G.14 Fibre, plastic and amorphous plugging forfixing screw nails.




Fig. G.15 On the left are two bolts, one with a hexhead and the longer one with a Torx head. These aredesigned to be screwed into concrete requiring a pre-cisely drilled and clean pilot hole. On the right are im-proved shank bronze nails, again used to fix weatherboarding etc. in aggressive situations such as near thesea.

fibre bound with bitumen. Various lengths anddiameters were made to accommodate varioussizes of wood screw. Later versions were made inbronze wire woven into tubes of various lengthsand diameters. These could be used where heatwould damage the fibre-based plug. The holedrilled in the timber allowed the wood screw to

pass through, but a larger hole was drilled in thewall to take the plug.

Also introduced was a dry mixture of silica-based fibre (originally asbestos fibre) and OPC.This could be used to give a fastening in holesof, in theory, any shape or size. Mix sufficient ofthe material with water, pack the stiff paste intothe hole, make a pilot hole in the damp mixturewith a bradawl, and drive in a wood screw. Oncethe OPC had set you had a strong fixture.

A large number of companies produce ‘wallplugs’ and other patent fixings, and informa-tion is available by obtaining literature or vis-iting websites. Among those with good cata-logues and literature are Rawlplug, Fischer, Hilti(who also make the cartridge guns etc.). Trythe websites: http://www.artex-rawlplug.co.ukand http://www.hilti.co.uk

For heavier load applications, there are ex-panding bolts which fit into holes in the ma-sonry and when tightened up expand a shell ora sleeve which grips the masonry (Figures G.16and G.17). Some expand for the full depth of thehole, others only at the bottom. No matter whatthe situation, there will be an expanding bolt tosuit your purpose and in a size to take the loadsto be applied. Apart from the bolt or nut show-ing, there are also ring eyes and hooks which canbe bolted into masonry.

Hex bolt head Hex nut

Steelwasher

Light gaugepressed steel cap

Castmetal

split shield

Light gauge wirecirclet ingroove

Light gaugepressed steel

cap

Light gaugewire circlet ingroove

Tapered centralcavity in shield to

take 'wedges'

Holethreadedinternally

Wedge shaped nut forloose bolt type

Wedge shaped end of bolton bolt projecting typeLoose bolt type Bolt projecting type

Fig. G.16 Rawlbolts – schematic.



Appendix G 337

Fig. G.17 Rawlbolts M6 and M12 in bolt projectingand loose bolt types.

Fixing back to a hollow background is anotherarea where inventive minds have run riot. Allthe fixings manufactured depend on having adevice which expands behind the skin or panelto which you are fastening – except one, whichis the gravity toggle. This goes through the holeall aligned, then gravity takes over and part of itfalls over and can be drawn up tight against theback of the panel. Figures G.18, G19 and G.20show all this.

Fig. G.18 Spring toggle and expanding fasteners forblind fixing in hollow sections or panels.

Frame fixings

Frame fixings (Figures G.21 and G.22) were spe-cially developed to make the fixing of door andwindow frames into openings in masonry wallseasier than with the traditional plug or dook. Touse them, the frame has a clearance hole drilledfor each fixing. Then a masonry drill is usedthrough the clearance hole to drill correspond-ing holes in the masonry reveals. To do this

Captivenut Toggle

Toggle

Panel

Gravity togglehinges down

Gravity toggleOptionalwasher

Parallel thread, panhead, machine screw

Panel

Material tobe fixed

Fine wirespring opens

toggles behindpanel

Material tobe fixed

Spikes on togglebite into panel toprevent rotation

Gravity toggle fastener

Spring toggle fastener

Note relative sizes of holes in panel and material being fixedand the use of a washer where the holes are both enlarged.Either fastener can be fitted to the material through a holewhich allows the bolt only to pass, then offered up to the

panel and fixed in place.

Fig. G.19 Spring and gravity toggle fasteners – schematic.




Panel Timber to be fixed

Wood screw

Three thin legs join these partsof fastener together andcollapse and mushroom asscrew is tightened

Legs -- seecentre sketch

This hole makes woodscrew bite into plastic

Clearance hole

Captive metalinsert

Rubber bodyPan head

machine screw

Clearance holein timber batten

Panel

As machine screw istightened the rubber body

mushrooms

Fig. G.20 Rawlnuts and plastic mushroom fasteners – schematic.

successfully, the frame must be fastened securelyin the opening so that the jambs of the frame areplumb and the sill and head horizontal – notethat it would be easy to fix the jambs plumb andstill have the sill and head sloping off, or viceversa. To fix the frames for drilling, packers areforced between frame and reveals, head and sillas close to the clearance holes as possible if notdirectly over them. Once the holes in the revealshave been drilled, each frame fixing is tappedinto place and the screw for each is driven uptight with a hammer.

These devices are widely used in the indus-try but there appears to be little advantage overthe traditional plug or dook. The plug or dookcan be cut from waste timber, and waste from

Lip stops plug pushing

through frame

Slits allow plug toexpand as nail is

driven in

Fast thread on nail allows nail to be

hammered in and withdrawn with a

screw driver

Fin prevents plugrotating when

unscrewing nail

Frame

Clearancehole inframe

Clearance hole in masonry

Fig. G.21 Frame fixings – schematic.

treated joists etc. can be used. Ordinary nailscan be used with plugs or dooks, and for exter-nal work a galvanised nail is longer lasting thanthe plated screws used in frame fixings. Withplugs or dooks there is no need for packers, asthe frame is propped against the plugs, and theseare marked for plumb and level before being cutback to size.

Fig. G.22 Frame fixings – the large one on the right is150 long. Next to it is the drive screw which is used.Note the shape of the thread.



Appendix G 339

Hole drilled inmasonry wall

Two part glasscapsule, resin in one,hardener in the other

Ragged endof bolt Washer

Hex nut

Parallel threadedend of bolt

Resin and hardenermixed and allowed toset before load put on

bolt

Dust blown out

Fig. G.23 A chemical anchor schematic. A more up-to-date system uses a mastic gun and a cartridge of one partresin to fill the hole into which the anchor is set.

There are even proprietary brackets marketedin various thicknesses of plastic – more expensestill. Where plastic packers are not used, cuttingsof untreated plywood or even worse, hardboard,are used. Finally, there is always the chance thatthe packers are not packed tightly, and drivinghome the fixing will pull the jambs of the frameout of the plumb and distort the frame.

The last fixing is the ‘chemical anchor’ asillustrated in Figure G.23. A range of theseare manufactured, and Artex-Rawlplug imme-diately comes to mind. They work on the prin-ciple of setting a length of threaded rod into apredrilled hole in masonry, into which has beenplaced a compound which sets due to chemicalaction. Once setting has taken place, timber orsome other piece of construction can be placedover the threaded rod, and a washer and nutapplied and tightened.

Early models used a two-part capsule, one partholding synthetic resin and the other the chem-ical hardener. Pushing the threaded rod into thehole broke the capsule placed there, and rotat-ing the rod mixed the resin and hardener. Newervariations use a single-part resin, which hardenson contact with the air. The resin can be pack-aged in cartridges and injected into the hole inthe masonry – no capsule to break, no chemicalsto mix.

Figure G.24 shows a skeleton gun with a car-tridge of silicon sealer in it. This is the toolused with the cartridges of chemical anchoring

Fig. G.24 Skeleton gun for injecting compounds intoholes, grooves or recesses in construction work.




compound. The skeleton gun shown can be usedwith any standard cartridge to place a wide vari-ety of compounds, one of which is a glue whichsubstitutes for mechanical fasteners for use onitems such as skirtings, architraves and othertrims. This particular compound is a form ofcontact adhesive with gap filling properties. This

type must not be used for structural work of anykind.

It is used by simply applying a ribbon or seriesof spots of the compound on the substrate andthen pushing the trim firmly against it. Tempor-ary support may be required if the trim is heavyor overhead.


BY019-AppH BY019-Fleming-v6.cls September 14, 2004 20:10

APPENDIX H

Gypsum Wall Board

Most lay people know what is meant by plas-terboard – at least they think they do! It is alayer of white material with paper both sides.But that is too general a description of what isa truly versatile material and one which is usedin a wide variety of forms in virtually all theconstruction work in the Western world and nota little elsewhere. So we need to have a gen-eral discussion of what the material really is,how it can be fixed to different backgrounds,and what it can do for the building we areconstructing.

At its most basic, this sheet material is a sand-wich of two layers of special paper with a layer ofgypsum (calcium sulphate) between, which canbe nailed or screwed to a framed timber or lightsteel section background. Thus it can be used tocover the underside of upper floor joists to pro-vide a ceiling, or applied to walls and partitionsto provide a smooth wall surface.

The website to visit is http://www.british-gypsum.com

There you will find that much of their pub-lication, the White Book, is available and canbe viewed and printed out with Adobe Acro-bat. Another website to visit is the internationalcompany Lafarge, at http://www.Lafarge.com

Another company, Knauf, has very good on-line literature, particularly their 32 pages onthe use of plasterboard in housing. AdobeAcrobat is needed to download and printthis but it is very worthwhile. Find them athttp://www.knauf.co.uk

All the companies manufacture a wide rangeof products for wall panelling or dry liningwork, all with special features which make themsuitable for the wide range of conditions met inany type of building work. Among the types ofboard made are those suitable for acoustic and

thermal moderation, moisture resistant board,fire resistant board, flexible board, etc.

Concentrating on British Gypsum, in generalthe type of board the domestic market will useis plain plasterboard, Gyproc Wallboard. Thistype of board, the most basic, is perfectly ad-equate for domestic construction. It has goodstrength for covering walls and partitions whereimpact damage might occur, and by building uplayers it can fulfil all the fire resistance require-ments in low rise housing – protection of struc-tural elements such as steel beams and columns,timber structural members and so on. Figure H.1illustrates a double plasterboard layer.

The board is available in a variety of lengths –1800, 2370, 2400, 2500 and 3000 – and in widths of900 and 1200 plus the thicknesses 9.5, 12.5 and 15.Note that not every permutation of these sizesis manufactured. In addition to the range of di-mensions, the boards can be square edged ortaper edged. Again, not every board size manu-factured is available in both types.

As the name suggests, square edged meansthat the board is an even thickness across its en-tire area. Taper edged board has the margin ofthe long edges reduced in thickness to accom-modate tape and jointing compound or simplyjointing in a dry lined wall or ceiling system,thus giving a flush finish which is ready for paintwork as soon as the jointing is dry. Figure H.2shows taping and jointing of both types of board.

The board is supplied with a different paperon either side. One side is ivory coloured and it isthis side which has the taper edge. This paper isspecially formulated to receive decoration directwithout any further preparation. That doesn’tstop people applying further layers of this orthat in an attempt to improve. The reverse sideis covered with a grey coloured paper which is

341




Two layers of board laid with staggered joints on background framed at 600 centres. Dotted lines

indicate first layer, solid line the final layer. Diagonal lines indicate full sheet sizes.

Fig. H.1 Double plasterboard layer with board jointsstaggered.

specially formulated to receive a layer of plas-ter. The plaster would be a gypsum-based plas-ter about 3 thick – what is referred to as a skimcoat. Skim coating does give a superior finish toany wall or ceiling but perhaps defeats the ob-ject of dry lining in the first instance, which isthe elimination of a ‘wet trade’, plastering.

Concentrating on domestic application, thebackground to which plasterboard is fixed isusually timbers at centres with dwangs and nog-gings at intervals of around two per 2400 longboard. Pressed galvanised steel framing is occa-sionally used but is more common in commer-cial work and in alterations work. Fixing backto timber can be done with nails – plasterboardnails, which are galvanised or plated and gener-ally have some form of improved shank, a jaggedshank being common.

Alternatively, fixing can be with screw nails –dry lining screws – which are made of hardenedsteel and have a specially shaped head with aPhillips or Pozidriv slot, a thread which has a fast

Paper tape

Jointing over tape andfeathered offPaper tape

Jointing over tape andfeathered off

Square edge board

Taper edge board

Fig. H.2 Taping and jointing board joints.

drive into the timber, and a point which is self-drilling. The latter is of greater importance whenfixing to the pressed steel framing. The shape ofthe head where it penetrates the plasterboard isimportant as what is required is to depress thepaper surface of the board and allow the headof the screw to penetrate just below the generalsurface and still prevent the paper from tearingaway. This is also helped by the use of powerscrewdrivers which have an adjustable clutchthat slips at a predetermined loading and stopsturning the screw into the plasterboard and tim-ber or metal backing. Another device which canbe used is a special screwdriver bit which hasan outer annular ring to press on the board sur-face when the screw is at the correct depth, andwhich then makes the bit ‘cam out’ of the slot.Figures H.3 and H.4 show dry lining screws andbits.

For small jobs, hammering in nails with ajoiner’s hammer is not too tedious but to speedup the process and reduce the physical effortfixing with screws is extremely good. Batterypowered tools have changed attitudes in the in-dustry, and the use of collated dry lining screwsin a portable tool makes fixing large sheets of

Dry lining screw in lengthsof 30, 40 and 50 and 4.00

mm diameter

Hexagonal shank to fit in

power tool

Phillips or Pozidriv point Special bit for driving

dry lining screws

Smooth 'stop' tomake screwdriver

point 'cam' out

Jagged plasterboard nail inlengths of 30, 40 and 50 and

2.5 or 3.00 mm diameter

150

A plasterer's hawk

SpatulaUnderside showing

single central handle

TopTop

350

350

The plate can be of timber, aluminium or plastic

A wooden orplastic handlewith a flexiblesteel or plastic

blade

Fig. H.3 Dry lining screw, nail and screwdriver bit.



Appendix H 343

Fig. H.4 Dry lining screws and driver bit.

plasterboard the work of a few minutes and byone operative, 20–25 m2 per man/hour being thenorm for plain wall and partition work.

The frequency of the fastenings, whetherscrews or nails, is usually about 150–200 apartto every timber or framing member behind theboard. The framing needs to be in rectangles notexceeding 600 × 400 to maintain a flat surface,particularly for ceiling boarding. Fixing backwith plaster dabs or stripes to a masonry wall hasbeen used a lot in the past in domestic work butnow rarely finds a use except in refurbishmentwork. All the manufacturers mentioned abovegive details of the technique(s) in their literature.

Finishing off the board ready for decorationmeans applying jointing compound to all thejoints, bedding in a paper tape or scrim, and thenapplying a final layer of jointing compound, allin a thin layer and with the edges feathered tomake the joints disappear. The paper tape is sup-plied in rolls of around 300 m and is 50 widewith a centre crease to facilitate folding intocorner joints. A reinforced paper tape is madewhich can be applied to external corners. Thereinforcement is two strips of sherardised steelstrip. Scrim is a woven tape of either cotton orjute fibre, the cotton being the finer texture. Itis made in 50, 75 and 100 widths. Some decor-ators apply a coat of the jointing well watereddown to the whole of the boards and then dec-orate, but this is not really necessary. Emulsion

paint, oil paints, wallpaper, etc. can all be applieddirect to the board. Some decorators will applyan oil undercoat before wall papering to facil-itate removal when redecoration takes placeothers even go to the extent of using alu-minium paint, although to what purpose is notclear.

The taping and jointing was originally carriedout by both plasterers or painters and decoratorsbut is now carried out by specialist tapers andjointers, recognised as such in most WorkingRule agreements. These men will do the workwith hand tools such as spatulas and hawks(see figure H.3), finishing off with sponges etc.to give the smooth finish desired. The bulk oftheir work is carried out using more mechani-cal means, even if still powered by human mus-cle. The process is then known as Ames Tapingand comprises a number of tools with which tospread an even amount of jointing compoundinto straight and corner joints, and a tool whichholds a roll of paper joint tape and applies it intothe straight and corner joints, and then finallyspreads the jointing compound across the joint,feathering down the edges.

Fire protection using wallboard is basicallyabout putting a layer of a fire resistant materialbetween the source of the fire and the mater-ial to be protected. Generally one or two layersof 12.5 thick board are sufficient. In Chapter 4,Floors, reference is made to ceilings with twolayers of plasterboard when the floor dividesflatted housing. The combination of plaster-board and sound deadening materials gives afire resistance in excess of half an hour. Also inthat chapter, literally wrapping a steel beam withtwo layers of plasterboard has the same effect.Loadbearing timber or pressed steel stud par-titions always have any faces next to halls andstaircases clad in one layer of 12.5 plasterboardto protect the framing.

In all these examples thick or multiple layers ofplasterboard can slow down penetration of thepartition, ceiling, etc. by fire or hot gases to giveprotection to occupants using the hall or stair-case to escape or who live in the neighbouringflat.

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APPENDIX I

DPCs, DPMs, Ventilation of GroundFloor Voids, Weeps

This is a somewhat mixed bag of an appen-dix but is an attempt to take out a lot of repetitivedetail from the earlier chapters and explain somebasic materials and techniques common acrossa number of chapters.

DPCs and DPMs

Damp proof courses and damp proof mem-branes are layers of material, impervious to wa-ter, which are placed in the construction to pre-vent penetration by dampness, rain, etc. DPCsare built horizontally into walls near groundlevel, over lintels and under sills and verticallyat door and window jambs. DPMs are laid un-der floors or on or within the vertical thicknessof walls. We will examine these situations anddetermine what is the best from a wide varietyof materials to be used in the circumstances.

The conventional symbol for damp proof lay-ers is shown in Figure I.1. Depending on thebase material, these layers can vary from 150 mmthick down to a fraction of a millimetre. For thoseof a few millimetres and less it is impossible toshow them to scale or in proportion.

The answer to this problem is quite simple.These layers are very important in the construc-tion, therefore the symbolism must be clear onthe drawings. The layers are shown larger thanscale or proportion allows and the additionalspace required is taken off the adjacent lay-ers – but only from one side. Dimensions aregiven to one face or side of the damp prooflayer where the adjacent material is shown fullsize.

The detail shown in Figure I.2 was studiedin connection with solid concrete floors (Chap-ter 2). Note the placement of DPC and DPM.

Note that neither is given a thickness. The DPCwould be contained entirely within the mortarbed of the brickwork. The Visqueen sheet usedfor the DPM is only 250 microns thick.

The annotation on Figure I.2 states that theDPC and DPM overlap. Though not absolutelynecessary, it is best if they overlap in the verti-cal plane alongside the wall. The DPM is laidfirst and turned up at the edges. The DPC islaid next and is turned down over the DPM.With the joint horizontal, any settlement of thefloor slab neatly slices the join like a pair of scis-sors. With the lap vertical, the joint slides apartwith the settlement, especially when edge insu-lation is in place. These points are illustrated inFigure I.3.

Only DPCs are built using thicker materialssuch as bricks, tiles and slates. In the case of thesematerials, they can be shown to proper scale asshown in Figure I.4.

Materials for DPCs and DPMs are the subjectof several British Standards:

� BS 743 – Materials for damp proof courses� BS 6398 – Bitumen damp proof courses for ma-

sonry� BS 6515 – Polyethylene damp proof courses for

masonry.

See also the precis of British Standards in Ap-pendix L.

CP 102 is a Code of Practice, also published bythe BSI, and is entitled Code of Practice for protec-tion of buildings against water from the ground. BS8215 is also a code of practice entitled Design andinstallation of damp proof courses in masonry con-struction, which deals with damp proof coursesin the superstructure of the building such asaround door and window openings.

344

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Appendix I 345

Fig. I.1 The conventional symbol for damp prooflayers.

DPC and DPM materials, all listed in BS 743,can be classified as:� Bitumen sheet material

� Plain – BS 6398� Plastic sheet material – BS 6515� Metals� Mastic asphalt

� Applied hot� Masonry materials.

In addition to the materials listed in BS 743 thereare:� Self adhesive bitumen sheet materials – not

studied here� Cold applied liquid materials� Tiles of concrete or fired clay.

Bitumen sheet DPCs are generally classified bythe ‘base’ materials used in their manufacture,and are detailed in BS 6398. Base materials avail-able are:� Hessian – a single layer of plain woven jute

yarn� Fibre – one or more absorbent sheets of felt

made from a mixture of animal and vegetablefibres

� Asbestos – absorbent sheet made from 80%asbestos fibres

� Lead – sheet or strip milled lead with solderedjoints.

215

515

150

300 min.

150 FGL

DPC90

210

Facing brick to be Bloggs

heather mixture wire cut brick built in cement/

lime mortar 1:2:9, weather

struck jointed

Brickwork raked out to receive

rendered finishConcrete floor to be power floated smooth to receive

paint finish

Concrete Mix C

DPC and DPM overlap

250g VisqueenDPM

Hardcore of broken brick or stone all to pass a 100 ring, blinded with sand

Backfill with selected subsoil from the excavations

ConcreteMix A

Fig. I.2 Placement and thickness of DPC and DPM ina detail drawing.

Lap will tear or slice here

Overlap here will slide apart

Fig. I.3 Laying and overlapping of DPC and DPM.

Table 1 of BS 6398 gives the Class and Descrip-tion of each type of bitumen DPC (an exampleis shown in Table I.1) together with a full listingof the mass/m2 of materials used in their manu-facture. Classes are:

A Hessian baseB Fibre baseC Asbestos baseD Hessian base laminated with lead (also

known as ‘lead core DPC’)E Fibre base laminated with leadF Asbestos base laminated with lead

215

Bloggs heather rusticfacing brick

Two courses of Class C Engineering brick

in cement/lime mortar 1: :3

Commonbrickwork

VisqueenDPM

Hardcore of brokenbrick to pass 100ring and blinded

with ashes.

Concrete Mix Awith power floated

finish

34

Fig. I.4 Brick and tile DPCs to scale.

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Table I.1 Class and description of each type of bitumen DPX from Table 1 of BS 6398

Type Application Specification Detail Use

Sheet Built in Bitumenimpregnated

Fibre bases with andwithout metal cores

Generally DPCs,rarely DPMs

Plastic Polythene andpolypropylene

DPCs and DPMs

Metal Lead and copper DPCs only

Liquid Cold applied Bituminousemulsions

Proprietary types DPMs only

Hot applied Mastic asphalt Natural or limestoneaggregate

DPCs only and‘tanking’

Coal tar pitch DPMs only

Others Built in twocourses in 1:3cement mortar

Brick As British Standard 3291for DPC brick

DPCs only

Slate and tile To appropriate BS forroofing material

DPCs only

Table 2 of BS 6398 lists suitable locations for theuse of these DPCs, with the recommendationsbased on the following:

� Compressive loads� Flexural loads� Shear loads� Water movement – up, down and horizontally.

While polyethylene DPC is listed in BS 743, thedetailed standard is laid out in BS 6515. The ma-terial to be used is described as follows: black,low density, polythene sheet with a mass in therange 0.425 to 0.60 kg/m2. The composition toinclude 2% evenly distributed carbon black andno more than 5% other material both by mass.No air bubbles and no visible pin holes. Table1 in Appendix D of the Standard lists uses etc.using the same criteria as bitumen DPCs.

This material is supplied in rolls of 10 m2 andin widths which are generally multiples of half abrick plus 200, 300 and at least 1000. There maybe others – it depends on the manufacturer. Thismaterial is used in walls at DPC level, rounddoor and window openings, etc.

Much of the polythene seen used as DPM ma-terial on building sites today is coloured blue.Figure I.5 is a photograph of a substructure andshows a polythene DPM laid over a solum. Onthe left-hand side it is ready to receive a con-crete garage floor, which is reinforced with thesteel fabric lying there. On the right-hand sidethe polythene has been laid and weighed down

with a layer of concrete. This will be under thetimber hung ground floor of the house.

For more information try the websitehttp://www.visqueenbuilding.co.uk

Other materials are specified in detail in BS743:

� Lead should be Code 4 (i.e. 1.80 mm thick or4 lb/ft2) and should be milled sheet or striplead for building purposes complying with BS1178.

Fig. I.5 Polythene DPM.

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Appendix I 347

� Copper should be 0.25 mm thick complyingwith BS 2870 sections 2 and 7, Temper GradeO, dead soft or fully annealed.

� Mastic asphalt should comply with either� BS 1097, Mastic asphalt for tanking and

damp proofing (limestone aggregate), or� BS 1418, Mastic asphalt for tanking and

damp proofing (natural rock asphalt aggre-gate).

Whichever asphalt is used it is applied hot – gen-erally around 230◦C – in one or two layers from12 to 20 mm total thickness. It can be applied tohorizontal and vertical surfaces. It can be appliedto the workman’s side of a face or to the oppo-site side – overhand. It cannot be applied to over-head surfaces. It requires specially skilled labourand special plant for successful application.

Tanking is a technique applied to under-ground basements and is beyond the scope ofthis text. Materials other than mastic asphalthave been used for tanking but this is the bestalthough the most expensive in capital outlayterms – what it would cost to redo the water-proofing of a basement wall carried out with in-ferior materials doesn’t bear thinking about.

Slates should be not less than 230 mm long orless than 4 mm thick. They should comply withBS 5642 with regard to the wetting and dryingtest and resistance to sulphuric acid. A minimumof two courses should be laid to break joint in amortar of 1:0– 1

4 :3 cement/lime/sand.Bricks should be of fired clay or brick earth.

They should comply with BS 3921, DPC bricks. Aminimum of two courses should be laid to breakjoint in a mortar of 1:0– 1

4 :3 cement/lime/sand.

Other DPM material

Liquid, cold applied, bitumen-based materialsare frequently used to provide damp proof mem-branes. They are principally ‘emulsions’ – a bitu-men/rubber latex which while drying by evap-oration of water give a waterproof seal. They areapplied to a concrete or masonry surface in twocoats by brush, the second coat brushed on atright angles to the first to cover pinholes.

They are frequently used in sandwich floorconstruction and on concrete bridge abutments.They can also combine the properties of an adhe-sive for block or parquetry flooring. Applicationrate is generally around 2.5 litres/m2. RIW, Syn-

thaprufe and FEB Hyprufe are three well knowntrade names.

Two websites to try:

http://www.ruberoid.iehttp://www.degussa-cc.co.uk

Hot bitumen is frequently specified as a DPMmaterial. Originally this was ‘coal tar pitch’ ob-tained from the ‘gas works’ as a by-product ofthe destructive distillation of coal. It was deliv-ered to site in insulated and heated tanker lorriesand the hot pitch was spread over a solum froma watering can with a spout shaped like a fan,which would pour a wide ribbon of the hot liq-uid. The thickness of the ribbon could be set byvarying the rate of pour.

‘Bitumen’ is now obtained from the distillationor ‘cracking’ of crude oil and this bitumen canbe used in exactly the same way as the coal tarpitch. Delivery to the site is still in insulated andheated tanker lorries. Spreading is still carriedout with the special watering cans.

Hot applied bitumen DPMs are generallylaid over a blinded hardcore bed under hungground floors. The blinding is of sand or prefer-ably furnace ash, which is wetted and beatendown to a smooth hard surface. The ash be-ing slightly pozzolanic (setting like a cement inthe presence of water) gives the better finish onwhich to work.

A 10 mm thickness of pitch or bitumen is gen-erally satisfactory. When the workman reachesa wall it is usual to allow the bitumen to splashup the wall, thus reducing the area of wall fromwhich moisture can evaporate.

Plain clay or concrete roofing tiles can be usedfor damp proof courses in ancillary construc-tion works such as screen walls and gardenwalls. A minimum of two courses should belaid to break joint in a mortar of 1:0– 1

4 :3 ce-ment/lime/sand. Materials used for mortar –cement(s), lime and sand – should comply withtheir respective and appropriate British Stan-dards.

Table I.1 classifies the information on DPCsand DPMs by basic material.

Weeps

Weeps were originally used to allow thedrainage of water trapped in a cavity in wallconstruction. This can occur in several areas of abuilding:

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Cavity with weak concrete fill up to

ground level

Moisture penetratingouter leaf runs

down and gathers here to be drained

out through the weep

Mortar omitted from the perpend or plastic device built in every

fifth brick

Fig. I.6 Schematic of a weep formed at ground levelin a masonry wall.

� The substructure walls, just above finishedground level

� Above the DPC laid over a lintel� At string courses� At fire stops in timber frame construction.

Weeps can be formed by simply omitting themortar from the perpend or by inserting a pur-pose made plastic weep.

Figure I.6 is a schematic of a weep formed atground level in a masonry wall. Note that it is abasic weep with mortar left out of the perpend.Note the slope outwards of the cavity filling,which is standard on every detail you look at butmust be well nigh impossible to form. The au-thor has looked down a few cavities in his timeand has yet to see this being done. Mostly theconcrete is left quite rough; occasionally there isan attempt to level it off.

Figure I.7 is a photograph of a proprietaryplastic weep merely set in place in some loosebricks to show how it is positioned. There mustbe at least a dozen manufacturers making theirown version. The point about using these weepsis to see that the perpend does not get blockedwith mortar as building goes on round aboutit. The internal part of the weep is specially de-signed to take water out and not let it be blownback in again. Figure I.8 is one of these weepsopened up to show the baffles and water chan-nels. The bottom of the weep is at the bottom

Fig. I.7 A proprietary plastic weep.

Fig. I.8 Weep opened up to show the baffles and waterchannels.

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Appendix I 349

of the picture with the gridded edge on the leftshowing on the face of the wall. Note that thisparticular weep has a drip built into it to throwwater clear of the wall surface.

Weeps are made for building into walls whichwill eventually be rendered. The outer face hasa plastic cover over the outlet. The weep is builtinto the brickwork, projecting the thickness ofthe render. The render is applied and, when set,the plastic cover is peeled off the weep. The re-sult is a clean, unclogged weep and a neat finish.

Weeps are also used in timber frame construc-tion, not just to take water away but also to pro-vide a minimum amount of ventilation of thecavity between masonry cladding and timberpanels. This has already been covered in Chapter3, Walls. There it was stated that weeps had to bebuilt in above and below the horizontal fire stop-ping at first floor levels. That fire stopping hasto be the full cavity thickness and covered witha DPC to stop water getting across the cavity.Small particles of dust, waste material and deadinsects and spider webs can and do build up inthese cavities and especially over the horizontalfire stops. This waste material can block the flowof water down the inner surface of the masonrycladding and can even seal off the pores in themasonry. So water can build up and can run backinto the timber frame. This is where the weepsabove the fire stop act not just as ventilators butas weeps!

Ventilators

The ventilators being discussed here are exclu-sively those built into substructure walls to ven-tilate the void under a hung floor. They mustlet a minimum amount of air pass and this iscovered in the British Standard. They must ex-clude vermin, birds and pets or other domesticanimals and even small children. They must ex-clude water, so the liner should be built in witha slight slope to the outside and the ventilatorsthemselves have the holes or slots arranged toshed water to the outside

Figure I.9 is a photograph of ventilatorsand liners made in terracotta, 215 × 65 and215 × 140.

BS 493 lists two categories of air brick or ven-tilator, and the reader is referred to the precisof this Standard in Appendix L. The sizes of airbricks and ventilation gratings for use externally

Fig. I.9 Ventilators and liners made in terracotta.

always equate to the work size of full stretcherface multiples, the most common sizes being:

� 215 × 65 and� 215 × 140

and so on to suit different brick sizes.Cavities must be closed to duct ventilating air

through the wall. This was done traditionallyby closing all four sides with cuttings of roof-ing slate, as shown in Figure I.10. They must notventilate the cavity between the masonry skins,so now liners are built in, as shown in the ac-companying figures. This prevents the spreadof fire from timber structures built into the cav-ity, where the cavity would act as a ‘chimney’,spreading the fire up to the next floor(s) and fi-nally the roof.

Liners for use with external airbricks and grat-ings are made from clay or plastics. Off-cuts ofclay or plastic drainage piping are sometimesused but a purpose-made clay liner is to be pre-ferred.

One of the many gadgets made for the indus-try is a telescopic liner for underfloor ventilators.They can be used to duct air across and up ordown a cavity where there is a non-conventionalarrangement of ground and floor levels. For ex-ample, a hung floor might be built which is

Onebrick wide

200

One or twocourses high

Slate closing cavity -- two sides omitted for

clarity

Fig. I.10 Closing all four sides of vent opening withcuttings of roofing slate.

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Onebrick

One or twocourses

Telescopicsection


Hung floor construction

Underfloor void

Solum finish

Telescopicliner

Vertical DPCrequired here

Fig. I.11 Telescopic liner for under floor ventilators.

below ground level, and a straight through ven-tilator above ground level would ventilate theroom, not the void under the floor. Figure I.11shows how this might work. It is used in re-furbishment work where there is no way of al-tering the relative floor and ground levels, butone wonders what is done about the lack of aDPC vertically in the walls joining the under-floor DPC to the wall DPC. It can hardly be aprocedure to be advocated for new build work,not least because the unit is made of plastic andso is not fireproof as a clay liner would be.

Figure I.12 is a photograph of a terracotta linerbuilt into a blockwork substructure wall. Thereis another course of blockwork for the outer leafof the wall to be built, and then a timber framekit is to be erected above this. Note that the lineris sealed to the blockwork with mortar. This willbe repeated when the outer leaf course is built.The large clay pipe immediately below the ventliner is a duct for service pipework. In this caseit is a 100 nominal soil and waste pipe.

Figure I.13 is a photograph of a similar linerbuilt into a cavity wall in the wrong place. Itis built two brick courses too high and is ac-

Fig. I.12 Terracotta liner built into a blockwork sub-structure wall.

tually opposite the ground floor construction.Once the insulation for the ground floor is put inplace, the ventilator will be completely blocked.Note also that the liner is not bedded all roundwith mortar. It should be ‘sealed’ to both leavesof masonry. The other vents in this house werein a similar situation, and unless the work wasseen at this particular stage, no-one would everknow – until the floor started to rot.

Figure I.14 shows a typical vent in a cavity wallwith a 100 thick outer leaf of artificial stone. Theair brick is of clay.

Fig. I.13 A similar liner built into a cavity wall in thewrong place.

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Appendix I 351

Fig. I.14 A typical vent in a cavity wall with a 100thick outer leaf of artificial stone.

How much ventilation?

Although all regulations require ventilation ofthe underfloor space in hung floor construction,it would appear that only the Scottish Regula-tions give precise figures for its calculation, al-though these figures give some peculiar results,as we shall demonstrate.

Each ventilator built in will admit a givenamount of air, and the amount required is calcu-lated on allowing a free air flow through a givenarea per metre of wall – 1500 mm2 per metre run;or on a given area of ventilator per square metreof floor area – 500 mm2 per m2 of floor area.

Note: the area of the ventilator in this part ofthe exercise is assessed on the size of the hole inthe wall and not on the clear opening size of anygrid or air-brick placed over the hole or any linerplaced in the hole. This restriction is allowed forby requiring grids or air bricks to comply withBS 493.

Let’s look at the problems involved in ven-tilating the underfloor void of a building witha ground floor area of approximately 45 m2. Ahole made by omitting two courses high and onebrick wide has an area of 225 × 150, i.e. 33 750mm2. Divide this by 1500 and we arrive at a spac-ing of 22.5 metres along the wall between venti-lators; or a typical ground floor area would be 45m2, which multiplied by 500 mm2 gives 22 500.Divide by 33 750 and we have the equivalent oftwo-thirds of an air brick.

So for a building of approximately 7 m × 6.5 mon plan, using the first method we calculate thatit will require one and a bit ventilators in thetotal run of the walls, and only part of a ventusing the second method. Clearly impractical.Most buildings end up with a far greater number

XXX

X X

X X

X

X = ventilator

Fig. I.15 Typical house plan with the position of vents.

of vents, so what are the criteria for deciding onhow many and where to put them?

The placing of a single vent would give riseto ‘dead spots’ – areas of the underfloor spacewhere the air would be stagnant and, if moist,would raise the moisture content of the build-ing fabric. Figure I.15 shows a typical house planwith the position of vents marked as X. Ideallythe vents should be placed within a metre of cor-ners and should not be more than 4–5 m apart. Itis best to place the ventilators in opposite wallsand start and finish as close to the corners as pos-sible. Observing these simple rules will avoiddead spots, i.e. parts where the void does notget proper ventilation.

Figure I.16 shows a house with part of the floorof solid concrete on upfill construction. This

X

X X X

X

XX

A

Solid concrete floor

Fig. I.16 A house with part of the floor of solid con-crete on upfill construction.

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X

X X

Deadspot

Solid concrete floor

Fig. I.17 A house with a small area of hung floor –inadequate ventilation.

effectively blocks off the possibility of ventilat-ing a corner, so we break the rule about ventila-tors being opposite each other, and ventilate theodd corner and take out a potential dead spot.

Figure I.17 shows a house with a small areaof hung floor, with two sides bounded by asolid floor construction. Three vents should beadequate following the rules of the Regula-tions but this leaves a large dead spot. Theonly way round it is to adopt the idea inFigure I.18, where ducts are placed under the

X

X

X X

X

Fig. I.18 A house with a small area of hung floor –adequate ventilation.

solid floor (in the upfill) and begin with air bricksin the outer wall. The ducts are usually madefrom 150 diameter plastic or clay drainage pipe(the pipes must be strong enough not to col-lapse under the weight of the floor structure).The reason for putting in two pipes is that it willhelp the air pressure under the floor to balancewhen strong winds are blowing. Evening out thepressure in this way prevents any uneven pres-sure on the floor structure causing unwelcomedraughts in the house above.

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APPENDIX J

Drawing Symbols and Conventions

As one would expect, the drawing of archi-tectural details requires a common methodo-logy for the representation of building materials

Break line

Common brickwork

Facing brickwork

Natural stonework

Concrete blockwork

No-fines concrete

In-situ dense concrete

Fig. J.1 Architectural drawing symbols (1).

Sawn timber

Wrot or dressed timber

Damp proof course ordamp proof membrane

Note that DPC or DPM is never to scale. In practice they are so thin that they are always exaggerated on

drawings because of their importance.

Plaster or mortar render

Earth or soil

Hardcore bed or layer

Upfill either with selectedmaterial from excavationsor with imported material

Membranes generally, roofing felts, plastic,

breather paper

Fig. J.2 Architectural drawing symbols (2).

and construction forms. The samples given inthis appendix are those used in this text; somemay not be ‘official’ or their use may be slightlyunorthodox. The author apologises if that upsetsanyone.

While DPCs and DPMs are always shownto an exaggerated scale, the problem alsoarises with membranes. Where plastic, paper

353

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or felt layers are used, they are generallyshown adjacent to the layer on which they areapplied.

The British Standards Institution did publisha document some years ago which dealt exclu-sively with architectural drawings – everythingfrom paper sizes and folding drawings to titles,

line thickness and conventional symbols. Alas,this seems to have been withdrawn. There are,however, several good textbooks on the marketwhich cover the topic. One which can be recom-mended is An Introduction to Construction Draw-ing by Arthur Thompson, published by Arnolds(ISBN 0-340-56823-2).


BY019-AppK BY019-Fleming-v6.cls September 14, 2004 20:12

APPENDIX K

Conservation of Energy

Heat loss calculation usinga spreadsheet

Conservation of energy has long been a require-ment of Building Regulations although manyargue that even the current requirements don’tgo far enough. Whether one agrees with thator not, the fact is that calculations have to bedone to assess the energy efficiency of the build-ing proposed.

One of the features of these calculations isthe need to calculate how much heat will passthrough a piece of construction – a floor, a wallor a roof, a door or a window. Then the areas ofthose features with a high heat loss have to bebalanced out and items which cause cold bridg-ing have to be assessed. Areas where conden-sation might occur within the building struc-ture have to be identified and eliminated orprevented. The whole exercise is beyond thescope of this simple text but the student wouldbe advised to come to terms with the one or twosimple ideas put forward in this appendix, witha view to a more advanced understanding laterin the course he or she is pursuing.

We must start with a few definitions. Energyis measured in watts. Unit area is one squaremetre. Unit thickness is one metre. Unit tem-perature difference is one degree kelvin.

Emissivity is the ability of a surface to passor receive energy to or from the atmosphere inwhich it exists in response to a unit differencein temperature. It varies with the exposure ofthe surface, its texture and colour. Texture andcolour have so little effect that rough/smoothand light/dark are sufficient to describe it.

Exposure, on the other hand, has a far greatereffect. Three categories are generally used, shel-tered, normal and severe, severe exposure being

at least three times more emissive than one cat-egorised as sheltered.

Thermal conductivity is the amount of energywhich will pass through unit area, across unitthickness of a material in response to a unit dif-ference in temperature, and is represented by theletter k.

Thermal resistance, r, is the reciprocal of ther-mal conductivity, i.e. 1/k, and is the resistance ofunit thickness of unit area of the material.

Thermal conductivity of a material of thick-ness l is given by dividing l by the conductivityk. l is always used in metres but may be givenin millimetres when taken from an architecturaldrawing where only millimetres are supposedto be used. Take care, then, when transferringdata from detail sheets to calculations.

U-value, or thermal transmittance, is theamount of energy which will pass through unitarea of a piece of construction of given thicknessand composition in response to unit differencein temperature, so it is the reciprocal of the sumof all the individual thermal resistances plus ex-ternal and internal emissivities. It is measuredas W/m2 K:

U-value =1

external internalsurface +R1 + R2 + R3 + R4 + Rn+ surfaceresistance resistance

An Excel spreadsheet has been saved on thisbook’s web page, www.thatconstructionsite.com, in ‘Sample pages’, where you can down-load it and use it. If you find it useful please feelfree to use it; the only condition is an acknowl-edgement of the source and the author as printedon the first sheet of the worksheet WALUVAL2.

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BY019-APPL BY019-Fleming-v6.cls September 14, 2004 20:56

APPENDIX L

Short Precis of SelectedBritish Standards

General A brief summary of the history and working of the British Standards InstitutionBS 187 Calcium silicate bricksBS 493 Air bricks and gratings for wall ventilationBS 743 Materials for damp-proof coursesBS 1199 With BS 1200 Sands for mortars, plasters and rendersBS 1282 Wood preservativesBS 3921 Clay bricksBS 4449 Carbon steel rods and bars for reinforcing concreteBS 4471 Sizes of sawn and processed timberBS 4483 Welded steel fabric reinforcementBS 6398 Bituminous damp-proof courses for masonryBS 6515 Polyethylene damp-proof courses for masonry

British Standards

GeneralThe British Standards Institution was founded in1901 and incorporated by Royal Charter in 1929.The objectives set out in its charter include:

� co-ordination of the efforts of producers andusers for the improvement, standardisationand simplification of, in our field, construc-tion materials and components

� to simplify production and distribution� to eliminate the unnecessary production of a

great variety of sizes of products and compo-nents

� to set up standards for quality and dimensions(in many cases these are generally consideredto be minimum standards of quality, nor dothe standards preclude manufacture of mater-ials or components to other dimensions)

� to promote the adoption of British Standards.

The Institution is a non-profit making or-ganisation funded principally by government

grant, sale of publications and subscriptionsfrom members. Any company, organisation orindividual can be a member.

Individual British Standards are under con-stant review, and the pace of this accelerateswith the introduction of new materials and tech-niques and, of recent years, the harmonisationwith European Standards.

This constant updating has an important ram-ification for the designer of any building andother colleagues in the team of professionals run-ning the contract to build it. When letting a con-tract to build, the team must agree a point in timepast which no further amendments to Standardsor new Standards will affect the contract. It isusual to state in the contract documents (in thePreambles) that reference to British Standardswill include all amendments to the Standardspromulgated before a date some time before thedue date for receipt of tenders – usually at least

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Appendix L 357

4 months. As the contract proceeds there will beamendments to some of the Standards but thesecannot be introduced on the contract. The de-sign team must stick with the amendments up

to the date before tendering or they must negoti-ate with the contractor to have the new Standardintroduced.

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British Standard 187

Calcium silicate (sand-lime and flint-lime) bricks

This precis is intended only as an introduction to aparticular British Standard to place particular in-formation in the correct context within this text.The precis, therefore, does not include reference tothe entire technical content of the Standard. Tablesincluded in the precis are NOT the tables from theBritish Standard but may follow the same general

pattern, including additional or excluding super-fluous information as is thought appropriate for thistext. At the appropriate stage in any course, stu-dents will be referred to the full Standard. Otherreaders should have recourse to their local pub-lic library or technical college/university library.Additional comment is given in italics.

Note that this Standard is no longer current but isincluded here as it is referred to in current editions ofthe Building Regulations.

This Standard is divided into two sections, Spec-ification and Appendices, preceded by a fore-word and listing of the ‘Cooperating Organisa-tions’.

The Specification includes: Introduction;Scope; References; Definitions; Materials; Form;Sizes; Information to be provided by the pur-chaser; Appearance; Classification and compres-sive strength; Marking; Manufacturer’s certifi-cate; Independent tests; Samples for tests.

The Appendices are taken up with a detaileddescription of the tests to which samples ofbricks must be put in order to comply with theStandard. These form no part of this text butwould be studied under materials science andreplicated in the laboratory by students in a laterpart of their course.

Introduction. This stresses changes made sincethe previous Standard was written, in particu-lar the necessity of using the strength of brickstaken when they are wet since this is the lowervalue. Also highlighted is the omission of a wa-ter absorption test since for these bricks it bearsno relationship to durability nor is it an easy in-dicator of weather resistance as rain penetrationis more likely through the mortar.

Scope emphasises that the Standard covers onlycalcium silicate bricks laid on mortar and to acoordinating size of 225 × 112.5 × 75.

Definitions important to the student at this stageare similar to BS 3921: frog, solid brick, cavity,cellular brick, frogged brick.

Materials for the manufacture of these bricksinclude sand of siliceous gravel or crushedsiliceous gravel or rock with a lesser proportionof lime. The mixture is pressed into shape andautoclaved (subjected to steam and pressure).Suitable pigments may be added.

Form or Type. The bricks may be solid orfrogged. See BS 3921 for a description.

Sizes. Coordinating and work sizes similar tothose for clay bricks, BS 3921. Note that sam-ple size for checking the dimensions is only10 bricks.

Information to be provided by the purchaser.The purchaser must state whether loadbear-ing or facing bricks are required. In additiona strength classification from Table 2 may bestated. If not stated then the lowest strength classfor the type and kind of brick may be inferredby the supplier.

Appearance. Bricks must be free from visiblecracks and noticeable balls of clay, loam or lime.Facing bricks shall be of the colour and tex-ture required by the purchaser. Arrises of bricksshould be relatively free of damage.

Classification and compressive strength. Thereare five classes of bricks given in Table 2, Class 7,6, 5, 4 and 3, with compressive strengths rang-ing from 48.5 N/mm2 down to 20.5 N/mm2. Afurther class, Class 2 for Facing brick and com-mon brick, has no minimum required compres-sive strength listed.

Marking. This refers to the data which must besupplied by the manufacturer with a consign-ment of bricks and includes:

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Appendix L 359

� Manufacturer’s identification� The BS reference� The type of brick, i.e. solid or frogged, and the

work size� The strength class as given in Table 2. This can

be indicated by colour coding: 7 green, 6 blue,5 yellow, 4 red, and 3 black. Class 2 has nocolour code.

Manufacturer’s Certificate. The purchaser can

request a certificate of compliance with this Stan-dard.

Independent tests. Again an option for the pur-chaser to agree with the supplier. Both the testsrequired and the laboratory carrying out thework must be mutually agreed.

Samples for test. Gives the number of bricks re-quired for testing, and the method by which thesample is taken.

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Airbricks and gratings for wall ventilation


pattern including additional or excluding superflu-ous information as is thought appropriate for thistext. At the appropriate stage in any course, stu-dents will be referred to the full Standard. Otherreaders should have recourse to their local pub-lic library or technical college/university library.Additional comment is given in italics.

The Standard comprises a Specification in eightparts including tables and figures, followed bytwo annexes. The eight parts are: Scope; Ref-erences; Definitions; Materials; Manufacturer’scertificate; Marking; Airbricks for external use(Class 1 units); Wall ventilators or gratings forinternal use (Class 2 units)

Scope is more concerned with what is not cov-ered, i.e. durability of air bricks or gratings (otherthan colour change in plastic materials), fireproperties of thermoplastic materials and com-pressive strength of thermoplastic units.

References to other British Standards or otherpublications.

Definitions. An airbrick is defined as a unitwhich is built into a wall, i.e. it actually displacesa brick or block or a part of one of these. A wallventilator or grating is defined as being eitherbuilt into or fixed to internal walling, usually inconjunction with an airbrick. The wall hold ofthe brick or grating is defined as the area in con-tact with the wall.

Materials. The list of materials includes a note ofthe test(s) with which the materials must comply,these being from the British Standards for these

Minimum clearCoordinating size Work size air space∗

225 × 75 215 × 65 One brick long and one course high 1290 mm2

225 × 150 215 × 140 One brick long and two courses high 2580 mm2

225 × 225 215 × 215 One brick long and three courses high 4500 mm2

∗Minimum clear air space refers to the aggregate of the area of all the holes or slots in the brick. It is not the areaused when determining the number of air bricks required in a wall by the Building Regulations.

materials. There is no need to refer to these otherstandards at this time. The list is:� Concrete� Clay, fireclay or terracotta� Thermoplastics� Copper or copper alloy� Aluminium� Plaster (internal use only)� Steel, carbon steel sheet or plate, which must

be galvanised or plastic coated or stainlesssteel.

Manufacturer’s certificate. Must be producedon request by purchaser and to the effect thatthe product complies with this Standard in allrespects.

Marking. Airbricks should be marked with ‘BS493 Class 1’ and gratings with ‘BS 493 Class 2’.

Airbricks for external use (class 1 units). Thispart covers the ‘design’ of the airbrick, i.e. thehole or slot made in the brick for the passageof air can be circular, square, rectangular or of alouvred design. Reference is made to Figures 1,2 and 3. Ten coordinating and work sizes aregiven, three of which are designed for use withstandard metric bricks (see BS 3921). These threesizes are as listed below.

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Appendix L 361

Other sizes listed can be used in blockwork orwith modular brick walls.

Wall ventilators or gratings for internal use(class 2 units). This part repeats the referenceto the design of the apertures as for ‘airbricks’.

The Annexes cover the information to be sup-plied by a purchaser when ordering, and detailsof an impact test which airbricks and ventilatorgratings must pass in order to comply with thisStandard.

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Materials for damp-proof courses



Note that this Standard is partly superseded but re-mains current and it is referred to in current editionsof the Building Regulations.

This Standard comprises a brief Specificationfor a number of materials used for the manufac-ture of damp-proof courses. This specificationis preceded by a foreword and a list of cooper-ating organisations. Early editions of the standarddid have appendices but these have been deleted in thecurrent edition, tests etc. on materials being the sub-ject of individual Standards. Reference to these Stan-dards is not required for this text.

The sections in the Specification are:Scope, Lead, Copper, Bitumen, Mastic Asphalt,Polyethylene, Slates, Bricks, Mortar, Designa-tion for ordering.

Scope simply lists the materials already givenabove.

Lead was traditionally described with reference to itsmass/unit area but is now described with a referencecode. For a damp-proof course, the lead shouldweigh not less than 19.5 kg/m2. This is equiva-lent to 1.80 mm thick and has the reference ‘Code4’. Code 4 may seem a strange reference but was thelead industry’s way round changing the thickness oftheir products when the building industry changed tothe metric system of measurement Prior to that timelead was described, for example, as ‘4 pound lead’, i.e.weighing 4 pounds per square foot; 4, 5 and 6 poundlead were common ‘sizes’ used by plumbers for roofwork and in DPCs. After metrication, the industrycould no longer refer to pound avoirdupois or feet andinches so ‘4 pound lead’ became Code 4, ‘5 poundlead’ became Code 5 and so on. The reference to BS1178 is for milled sheet lead for building pur-poses. There is no need to refer to that Standardat this time.

Copper. Generally an ‘annealed’ material is re-quired. When ductile metals are rolled out to formsheet or strip, the metal becomes brittle or ‘workhardened’. This is the principle behind continuouslybending a piece of wire to get it to break. To softenor ‘anneal’ the metal it is heated to a cherry red andallowed to cool slowly. There is no need to refer toBS 2870 at this time.

Bitumen. A short but detailed specification ofthe technical requirements of a DPC materialmade by impregnating a ‘base’ material, e.g. hes-sian, with bitumen. There is no need to refer tothe materials Standards at this time.

Mastic asphalt. Two types of mastic asphalt aregiven:

� Limestone aggregate mastic asphalt� Natural rock asphalt aggregate.

There is no need to refer to the materials Stan-dards at this time. Sufficient to know that the ma-terial is supplied in large hardened lumps or ‘cakes’of around 25 kg and melted in a ‘pot’, usually heatedwith LPG, and spread while molten. It hardens aftera short time but still has a measure of plasticity de-pending on the ambient temperature. It is possible toget different grades of the material made from arti-ficial bitumens or modified natural bitumens whichwill remain relatively stable in higher/lower temper-atures. For DPC work it is spread in one coat with athickness of 13 mm.

Polyethylene. A plastic material, thin and verytough. There is no need to refer to the materialsStandard at this time.

Slates used as a DPC must be at least 230 mmlong and at least 4 mm thick. Two courses mustbe laid, the full width of the wall and to break

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Appendix L 363

bond. The mortar used must be 1:0–3/4:3, cement,lime, sand. There is no need to refer to the ma-terials Standards at this time

Bricks used as a DPC must comply with BS 3921,clay damp-proof course bricks. Two coursesmust be laid, the full width of the wall and tobreak bond. The mortar used must be 1:0–3/4:3,cement/lime/sand. There is no need to refer tothe materials Standards at this time.

Mortar. Reference is made here to the Standardsfor the materials in the mortar used for slate andbrick DPCs.

Designation for ordering. When ordering, thefollowing should be given:

� British Standard number� Material to be used� Width, length, weight or number as appropri-

ate.

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British Standard 1199 and British Standard 1200

Building sands from natural sources



Note that this Standard, although superseded, is re-maining current as it is referred to in current editionsof the Building Regulations.

This Standard comprises Specifications for:� Sands for external renderings and internal

plastering with lime and Portland cement –BS 1199

� Sands for mortar for plain and reinforcedbrickwork, block walling and masonry –BS 1200.The sections in both Specifications are: In-

troduction, Scope, References, Definitions, Sam-pling and Testing, Quality of sands, Grading,Supplier’s Certificate and cost of tests, Addi-tional information to be furnished by supplier,Table 1 – Sands for as appropriate.

BS 1199 – Sands for external renderings andinternal plastering with lime and Portlandcement

Introduction. This emphasises that the gradingof sands is based on the process of washing, de-cantation and dry sieving; the determination ofclay and silt content is now part of the gradingprocess; two grading ranges of sand are given.Either grade can be used for rendering; however,where there is a choice, the coarser grade shouldbe selected as this leads to lower water/cementratios (fine sands require higher water/cement ra-tios) leading in turn to lower rates of shrinkageon drying (and therefore less cracking/crazing ondrying).

Scope. This part of the Standard refers to theuse of mixtures containing naturally occurringsands, crushed stone sands and crushed gravelsands with lime, lime and cement, lime withgypsum, and cement as binders.

References. A list of appropriate Standards isreferred to at the end of the Standard relating totesting equipment and related materials.

Definitions. Definitions are given for sand, natu-ral sand, crushed stone sand and crushed gravelsand. Sand is defined as any material whichmainly passes a 5.00 mm BS test sieve.

Sampling and testing. BS 812 gives the detailsof how the sampling and testing of sands is tobe carried out. The results are obtained by av-eraging the individual results from two tests oftwo different samples. There is no need to refer toBS 812 at the present time.

Quality of sands. Basically sands can be from asingle source or a mixture of any two or all three.They must be free from deleterious material ei-ther in the mix or adhering to the sand particles.Deleterious materials include iron pyrites, salts(particularly chlorides and sulphates), coal or otherorganic impurities, mica, clay silt, shale or otherlaminated materials or flaky or elongated parti-cles in form or quantity which could affect thestrength, durability or appearance of the mortaror of any materials in contact with it.

Very important – The various sizes of particlesin the sand must be evenly distributed through-out the mass of sand.

Grading. Two grades, A and B, are defined inTable 1, and sand should fall within the crite-ria given in the table when tested according toBS 812 – see Sampling and testing above. It isworth noting that the largest sieve size throughwhich all particles for both grades must pass is6.3 mm, and that about 2% is retained on the nextsize, which is the 5.00 mm sieve. See Definitionsabove.

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Appendix L 365

At this stage in the student’s progress there is noneed for further detail on the gradings; however, anexplanation of what grading is and what its effectis on renders etc. is appropriate. If one could imag-ine a loose material made from spheres which are allthe same size – say 20 mm in diameter – and it wasto be bound together with a powder suspended inwater where the binder particles were measured inmicrons, then two possible scenarios arise. The wa-ter/binder paste will fill the voids between the largespheres or the paste will merely coat the spheres. Inboth cases the amount of binder used per unit of fi-nal material produced will be very high, much higherthan if the material had fewer/smaller spaces betweenthe spheres. Also, lots of spaces left will produce amaterial which is weak in compression and adhesion.Filling excessively large spaces while being strong incompression will result in a brittle material.

To obtain a reduction in space and an improvementin performance we must therefore use some smallerspheres to fill the spaces up. In fact it would helpif the smaller pieces were not spheres at all but lit-tle cubes with concave faces which fitted between thespheres exactly – always assuming we could align thelittle cubes in one particular way every time. Noneof that is possible. We generally use naturally occur-ring or randomly produced materials as aggregatesor ‘filler’ in mortars, renders (and concretes), so wemust arrange for the reduction in free space by othermeans. This is done by producing a balance of theproportion of any one particle size in the aggregateused. The sizes and proportions are measured by de-termining the quantity of material passing througha set of sieves, each with apertures in a progres-sively smaller fixed sequence. If the quantities re-tained on each sieve fall within predefined limits, thenthe sand can be classed as Type A or B or simply ‘notcompliant’.

Old specifications and bills of quantities may referto ‘Zones’, which was the terminology used in previ-ous Standards.

Supplier’s certificate and cost of tests. The sup-plier of the sand is responsible for providing atest certificate to the purchaser if requested to doso and is always responsible for ensuring thatthe supply conforms at all times with the appro-priate parts of the Standard. The costs of testsrequested by a purchaser are paid by him if the

material does comply with the Standard and bythe supplier if it does not comply.

Additional information to be furnished by thesupplier. The following must be given by thesupplier if requested by the purchaser: source ofsupply – county, parish, name of quarry or pitand for dredged material the precise location onthe river or estuary; group classification accord-ing to BS 812; external characteristics – shape andsurface texture of the particles according to BS812; physical properties – relative density, bulkdensity in kg/m3, water absorption, grading bysieve analysis according to BS 812.

BS 1200 – Sands for mortars, plain and rein-forced brickwork, block walling and masonry

Introduction. See comment for BS 1199 above.

Scope. This part of the Standard refers to the useof mortars containing naturally occurring sands,crushed stone sands and crushed gravel sandswith binders for building plain and reinforcedbrickwork and blockwork of clay or concreteunits.

References. See comment for BS 1199 above.

Sampling and testing. See comment for BS 1199above.

Quality of sands. See comment for BS 1199above.

Grading. Two grades, S and G, are defined inTable 1 and sand should fall within the criteriagiven in the table when tested according to BS812. It is worth noting that the largest sieve sizethrough which all particles for both grades mustpass is 6.3 mm and that about 2% is retained onthe next size, which is the 5.00 mm sieve. See Defi-nitions above. It should also be noted that the pro-portions retained on each sieve are very differentfrom BS 1199, reflecting the differing needs withregard to mortars and renders or plasters. Theset of sieves used is identical and the method oftest is still BS 812.

Supplier’s certificate and cost of tests. See com-ment for BS 1199 above.

Additional information to be furnished by thesupplier. See comment for BS 1199 above.

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Guide to the choice, use and application of wood preservatives




This Standard comprises a Guide for a numberof materials used for the preservation of tim-ber and timber-based products and components.This Guide is preceded by a foreword and a listof cooperating organisations. As this Standard isonly a Guide and not a specification one cannot an-notate drawings and details with the injunction thatpreservative treatment will ‘comply’ with BS 1282.

The sections in the Guide are: Scope, Ref-erences, General, Wood-destroying organismsin the United Kingdom, Wood-destroying or-ganisms abroad, Classification and descriptionof wood preservatives, Specification for woodpreservatives, Preservatives without publishedspecifications, Hazards to health, Preparation oftimber for treatment, Methods of treatment, Se-lection of appropriate preservative treatments,Codes and Standards for the preservative treat-ment of specific timber products, Checking thestandard of preservative treatment.

The Guide also includes two Tables:Table 1 – Published preservative specificationsTable 2 – Recommended codes and standardsfor the preservative treatment of specific timberproducts.

Scope reiterates that the Standard gives guid-ance only on selection of appropriate methodsof treatment, preservatives to use and how theseare affected by conditions in which the timber isused, etc. It contains references to other Stan-dards and codes of practice where additional in-formation can be found on the treatment of ma-terials such as plywood and components suchas windows and window frames.

References. The Standard refers to many otherStandards and codes of practice and these arelisted on the rear cover of this Standard.

General. Destruction of wood takes place nat-urally through attack by fungi, insects, marineborers and some bacteria – all described in laterparts of the Guide. Two ways to deal with suchattack:

� Select a naturally resistant timber species – notalways possible or always economical

� Treat a suitable, and generally cheaper, timberto resist the expected attack.

Two basic factors affect the efficacy of the treat-ment:

� the properties and the efficiency of the preser-vative

� the method by which it is applied.

Wood-destroying organisms in the UnitedKingdom. Fungi – the ‘wet’ and ‘dry’ rots re-quire a moisture content in excess of around 22%.Some timbers such as oak, teak and western redcedar are very much more resistant to such at-tack even at high moisture contents. Others, suchas beech and all sapwood of other species, arevery susceptible to attack.

Insects – the most common throughout the UKis the common furniture beetle, Anobium punc-tatum, which attacks the sapwood of timbers.Death watch beetle, powder post beetle and thehouse longhorn beetle also occur but are not sowidespread.

Marine borers are not of interest in terms ofthis text.

Wood-destroying organisms abroad are not ofinterest in terms of this text.

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Appendix L 367

Classification and description of wood preser-vatives. There are three main types: the tar oiltype, Type TO; the organic solvent type, TypeOS; and the water-borne type, Type WB.

These all have advantages and disadvantages andthe descriptions of each which follow attempt to high-light the more important features but of necessity aremuch condensed versions of the Standard, althoughsufficient for the student at this stage.

Type TO preservatives are obtained by the de-structive distillation of coal and are in effect coaltar oils – creosote. Their chemistry is very com-plex. Some features are:

� Suitable for exterior work� Their odour generally makes them unsuitable

for interior use� Resistant to leaching� Degree of water ‘repellency’� Non-corrosive to metals� Help protect iron and steel� Not readily flammable� Cannot normally be painted over� Can be glued provided the surface is free of

creosote� Porous material in contact can ‘wick’ out the

creosote� Odour can be picked up by other materials

and foodstuffs in the vicinity – not necessarilyin contact with the timber.

� Timber which has been pressure treated can‘bleed’ in hot weather.

Type OS preservatives come as a basic materialand in two further variants. The basic materialis a solution of one or more organic fungicidesand sometimes an insecticide in an organic sol-vent, usually a petroleum distillate such as whitespirit. The active ingredients are copper naph-thenate, zinc naphthenate, pentachlorophenol,pentachlorophenyl laurate, orthophenyl phenol,tributylin oxide, chlorinated naphthalene, dield-rin and gamma-BHC. Some features are:

� Suitable for exterior and interior use� Most are resistant to leaching but some loss

may occur due to evaporation. Generally notcorrosive to metals but copper compoundsshould not be used where there will be incontact with aluminium or aluminium alloys,plastics or rubber

� Treatment leaves the timber with a clean ap-pearance

� Once the solvent has dried off timber can beglued and painted

� No raising of the grain or swelling of timber socan be used on accurately machined timbers

� Once the flammable solvent has evaporated,the timber is no more flammable than un-treated timber

� May taint nearby foodstuffs.

The first variant will additionally contain a waxor a resin as a water repellent, which can makethe timber more difficult to glue or paint over.The second variant consists of the preservativewith a water repellent and a pigment eitherto enhance or maintain the appearance of thetimber.

Type WB preservatives are a solution ofinorganic preservative salts and a ‘fixing’agent, usually a dichromate all in water. Themost common solution contains copper sul-phate, sodium dichromate and arsenic pentox-ide. Other mixtures are: copper sulphate andpotassium dichromate; sodium fluoride, potas-sium dichromate, sodium arsenate and dinitro-phenol; potassium and ammonium bifluorides;disodium octoborate on its own. They feature:

� Copper/chrome and copper/chrome/arsenatesolutions undergo chemical change in thetimber and ‘lock’ themselves to the timberand so cannot leach out – they are suitable forexposed exterior use

� Other solutions leach out to some degree orother and so can only be used where protectedfrom moist conditions – painted, under cover,etc.

� Generally not corrosive to metals but solu-tions containing copper compound shouldnot be used where there will be contact withaluminium or aluminium alloys, plastics orrubber

� The preserved timber is usually clean, odour-less and can be glued and painted. No stainingof adjacent materials

� Non-inflammable� Treatment with water-borne preservatives

means the timber has to be dried out and willswell and shrink, grain will be raised and dis-tortion may take place.

Specification for wood preservatives. Refer-ence is made in Table 1 to the relevant BritishStandards and documentation supplied by

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the British Wood Preserving Association andgovernment departments.

Preservatives without published specifi-cations. Refers to new formulations beingmarketed by the industry and the need tospecify by ‘performance’.

Hazards to health. Warns that many of the com-pounds used are hazardous to the health ofanimals and humans, being toxic to fungi, in-sects and bacteria. Reference to various sourcesfor advice on handling and other precautionsto be taken for the safe use and applicationof the preservatives and the timber and timberproducts.

Preparation of timber for treatment. All barkto be removed. Timber should be seasoned ex-cept where treated by the diffusion process. Allcutting and machining, notching, boring, etc.should be done before treatment. Some timberssuch as Douglas Fir must be ‘incised’ (slit-likecuts 20 mm deep in the direction of the grain) toassist penetration of the preservative.

Methods of treatment. These are described inbroad detail in Appendix C and so only thosegiven in the Standard are listed here:

� Pressure impregnation� Low pressure impregnation� Vacuum impregnation, double vacuum pro-

cess

� Liquefied gas process� Hot and cold open tank process� Immersion, deluging, spraying and brushing� Diffusion process� Boucherie process*� Gewecke processes*� Oscillating pressure method*� Boulton process*

*Rarely used in the UK

Selection of appropriate preservative treat-ments. Factors affecting the choice are:

� The effectiveness of the preservative� Method of application� How the timber absorbs the preservative� The timber’s natural durability� Where the timber is to be used.

Codes and Standards for the preservative treat-ment of specific timber products. See Table 2.

Checking the standard of preservative treat-ment. The principal point for the ordinary useris the need to use a reputable supplier whois a member of the appropriate trade associ-ation, is licensed to provide the treatment oftimber and can give a certificate guaranteeingthat the treatment has been carried out properly.Larger organisations such as British Telecom canafford to employ inspectors to carry out suchwork.

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Appendix L 369


Clay bricks



Note that this Standard has been partly supersededbut is included here as it is referred to in current edi-tions of the Building Regulations.

This Standard is divided into two sections, Spec-ification and Appendices, preceded by a fore-word and a listing of the committees responsiblefor its production.

The Specification includes: Introduction;Scope; Definitions; Sizes; Dimensional devia-tions; Durability; Compressive strength; Waterabsorption; Sampling for tests; Marking.

The Appendices are taken up with a detaileddescription of the tests to which samples ofbricks must be put in order to comply with theStandard. These form no part of this text butwould be studied under materials science andreplicated in a laboratory by students in a laterpart of their courses.

Introduction. This stresses the importance ofthree features of clay bricks and which form partof the testing process for compliance with theStandard. They are water absorption, durability(frost resistance and soluble salt content) and toler-ance (the need to have the bricks within a uniformoverall size).

Scope repeats the listing of requirements in theIntroduction, adding that the Standard refers tobricks intended to be laid on a bed of mortar andonly to a ‘format’ of 225 × 125 × 75 mm (com-monly referred to as a ‘Standard Metric Brick’).

Definitions are given for compressive strength,water absorption, coordinating size and worksize. The first two refer to the testing proceduresin the appendix. The reference to sizes is:

� Coordinating size. The size of a coordinat-ing space allocated to a brick, including al-lowances for joints and tolerances.

� Work size. The size of a brick specified for itsmanufacture, to which its actual size shouldconform within specified permissible devia-tions.

Sizes. External sizes and void sizes are given; theformer are given in Table 1, the coordinating size(the space for a brick and its mortar) being givenas 225 × 112.5 × 75. The work size is given as215 × 102.5 × 65, derived from the coordinatingsize by deducting a nominal 10 mm thickness ofmortar.

Void sizes are given as a percentage of the ex-ternal volume of the brick and divide bricks upinto four ‘types’:

� Solid bricks have no holes, cavities or depres-sions. (Cavities are described as hole closed atone end. A depression would be a ‘frog’)

� Cellular bricks shall not have holes but onlycavities or frogs exceeding 20% of the externalvolume of the brick

� Perforated bricks have holes, the area of eachnot more than 10% of the area of the brick. Thethickness of material left across the width ofthe brick must not be less than 30% in total.The volume of the holes shall not total morethan 25% of the external volume of the brick

� Frogged bricks have only cavities or depres-sions on one or two beds, which should notexceed 20% of the external volume of the brick.

Dimensional deviations. The ‘Limits of Size’are given in Table 2 for a sample of 24 bricks.

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On the length, the difference between the maxi-mum and minimum overall sizes is 150 mm. Dis-tributed evenly over all 24 bricks, this approxi-mates to 6 mm per brick. Such a large differencecannot be taken up simply by varying the thicknessof the mortar joints but must be allowed for when set-ting out the brickwork and the bonding for the struc-ture.

Durability of brickwork depends on two factorswhich arise from the use of any particular brick:resistance to frost and the soluble salts content.Frost resistance falls into three classes: Frost re-sistant (F), Moderately Frost Resistant (M) andNot Frost Resistant (O). Soluble salts content isclassed as either Low (L) or Normal (N).

So, one could have a brick which is frost resis-tant with normal soluble salt content and thiswould be classified as FN. Similarly, a brickwhich had no frost resistance and had low solu-ble salt content would be classed as OL.

Compressive strengths for five ‘kinds’ of bricksare listed in Table 4. The ‘kinds’ are given asEngineering A, Engineering B, Damp-proofcourse 1, Damp-proof course 2 and ‘All others’.

Water absorption as a percentage by mass isgiven in Table 4 for each of the five ‘kinds’ ofbrick.

Taking samples of bricks for the test given inthe appendix is described in detail. The numberrequired for each test is given in Table 5.

Marking. This refers to the data which must besupplied by the manufacturer with a consign-ment of bricks and includes:

� Manufacturer’s identification� The BS reference� The type of brick, i.e. solid, cellular, perfor-

ated, frogged� The name of the brick, e.g. sand faced buff.

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Appendix L 371


Carbon steel rods and bars for the reinforcement of concrete



The standard has two main parts, a Specifica-tion and Appendices, both preceded by a shortforeword and a list of the responsible Commit-tees.

Specification. It is important for the student torealise that the Standard covers bars which can bepurely circular in cross-section or which can be ‘de-formed’, i.e. the general cross-section is (1) circularwith projections formed on the surface of the bar or(2) square section bars which are twisted and thecorners are left plain or chamfered, the purpose of1 and 2 being to improve the ‘grip’ in the concrete.The steel used for plain round bar is 250 Grade andfor deformed bar is 460 Grade, this being termedhigh yield steel. These grade numbers refer to thetensile strength of the steel. Grade 460 for exam-ple has a tensile strength of 460 N/mm2. Deformedbars can be produced by either hot rolling or coldworking.

A number of tables are included in the Speci-fication. The most useful of these to the studentat this early stage is Table 2, Cross-sectional areaand mass of bars of nominal diameter 6, 8, 10,12, 16, 20, 25, 32, 40 and 50 mm. Note that therange 8 to 40 mm are the ‘preferred sizes’ andare those readily available from the manufactur-ers. See also ‘Sizes’ below.

Also included are sections on Scope; Defi-nitions; Sizes; Cross-sectional area and mass;Length; Steel-making process; Chemical com-position; Bond classification of deformed bars;Routine inspection and testing; Mechanicalproperties; Fatigue properties of deformed bar;

Retests; Verification of characteristic strength;Marking.

Scope. Much of the important information fromscope has been explained above.

Sizes. Table 1 gives the preferred sizes for barsof 250 Grade steel in the range 8 to 16 mm andfor 460 Grade steel in the range 8 to 40 mm. Tol-erances are also given.

Cross-sectional area and mass. Reference has al-ready been made to Table 2 included in this sec-tion.

Length. This section sets out tolerances on cutlengths of bar.

Steel-making process. Refining molten iron ei-ther in a top-blown basic oxygen converter or bymelting in a basic-lined electric arc furnace.

Chemical composition. Well beyond the scopeof this text.

Bond classification of deformed bars. Gener-ally square twisted bars are classified as Type 1and bars with proturbences on the surface asType 2.

Marking. Bars should carry ‘rolled-on’ legiblemarks no further apart than 1.50 m to indicatethe origin of manufacture, i.e. a maker’s mark.

The remaining sections in the specification aretaken up with the provision of a number of tests,and refer to the appendices, where the tests aredescribed in detail.

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Sizes of sawn and processed softwood




This Standard comprises a brief Specificationand an Appendix dealing with ‘Surfaced soft-woods of North American Origin’ (i.e. fromCanada and the USA). This specification is pre-ceded by a foreword and a list of cooperatingorganisations.

The Specification comprises sections onScope; Definitions; Moisture Content; Sizes andmaximum permitted deviations; Maximum per-mitted reductions from basic sizes by planing.

Scope. It is important to note that this Stan-dard does not apply to softwood trim, softwoodflooring and softwood for structural purposes.This does not mean that timber complying with thisstandard cannot be used for trim (See Table C below)or structural purposes, and flooring board could be,but it is very unlikely, machined from it. The soft-wood trims given in BS 1186 Part III are of limiteduse. They do cover trims with simple splays and pen-cil rounds but do not include trims with torus orogee mouldings common in better class work. Thesewould be machined from softwood covered in thisStandard. If the softwood specified here was to be usedfor structural purposes it is likely that ordinary ‘rulesof thumb’ would be applied, whereas if the structurewas the subject of detailed design, then timber com-plying with BS EN 336 would be appropriate.

Definitions. Students should familiarise them-selves with the definitions for parcel, planing,moisture content, regularising. Particular noteshould be made that moisture content is calcu-lated as the mass of water against dry mass oftimber.

Moisture content. Measurement is to be carriedout using an electric moisture meter. This instru-ment consists of a pair of metal probes insulated alongtheir length, leaving only the sharpened tips bare. Theprobes are driven into the timber and the electrical re-sistance of the timber between the ends of the probesis measured. The resistance is taken to be a functionof the moisture contained in the timber.

Sizes and maximum permitted deviations. Ba-sic cross-sectional dimensions are given in Ta-ble A and basic lengths in Table B, both givenbelow. These tables include annotation on thecurrent economic effect of choosing a particularsize and length of softwood.

The maximum allowable deviations dependon which size is being measured: in cross-sectionsizes not exceeding 100 mm −1 mm, +3mm;and over 100 mm, −2 mm +6 mm. And onlength, there is no limit to oversize but undersizeis not permitted. Timber is frequently sawn alongits length – ‘rip sawing or ripping’– to give smallercross-sections. This practice is called re-sawing. Ob-viously the sawdust removed means a reduction inthe sizes of the pieces obtained. The ‘re-sawing’ al-lowance on each piece obtained is a maximum reduc-tion of 2 mm. For example, a 100 × 25 board is re-duced in width by re-sawing. We cannot obtain twopieces 50 × 25 because of the material lost in the sawcut but each piece must not be less than 48 × 25 .Actual sizes of timber vary with moisture con-tent, particularly the cross-sectional dimensions. Allsizes are taken to be at 20% MC. The allowanceto be made at different MCs is given in theStandard.

Maximum permitted reductions from basicsizes by planing. By planing is meant the

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Appendix L 373

machining of the opposite faces, generally with rotaryknives, to bring the surfaces to a smooth, even finishand the timbers to the same overall dimensions. Thesurfaces will only require sanding before a decorativefinish is applied – paint, varnish, stain, etc.

Table A Common cross-sections with availability and relative cost.

Width

Thickness 75 100 125 150 175 200 225 250 30016 R 40 R 48 R 60 S 7819 R 40 R 48 R 60 G 7822 R 48 G 54 R 70 G 82 S 101 S 120 S 13925 S 48 S 63 R 85 R 99 S 120 S 140 S 160 S 190 S 22032 S 62 S 87 R 110 R 140 S 170 S 200 S 220 S 260 S 30036 S 62 S 87 R 110 R 14038 S 62 G 87 R 110 R 140 S 170 S 200 S 220 S 260 S 30044 S 81 S 106 S 132 S 159 S 185 S 212 S 238 S 270 S 34047 G 81 G 106 G 132 G 159 G 185 G 212 G 238 G 270 S 34050 S 106 S 125 S 150 S 175 S 200 S 225 S 250 S 300 S 35063 S 125 S 150 S 175 S 200 S 225 S 250 S 300 S 35075 G 180 S 230 G 280 S 330 G 380 G 430 S 500 S 580

100 S 250 G 400 G 580 G 690 S 750 S 830150 S S S200 S250 S300 S

R – Re-sawn as requiredG – Generally held in stockS – Seldom held in stock, ordered if necessary123 – Cost per metre in pence at first quarter 2004, local delivery free, no price shown – subject to quotation forbest cost at the time

Table B Common lengths showing availability and effect on cost.

1.80 2.10 3.00 4.20 5.10 6.00 7.20W DL W DL G N G N G N G N G DL

2.40 3.30 4.50 5.40 6.30W DL G N G N G N G DL2.70 3.60 4.80 5.70 6.60

W DL G N G N G N G DL3.90 6.90G N G DL

W – Sawn from longer lengths at an additional chargeG – Generally held in stockN – Normal costDL – More expensive because of length

This reduction in overall size is controlledwithin the limits set out in Table C and de-pends on the end use of the timber and the ba-sic sawn dimensions. The reductions vary from4 to 13 mm and are inclusive of any ‘re-sawing’

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Table C Maximum permitted reductions from basic sawn sizes to finished sizes by planing two opposingfaces (mm).

Basic sizes

15 up to and Over 35 and Over 100 andApplications including 35 including 100 including 150 Over 150

Matching and interlocking boards 4 4 6 6Wood trim other than specified in BS 1186 Part 3 5 7 7 9Joinery and cabinet work 7 9 11 13

allowance described above. The variation in sizeafter planing shall be −0 mm, +1 mm.

Note that this is the overall reduction in the cross-sectional dimension, e.g. a 50 × 50 sawn timber isplaned for wood trim and becomes 46 × 46 overall.

Match and interlocking boards generally havesome form of joint such as a tongue and groove ma-chined on the edge. The allowances above do not in-clude any reduction in the dimensions caused by themachining of the joint.

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Appendix L 375


Welded steel fabric reinforcement



This Standard is divided into two sections, Spec-ification and Appendices, both preceded by ashort foreword and list of Committees responsi-ble for the Standard.

The Specification is further divided intoa number of paragraphs: Scope; Definitions;Information to be supplied by the purchaser; Di-mensions; Cross-sectional areas and mass; Pro-cess of manufacture; Quality; Fabric classifica-tion; Tolerances on mass, dimensions and pitch;Bond classification of fabric; Testing and manu-facturer’s certificate; Mechanical tests; Retest.

The Appendices detail the mechanical testsand the notation and classification of the fabric.

Scope. The Standard covers fabric or mesh madefrom plain or deformed wire, rod or bar com-plying with BS 4449, BS 4461 or BS 4482 for thereinforcement of concrete.

Definitions are given for all terms used indescribing fabric reinforcement. All terms areused in accordance with their common every-day meaning and so will not be repeated in thisprecis.

Information to be supplied by the purchaser.

� The number of this Standard� The number of the Standard referring to the

wire quality (see Scope)� Whether the wire is plain or deformed� The wire sizes and mesh arrangement re-

quired� The dimensions of each sheet required� The quantity of each sheet required.

Dimensions. The dimensions of the individ-ual wires, rods or bars shall comply with the

appropriate standards listed above in Scope.While it is possible to have made up any com-bination of wire in any mesh size and configu-ration, it is more usual for the designer to spec-ify from the preferred range of designated meshtypes and sheet size listed in Table 1, from whichthe following information has been extracted.The stock sheet size is 4800 long × 2400 widegiving an area of 11.52 m2. The nominal size ofthe wires ranges from 2.5 mm to 12 mm dia-meter.

Cross-sectional areas and mass. The meshpitch range is 100 × 100, 100 × 200, 200 × 200and 100 × 400. Each combination of mesh pitchand wire size given in Table 1 has a unique refer-ence. Square meshes are designated A followedby a number(s); Structural meshes (100 × 200pitch) are designated B followed by a number(s);Long meshes (100 × 400) are designated C fol-lowed by a number(s); and Wrapping meshes(200 × 200 and 100 × 100 pitch) are designatedD followed by a number(s).

The cross-sectional area of the wires in eachdirection is given as the mass in kilograms persquare metre. Both of these latter pieces of informa-tion are used in detail reinforcing calculations and incosting by the builder or quantity surveyor.

Process of manufacture. The fabric is machine-made under factory controlled conditions, thewires being electrical resistance welded at everycrossing.

Quality. Wire, rod or bar must be of Grade 460steel except for wrapping mesh, which may beof Grade 250. These grade numbers refer to the ten-sile strength of the steel. Grade 460 for example hasa tensile strength of 46 N/mm2.

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Wires etc. may be butt welded in their lengthand broken welds not exceeding 4% of the totalin any sheet are permissible; however, these bro-ken welds should not exceed half the number ofwelds along the length of any one wire.

Fabric classification. If the designer does notwish to use one of the meshes listed in Table 1,this paragraph refers to the correct way to des-ignate the required fabric to the manufacturer.Appendix B explains how the designer should specifya specially designed fabric and also how to draw upa schedule of fabric reinforcement for concrete mem-bers. An understanding of the latter is importantlater in QS courses when measuring reinforcementfor bills of quantities and for costing purposes.

Tolerances on mass, dimension and pitch referto the maximum permissible deviation from themass per square metre of fabric, the maximumpermissible deviation on the length of the wiresand the maximum permissible deviation in thesize of the spacing of the wires.

Bond classification of fabric refers to the weldquality of the intersections of the wires. Reallyonly of interest to designers of reinforced concretestructures.

Testing, Mechanical test and Retests all refer tothe testing required by a fabric in order to com-ply with the requirements of the Standard, thetests being fully described in the appendices.

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Appendix L 377


Bitumen damp-proof courses for masonry



This Standard comprises a Specification fordamp-proof course material made from wovenor felted, fibrous material impregnated with bi-tumen. This specification is preceded by a fore-word and a list of cooperating organisations.Appendix A gives details of tests and testingprocedures. Appendix B includes a table listingthe recommended uses for various types andclasses of DPC; Appendix C covers high bondstrength bituminous DPCs, which are beyondthe scope of this text. This standard supersedesthe reference to bituminous DPCs in BS 743.

The sections in the Specification are: Scope,Definitions, Classification, Base materials, Bitu-minous materials and fillers, Assembly of damp-proof course, Marking and packaging.

Scope gives the all important information thatthe classification of these DPCs is by ‘base ma-terial’.

Definitions. Only one is given: nominal massper unit area, defined as ‘A numerical designa-tion of the mass per unit area, which is a conve-nient round number approximately equal to theactual mass per unit area expressed in kg/m2’.

Classification. Classification is given in columns1 and 2 of Table 1. There are six classes, A toF. Classes A to C are hessian-based, fibre-basedand asbestos fibre-based respectively. Classes Dto F are similar to these but with the inclusion ofa layer of lead.

Base materials. The hessian base is to be of asingle layer of woven jute fibre. The fibre base isto be of one or more absorbent sheets of mixed,felted animal and vegetable fibre. The asbestosbase is to be of an absorbent sheet containing

not less than 80% asbestos fibre. Presumably theremaining fibre can be of animal and/or vegetable ori-gin.

Bituminous materials and fillers. Reference ismade to minimum quantities of bitumen whichshould be used to saturate the base material, andhow this is to be tested and measured.

Two types of bituminous material are used:one for saturating the base and the other for coat-ing both sides of the saturated base. The secondtype of bitumen is mixed with a very finely di-vided mineral – talc, mica, etc. Once the coatingis applied it must be coated with a layer of min-eral dust to prevent the material from stickingto itself when rolled up for shipment. This ma-terial need not be so finely divided as the fillermaterial and can be made from sand, mica, slate,etc.

Assembly of damp-proof course. ‘The baseshall be impregnated completely with saturat-ing material. Any surplus saturant shall be re-moved, after which the coating material shall beapplied. When a lead sheet is included, this shallbe laminated with the base and the two sheetsshall be covered on both sides with coating ma-terial’.

Marking and packaging. The material is packedin rolls of at least 8 m length, each roll markedwith the BS number and the classification letterA to F.

Classification of bitumen damp-proof courses.Table 2 of Appendix B gives a table of situationsfor which bitumen DPCs are both suitable andunsuitable. In the context of this text, all classesare suitable for use where the compressive load

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Mass per unit areaClass Description of assembled DPC (kg/m2)

A Hessian base 3.8B Fibre base 3.3C Asbestos base 3.8D Hessian base laminated with lead 4.4E Fibre base laminated with lead 4.4F Asbestos base laminated with lead 4.9

is in the range 0.10 to 0.50 N/mm2. This meansbuildings up to four storeys in height. All classesare suitable for water movement in any direc-tion – up, down and horizontally. Where high

shear or flexural stresses are involved, these arenot suitable and reference should be made to themore stringent requirements for DPC laid downin Appendix C.

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Appendix L 379


Polyethylene damp-proof courses for masonry



This Standard comprises a brief Specificationof the material used for the manufacture ofpolyethylene damp-proof courses. This specifi-cation is preceded by a foreword and a list ofcooperating organisations. Appendices describea variety of tests which are beyond the scopeof this text. However, Appendix D includes atable of uses for polyethylene DPC and datafrom that table are summarised at the end of thisprecis.

The sections in the Specification are: Scope,Definitions, Composition, Thickness, Finish andimpermeability, Marking and packaging.

Scope lists the above subsections of the specifi-cation.

Definitions refers the reader to BS 6100, sec-tion 1.0.

Composition gives a technical description of thematerial forming the DPC. It is interesting to notethat to comply with the Standard, the DPC shouldcontain not less than 2% by mass of carbon black.This is mixed into the plastic as a finely divided pow-der, and so the DPC is black in colour. This impliesthat blue or green polyethylene DPCs do not comply

with this standard, although many rolls of such ma-terial are sold and used for that purpose.

Thickness. ‘Nine specimens . . . shall have a sin-gle layer of thickness not less than 0.46 mm.’

Finish and impermeability. The sheet shall befree from air bubbles and with no visible pin-holes. The test for the latter is outlined in Ap-pendix C and consists of viewing the sheetagainst a strong light. If the sheet is coloured black,pinholes would be readily seen in that test.

Marking and packaging. The material is packedin rolls of at least 8 m length, each roll markedwith the BS number and date.

Recommended uses for polyethylene damp-proof courses. Table 1 of Appendix D gives atable of situations for which these DPCs are bothsuitable and unsuitable. For the purposes of thepresent text, they are suitable for use with anycompressive load but not if there is any lateral,shear or flexural load or stress. They are suitablefor water movement upwards and horizontally,above ground level, but not for water movingdownwards such as at parapets and chimneysor in cavity trays.

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BY019-IND BY019-Fleming-v6.cls September 14, 2004 20:16

Index

blocks, 22autoclaved aerated concrete, 23concrete, 22

dense and lightweight, 23dimensions of standard metric block, 23materials, 22

blockwork substructure, 71bolts, 333

bulldog timber connectors, 333coach bolts, 333hexagonal-headed, 333self-locking nuts, 332

bonding, of bricks to form wallscommon bond, 9english bond, 10flemish bond, 10garden wall bond, 13quetta bond, 13rattrap bond, 14scotch bond, 13sectional bond, 6, 11, 12, 22stretcher bond, 9

brick, 2bricks, by function, 4brick and blockwork in superstructure, 81brick materials, 5brick sizes, 2

coordinating sizes, 3nominal sizing, 3

bricks and blocks standards and dimensions, 2British Standards, 356–79

air bricks and gratings for wall ventilation,360

bituminous damp proof courses for masonry,377

calcium silicate bricks, 358carbon steel rods and bars for reinforcing

concrete, 371clay bricks, 369general, 356materials for damp proof courses, 362polyethylene damp proof courses for masonry,

379sands for mortars, plasters and renders, 364sizes of sawn and processed timber, 372

welded steel fabric reinforcement, 375wood preservatives, 366

building masonry walls from foundation up toDPC level, 57

cavity fixings, 337, 338ceiling finishes, 124central heating, 252

emitters, 255piping for central heating systems, 253pressurised system, 254TVR, 255underfloor heating, 254

chemical anchors, 339coach screws, see screw nailscold bridging at wall openings, 132common and facing brickwork

bucket handle or grooved jointing or pointing,21

facing brickwork, 18flat jointing or pointing, 20flush jointing or pointing, 20keyed jointing or pointing, 21pointing and jointing, 19recessed jointing or pointing, 20reverse weather jointing or pointing, 21tuck pointing, 20

concrete, 316general,

formwork, 316in-situ concrete, 316moulds, 316plain concrete, 316precast concrete, 316reinforced concrete, 316shuttering, 316

materials and mixesaggregate

coarse, 316fine, 316grading, 316

concrete proportions, 317designed mixes, 318no-fines, 319prescribed mixes, ordinary, 318

380



Index 381

prescribed mixes, special, 318water, 317water/cement ratio, 317weigh batchers, 320

reinforcement, 318bent ends, 319cold rolled, 318deformed, 318fabric, 318grip, 319high yield steel, 318hooked ends, 319hot rolled, 318mild steel, 318rectangular mesh, 318round bar, 318square mesh, 318square twisted, 318

concrete screws, 336conservation of energy, 355contact adhesives, 340convention on thicknesses of walls, 8cutting bricks

bevelled closer, 8cutting bricks, 6half batt, 7king closer, 8quarter batt, 7queen closer, 8three-quarter batt, 8whole brick dimensions, 6

damp proof courses and membranes, 344–7materials, 345

dimensional stability of walls, 79dooks, 335door hanging, 190door ironmongery, 196door types,

15 pane doors, 190bound lining doors, 185fire resistant doors, 193flush panel doors, 187glazing, 196ledged and braced doors, 185panelled doors, 188pressed panel doors, 189smoke seals, 195

doors and windows,functions of doors and windows, 182

drawing symbols and conventions, 353–4dumpy level, 285durability of bricks, 3

earthwork support, 325electrical work, 266

accessories, 277circuits, radial, 271circuits, ring, 271flexible cord, 277fuses, 269IEE regulations, 266lamps, 277miniature circuit breakers, 270more on protective devices, 275phases, red, yellow and blue, 267power generation, 266residual current circuit breaker,

269sub-circuits, 270sub-mains and consumer control units,

268wiring diagrams, 276wiring installation types, 267work stages, 272

earth bonding, 273electrician’s roughing, 272final fix, 275testing and certification, 275

excavation, general, 34excavation, detail, 53

excavation for and placing concretefoundations, 53

marking out the excavation, 53poling boards, 325shoring, strutting & waling, 325steel sheet piling, 326steel trench support, 327strutting, 325timbering, 325timber sheet piling, 326walings, 325

fastenings and fixings, 328–40see also nails, screw nails and boltscavity fixings, 337, 338chemical anchors, 339concrete screws, 336contact adhesives, 340dooks, 335frame fixings, 338gravity toggle fasteners, 337plugs and plugging, 335rawlbolts, 336rawlnuts, 338rawlplugs, 335spring toggle fasteners, 337



382 Index

flat roofs in timber, 162insulation and vapour control layers, 164options for structure, 162voids and ventilation, 164

flitched beams, 108floor boarding

man-made board, laying, 313man-made board, material, 312timber, 312tongue and groove forms, 314

floor finishes, 124foundations, 37

see also substructuresframe fixings, 338frog up or frog down, 17

economics, 17

general principles of bonding, 21gravity toggle fasteners, 337ground floor construction, 59

detail drawings, 59gypsum wall board, 341–3

Ames taping, 343collated screws, 342dry lining screw bits, 342dry lining screws, 342paper faces, 341plasterboard nails, 342sheet sizes, 341square edged, 341taper edged, 341taping and jointing, 343

half brick thick walls, 16honeycomb brickwork, 16hung floors, 64

concrete floors, 67timber floor alternatives, 66timber floors, 64

insulation of external walls, 84internal partitions, 96

acoustic partition, 99foundations, 97, 98, 99stud partition details, 98

intersections of masonry walls, 9

levelling, 285booking readings, 288calculating levels from readings,

288collimation, 287dumpy level, 286

and staff, 287use of, 287

EPDM (electronic position and distancemeasuring) equipment, 290

setting to a level, 289stadia wire, 288

loadbearing and non-loadbearing internalpartitions, 96

maps and plans, 2791:1250, 1:2500 maps, 279Ordnance Survey, 279plans

1:500, 279details, 284house, 282scales, 280sections, 283

mortaradditives, 30cement, 25‘fat’ mixes, 28general rules for selection of mortar,

29joints, 3lime, 26mixing in additives, 30mixing mortar, 31sand, 27water, 27which mortar mix?, 27whys and wherefores of mortar, 25

nailsboat, 329collated, 330copper tacks, 330cut, 328Hilti gun, 335improved, 330masonry, 334materials, 328shot fired, 335star dowels, 334wire, 328

openings in upper floors, 113for flues, 114insulation of flue, 115integrity of flue material, 115isolation of flue, 115for pipes, 113for stairs, 116



Index 383

pipe sleeves, 126plasterboard, see gypsum wall boardplugs and plugging, 335plumbing, 233

air locking, 263appliances, 255

baths, 258kitchen sinks, 256showers, 258taps, 258WCs, 256wash hand basins, 257

capillary fittings, 236compression fittings, 234corrosion, 263equipment, 247

cold water cistern, 248feed and expansion cistern,

251hot water cylinder, 248

first fixings, 264fusion joints, 238hot and cold water services,

243insulation, 262joints to accessories, 238joints to appliances, 238overflows, 246pipe fittings, 234pipework, 233push-fit fittings, 236range of pipe fittings, 239services, 243soil and ventilation stacks, 246solvent weld fittings, 237valves and cocks, 241

schematics, 242waste disposal piping

air admittance valve, 261systems, 259traps, 260

water hammer, 263water supply from the main,

246

quoins, 9alternative definition, 16

rawlbolts, 336rawlnuts, 338rawlplugs, 335reveals, 9risbond joints, 14, 15

roof tile and slateabutment, 178–81bituminous shingles, 176eaves, 178–81interlocking tiles, 176materials, 171pantiles, 177plain tiles, 175ridge, 178–81slates, 175Spanish and Roman tiles, 177timber shingles, 176verge, 178–81

roof, 148Belfast truss, 153box beam purlin, 152bracing, 160classifications, 148forms, 149gang nail plate, 155hammer beam truss, 153insulation, 169king post truss, 152mansard truss, 152prefabrication, 149purlined roof, 151queen post truss, 152roof (gable) ladders, 158terminology, 149TRADA truss, 153, 154traditional, 167trussed, 150trussed rafter, 153truss shapes, 157trussed purlin, 152verges meet eaves, 159

screedsbonding agents, 323granolithic, 322heated screeds, 324laitance, 322laying

in bays, 323bonded screeds 1, 322bonded screeds 2, 323joint treatment in bays, 323monolithic screeds, 322unbonded screeds, 323

materialscement/granite dust mixes, 322cement/sand mixes, 322cement/whinstone dust mixes, 322



384 Index

typebonded, 322monolithic, 322unbonded, 322

screw nailsclearance hole, 331coach screws, 332collated screws, 331counterbores, 332countersinking, 331drywall screws, 342materials and finishes, 331pelleting over, 332Phillips heads, 330pilot hole, 331plug cutter, 332Pozidriv heads, 330slotted head, 330traditional wood screws, 330twin threaded screws, 331wood screws, 330

scuntions, 9setting out, 49

equipment required for basic setting out, 49procedure, 50the site plan, 49where do we put the building?, 49

solid concrete floorssingle and double layer concrete floors with

hollow masonry wall, 62spring toggle fasteners, 337stairs, 221

balusters, 223balustrade, 223flight, 221handrail, 223joining steps to stringer, 225kite steps, 227landings, 222measurements, 224newel, 224rise and going, 224rough carriage, 226staircase, 221steps, 222strings, 221winders, 227

subsoils, 36general categorisation of subsoils and their

loadbearing capacities, 37substructures

bearing strata, 48critical levels and depths, 46

depths and levels, 48failure of wide, thin, strip foundations, 44finished ground level, 47foundation width and thickness, 41level, 46mass of buildings, 39mass, load and bearing capacity, 40principal considerations, 38reinforced concrete foundations, 44simple foundation calculations, 39step in foundation, 49trench fill foundations, 45ventilators, 349–51

terminologyof bricks, 2of roofs, 149of wall openings, 129

testing of bricks, 5timber, 291

batten, 291baulk, 291board, 291conversion, 291cross sections available, 292deal, 291defects

natural, 295seasoning, 295

four sider machine, 293lengths available, 292moisture content, 294natural fire resistance, 298North American timber, 294planed, dressed or wrot timber, 292planing and thicknessing machine, 293plank, 291preservation, 296preservatives

fire resistance, 298organic solvent types, 296tar oil, 296treatment methods, 297water borne, 296

quarter sawn, 291regularised, 291, 292sawn, 291scantling, 291seasoning, 294

distortion, 295kiln, 295natural, 295

slab sawn, 291



Index 385

tolerancesregularised timber, 294sawn timber, 292wrot timber, 294

wane, wany edge, 291timber casement windows, 205

depth and height of glazing rebates, 206draught stripping materials, 206hanging the casements, 207joining the frame and casement members, 209timber for casement windows, 206

timber frame construction, 88balloon frame construction, 90breather membranes, 95cavity ventilation in masonry skin, 93DPC over fire stopping, 95fire stopping in cavity, 94, 95hold down straps to foundations, 93masonry skin materials, 93metal strapping tying at upper floors, 94, 95modem timber frame construction, 91platform frame construction, 89spread of fire in cavity, 94traditional timber frame, 88wall ties for masonry skins, 94

timber sash and case windows, 211the case, 212the sashes and case together, 214

timber vertical sliding sash windows, 214timber windows

for ordinary glazing work, 218glazing, 218

tipping, 17topsoil, 35type of bricks by shape, 4

upper floors, 103alternative materials for joisting, 118brandering for ceiling finish, 110ceiling finishes, 124cheek pieces for ceiling finish, 111floor finishes, 124herring bone strutting, 111, 112joist support

in masonry walls, 105in timber frame walls, 106on beams, 108on flitched beams, 108

linear and point loadings, 112modem sound and fire proofing, 121pugging, 120–22solid strutting, 111, 112sound proofing, 120

steel herring bone strutting, 111, 112support of masonry walls from floors,

123upper floor joist spacing, 110upper floor joists, 103

U-value, 355

ventilators in substructures,calculating number required, 351clay and plastic, external, 349clay liners, 349dead spots, 351positioning, 351telescopic liners, 350

vertical alignment in masonry, 14risbond, 15

wall-floor interfacesground floors, 62precautions, 62

walls, environmental control, 75air infiltration, 77expansion joints, 99fire, 79heat loss and thermal capacity, 75noise control, 79resistance to weather, precipitation, 75

wallsgeneral, 73

dimensional stability, 79insulation of external walls, 84requirements, 74support of masonry walls from floors,

123mutual, 228

calculation of surface density, 228fire resistance, 231transmission of sound, 228wall types, 229

openings, 126alternative lintelling, 134alternative sills, 136cold bridging, 132large openings in masonry walls, 127for larger pipes and ventilators, 127in partitions of masonry, 139in rainscreen cladding, 142for small pipes and cables, 126threshold arrangements, 137in timber frame walls, 141

weather for building, 32weeps, 347wood screws, see screw nails



386 Index

woodworking, 298circular saws, 299plunging router, 300stress grading, 310stress grading machines, 311stress grading marks, 312timber

barefaced mortice and tenon joint, 306blind mortice and tenon joint with foxtail

wedges, 307butt joint, 304cogging, 310dovetailing, 303, 310draw boring mortice and tenon joints,

308finger joints 1 and 2, 305grooving, 301half checked or halved joint, 304halving, 301joining, jointing or housing, 304

morticing, 302moulding, 303notching, 301operations on, 298plain housing, 305plain mortice and tenon joint, 306rebating, 301scarfed joints, 308scarfed joint variations 1, 309scarfed joint variations 2, 309shouldered housing, 305shouldered mortice and tenon joint, 306single and double notchings, 309splaying, 303tenoning, 302toe joint, 308tonguing, 301trenching, 301tusk tenoned joint, 307using a pressed steel connector, 310

Construction Technology: An Illustrated Introduction · Construction Technology an illustrated...

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