Rendering - A Practical Handbook

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Rendering- a practical handbook

Bill Monks


Contents

1 Introduction

1.1 Scope

1.2 Definitions

2 Building design

2.1 Walls to receive renderings

2.2 Building details

3 Specification of renderings

3.1 Matching mortar mixes with

their backgrounds

3.2 Number and thickness of coats

3.3 Controlling alignment and

thickness of undercoats

3.4 Controlling shrinkage

3.5 Controlling colour

3.6 Rendered finishes

2

2

2

3

3

3

5

5

6

6

7

7

7

4 Materials for rendering 10

4.1 Materials 10

4.2 Storage of materials on site 1 1

5 Workmanship 1 1

5.1 Planning the work 11

5.2 Preparation of walls to receive

renderings 1 1

5.3 Choosing the right designation

of mortar 12

5.4 Batching and mixing mortar 12

5.5 Applying renderings 13

5.6 Undercoats 14

5.7 Rendering onto metal lathing 15

5.8 Final coats 15

6 References and sources offurther information 16

Rendering - a practical handbook

Concrete Society Good Concrete Guide No. 3Order reference CS116ISBN 0 946691 62 2

The Concrete Society, 2000

Acknowledgement: Photographs in this Guide were provided by Bill Monks and by the British Cement Association.

Cover photo: Painted roughcast rendering.

Further copies of this publication and information about other Concrete Society publications may be obtained from:

The Concrete Society

Century House, Telford Avenue, Crowthorne,Berks RG45 6YS.UKTel: 01344 466007, Fax: 01344 466008, F- mail: [email protected],http://www.concrete.org.uk

All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, storedin a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in anyform or by any means, without the prior permission of the copyright owner. Inquiries should be addressed to TheConcrete Society.

Although The Concrete Society (limited by guarantee) does its best to ensure that any advice, recommendations orinformation it may give either in this publication or elsewhere is accurate, no liability or responsibility of any kind(including liability for negligence) howsoever and from whatsoever cause arising, is accepted in this respect by theSociety, its servants or agents.

Readers should note that all Concrete Society publications are subject to revision from time to time and shouldtherefore ensure that they are in possession of the latest version.



Bill Monks BA FCIOB FCIArbBuilding and Construction Consultant


Renderings evolved originally to improve the resis-tance of buildings to wind-driven rain. They still func-tion in this way, but they are also used to enhance theappearance of buildings by providing a range offinishes that give variety to external walls.

The traditional craft of rendering has changed verylittle over the years. Good examples of renderings thatare over a hundred years old are to be found through-out Europe. Studies of building design and materialshave led to the development of guidance in the formof a Code of Practice (1) which sets out basic principlesor 'rules' for the specification, materials selection andworkmanship for external renderings. As a result it isfair to say that renderings which are properly designedand properly applied should be as durable as thestructure of a building. When faults occur they areinvariably due to a failure to observe one or more ofthese basic rules. Identifying the cause (or causes) of adefect however is not always easy - for example it maystem from the use of an unsuitable material - but thedefect may be difficult to rectify and it will alwaysincur unplanned and unwelcome expense. It is bettertherefore to proceed in accordance with the slogan"Right First Time", on the basis that to get somethingright in the first place will always be less expensivethan to have to put it right afterwards.

2

emphases of the sections, but it should be appreciatedthat the subjects are inextricably interrelated.

The handbook deals with traditional mortar mixesbased on:

Cement: lime : sand

Masonry cement : sand

Cement : sand with a plasticizer.

It does not deal with the restoration and maintenanceof ancient or historic buildings. Specialist advice onthese should be sought from recognised authoritiessuch as Ashurst(2).Proprietary rendering systems, resin-based materialsand other coatings are not considered.

All the materials for rendering are specified in BritishStandards, and the authoritative reference on the sub-ject is the British Standard BS 5262 Code of practice forexternal renderings(1).

1.2 TerminologyRendering is the operation of covering brick orstonework with a coat of cement mortar or plaster.The term is also used to describe the coating itself.

Mortar is a mixture of clean, graded sand andcement, usually with building lime or a plasticizer,and water sufficient to give a workable consistence.

The freshly mixed mortar is applied or rendered ina thin layer onto a suitably prepared wall, where itsets and hardens.

Renderings usually comprise two or three separatelayers or coats. The work of rendering is a skilledcraft carried out by specialist plasterers.


1 Introduction


2.1 Walls to receive renderingsFor new work that is to be rendered, the walling mate-rial - e.g. bricks or blocks - should be specified to becompatible with the kind of rendering that is required.A basic principle is that a rendering should not be anystronger than the material to which it is applied. Thusa finish such as 'roughcast' (see Section 3.6) whichemploys a relatively rich mix would not be suitableover soft brick or low-strength insulating block. Metallathing would have to be used as a carrier for therendering (1).

2.2 Building details

At the foot of the wall it is usual to curtail the render-ing in line with the damp-proof course (dpc) so thatthe dpc is not bridged; this should be at least 150 mmabove finished ground level (Figure 2). The renderingwill be protected effectively at the top if the roof has agenerous overhang (Figure 3). On a gable wall, if theverge does not project in front of the rendering, it willbe necessary to install a downstand fascia. A 50 mmoverlap at the top of the rendering is a sensible provi-sion. Gutters and down-pipes must be fitted to slopingroofs (Figure 4).

Free-standing walls, because they are exposed to theweather on both sides, are liable to become very wet

Figure I: Dry dash, white calcined flint rendering with slate greyconcrete tile hanging.

The majority of buildings can be rendered success-fully using conventional materials and processes. Ren-derings can be used as a finish on their own or incombination with other materials (Figure I), and theyenable a range of materials to be used in the construc-tion of the wall; moreover, the rendering should beessentially maintenance free. The building details haveto be designed to keep rainwater from getting behindthe rendering, particularly at the vulnerable positionsaround its edges - at the top and bottom, and aroundopenings. The remainder of the rendering should notbe affected even by very severe weather.

Figure A- Renderings should he protected by overhangs at vergesand eves.

Figure 2: Renderings near ground level should not bridge thedamp-proof course. Figure 4: The absence of a gutter to the porch roof has allowed the

rendering to become splashed with soil.

3Rendering - a practical handbook


Figure 5: The rendering to an external, tree-standing wall, cracked byan expansive reaction between soluble sulfate in the bricks and cementin the mortar joints. The adjacent wall to the outhouses, built with thesame materials, is not affected.

(Figure 5). Boundary walls, retaining walls and para-pets should all be built with ventilated cavities topromote drying. A coping at the top should projectwell clear of the wall and include a generous drip orthroating on both sides; there should be a continuousdpc under the coping which should project to cover thetop edge of the rendering (Figures 6 and 7). Thesedetails are essential to prevent the wall from becomingsaturated, which can lead to problems with somecommon materials (Figure 5). With a saturated back-ground the rendering becomes vulnerable to damagefrom freezing. The backs of parapets should preferablynot be rendered. Chimneys are also best left unrendered.

The location of window and door frames in relation tothe outer face of the wall varies according to localpractice. In areas of severe exposure it is usual for theframe to be located behind the outer leaf (Figure 8),

Figure 6: A coping without a front overhung must include a flashingto protect the rendering.

Figure 8: The window frame is shielded by the outer leaf, the paintedconcrete sill and lintel have throatings to shed rainwater; the Tyroleanrendering is not painted.

rather than within the thickness of the outer leaf ascommonly found in South-East England (Figure 9).This is relevant to the treatment of the rendering atthe head and the reveals. If the building is sheltered, asimple butt joint will suffice, but if the building isseverely exposed it is better to install stainless steelstop beads, leaving a 6 mm gap to be filled later with aflexible sealant. Further consideration has to be givento heads and sills. There should be a projecting featureto shed rainwater at the head as well as at the sill, witha throating to prevent water from running backunderneath (Figure 10). Sills should not be rendered;they should extend into the wall at the reveals (Figure11). If this is not possible, purpose-made sectionsshould be fitted to the ends of the sills so that waterfrom the window area cannot spill over the ends (Fig-ure 12). This ensures that water is directed away fromthe vulnerable junction.

Figure 7: Rendered walls without copings may be acceptable in Mediterranean countries but they areunsuitable in the UK climate.



Figure 9: In sheltered positions, the window frame may he set towards Figure 11: Sloping, two-course tile sill, built into the window reveals,the outer face of the wall; the unpainted rendering has a wood-float

figure 10: Sloping protection and throating at head of window. Figure 12: Channel section fitted to the end of the sill to shed rain water

clear of the face of the wall.

3 Specification of renderings

3.1 Matching mortar mixes with theirbackgrounds, exposure and finish

In practice, in order to ensure that the rendering mor-tar is not'stronger' than the background onto which itis applied, it is generally safer to choose a designationof mortar that will be slightly weaker than the wallingmaterial. The principle applies equally to each indi-vidual coat of rendering which should not be strongerthan the one before. This requirement may be satisfiedby using the same mix proportions for each coat andensuring that successive coats are several millimetresthinner. Except when rendering over metal lathing,when the first undercoat should be a strong mix, it ismore convenient to proceed in this way rather than tohave mixes of different strengths in use at the same

time and having to make sure that a weaker mix isalways used over a stronger one.

There is a widely held misconception that strongermixes (i.e. those richer in cement) are better thanrelatively weaker, leaner ones. In fact, rich mixes arequite unsuitable for most situations. A mortar with ahigher cement content will need an increase in waterto maintain a workable mix, and this tends to increaseits shrinkage upon drying which increases the risk ofcracking in the rendering. In extreme cases, the surfaceof a low strength walling material can fail in shear,induced by shrinkage stresses from a rich mortar.

These potential problems are not resolved simply bychoosing a low-strength mix. Apart from the fact thatlow-strength mortars are liable to be less durable,

5Rendering - a practical handbookLicensed copy: unielon, , 10/12/2009, Uncontrolled Copy, ®The Concrete Society

finishes such as roughcast and dry dash, which affordgood resistance to rain penetration, are best whenmade with moderately strong, Designation II mortars;whereas wood float, scraped and textured finishes arebetter with medium strength, Designation III mortars.Hence, the need to match the mortar designation withthe background, the exposure and the required finish.

3.2 Number and thickness of coatsIn areas of the country subject to moderate or shel-tered exposure conditions, as assessed by the BREweather contour maps '•", two coats of rendering com-ply with normal recommended practice. The actualthicknesses, particularly of the undercoat, are boundto vary because walls are seldom perfectly flat. Anyunevenness in the wall should be taken out within thethickness of the undercoat, to produce a flat surface inorder that the final coat can be of uniform thickness.

The undercoat should be nominally 12 mm thick; thisprovides a tolerance for variations in the alignmentand plumb of the wall surface of ± 4 mm; the actualthickness could thus vary across the surface from 8 to16 mm; these are absolute values and should not beexceeded in any positions. If the wall varies by morethan 8 mm, it will be necessary to dub out any hollowareas first separately, or to trim back any projections,before applying the undercoat bearing in mindthat the final coat is to be of uniform thicknessthroughout.

In summary, with two-coat work, the final coat will beabout half as thick as the undercoat and the averagetotal thickness should be about 18 mm.

Three-coat work is recommended for all exposedpositions in coastal and hilly areas and for any build-ings not protected by their surroundings from drivingrain. The thickness of the undercoats should be speci-fied as 12 mm and 9 mm with a similar final coat of6 mm. The first undercoat will accommodate similarinaccuracies in the planeness of the wall as with two-coat work.

Single-coat work should be restricted to proprietaryrenderings designed to be used in this way. Theirapplication is usually confined to specialist firmsapproved by the manufacturer, whose advice shouldbe sought about the specification for each intendedproject.

3.3 Controlling alignment andthickness of undercoats

Once a rendering has been applied it is impossibleto determine its thickness except by drilling or byremoving a piece with a disc cutter, which would onlyshow its thickness in that position. A common faultwith rendering work is the application of undercoatsthat are too thin; these are often followed by final coatsof similar or greater thickness. This is in direct contra-vention of a fundamental principle.

Three methods are available to ensure that an ade-quate thickness of undercoat is provided.

1 The same mix that is to be used for the undercoat,is used to apply 'grounds' (i.e. strips) of mortar,vertically at about 2 m centres; these are setplumb with a straightedge, in line with oneanother, to give an average thickness from theface of the wall of 12 mm (Figure 13).

Figure i j : I'hmtbiug morttir 'grounds' with ti straightedge.

2 Timber battens 10 mm thick , or grounds (Figure14), with the edges splayed to facilitate their laterremoval, are fixed vertically at about 2 m centres,using masonry pins, with the battens packed outwhere necessary to give good alignment and anaverage thickness of 12 mm. If the surface of thewall is locally 'proud' behind the batten, then athinner batten (down to 8 mm) will be neededthere. The battens have to be removed when themortar has set and so the packing pieces shouldbe narrower than the batten. The grooves are thenfilled with mortar.

Figure 14: Fixing limber 'grounds' to regulate the thubiess oj anundercoat.

Rendering - a practical handbook6


3 Two lengths of piano wire are secured tightlystrung across the face of the wall, one near thebottom and one near the top of the 'lift', about2 m apart so that they can be spanned by the plas-terer's straightedge. Spacers at each end are usedto determine the thickness of the undercoat.

Whichever method is used, a check should be madethat adequate thickness can be obtained over thewhole area before the undercoat is applied.

3.4 Controlling shrinkageIn addition to the effect on shrinkage of the fineness ofthe sand (as noted in Section 4.1), three factors willaffect the shrinkage of the final coat:

1 The effect of initial drying shrinkage in theundercoat will be minimised by allowing theundercoat to stand for several days drying beforethe final coat is applied.

2 The final coat should be as thin as practicable:6 mm should be regarded as a sensible maximumthickness rather than a target mean value.

3 Excessive rates of drying of the final coat, eitherthrough high suction in the undercoat or througha high rate of evaporation in warm dry weather,should be avoided. The latter will need control-ling by using a mist spray and by shielding thewall from the direct heat of the sun for the firstfew days after application.

3.5 Controlling colourVariations in the moisture content or in the absorb-ency of backgrounds cause variations in the colour ofrenderings. A blotchy or shadow effect is likely toresult from applying a rendering over a backgroundwhich is made of different materials. Mortar jointsparticularly in blockwork will often 'grin through' arendering as a form of pattern staining (Figure 15).The more absorbent material will give a darkerappearance, and that part of the rendering will alsotend to be slightly more absorbent than lighter areas.Fortunately, when the undercoat is of adequate thick-ness, around 12 mm, pattern staining is unlikely to bereflected through the final coat.

3.6 Rendered finishes

The choice between different finishes is largely one ofpersonal preference. There are regional influences;some renderings provide better protection and aremore suited to exposed positions, but an unrestrictedchoice may be precluded by the nature or properties ofthe walling material, as discussed later.

Roughcast is rough-textured and is thrown or 'cast'onto the wall, and left untrowelled. The texture is pro-vided by the aggregate which may be 10 or 14 mmrounded shingle or angular, crushed rock. The finish isalso known by the term 'wet dash' because the aggre-gate is mixed into the mortar (Figure 16).

7

Figure 15: Pattern staining in an undercoat earned by the variation in absorbency between concrete blocks andthe mortar joints.

Figure 16 Roughcast rendering divided into panels

Figure 17: Dry dash rendering on part of a 2 m square sample panel,the aggregate has been blended to orderby the supplier.



Dry dash by comparison, uses similar materials, withthe difference that the aggregate is not part of the mixbut is 'dashed' onto the freshly-applied final coat whilethis'butter-coat' is still plastic (Figure 17).

The difference between the appearance of the two fin-ishes is that roughcast is the colour of the cementmortar paste coating whilst dry dash is predominatelythe colour of the chosen aggregate. Descriptions suchas 'pebble dash' and 'spar dash' describe the materialscommonly used. Calcined flint, spar, shingle and gran-ite are used throughout the UK.

Roughcast and dry dash are not usually prone todefects and they are unlikely to need any maintenance.However, if at some later date it is decided to paint a Figure 19: A Tyrolean rendering protected by an overhanging fascia.

Figure 18: Detail of the scraped finish texture.

roughcast rendering, the durability of the appearancewill become dependant on the properties of the paintthat is used; it may then require repainting tomaintain a satisfactory appearance at intervals ofbetween five and twenty years.As a generalisation, thrown finishes seem to bond bet-ter than trowelled finishes: thrown application proba-bly reduces the risk of air being entrapped at theinterface between layers. Elsewhere, incidently, thestandard method of application for all renderings is bythrowing, rather than by the method familiar in theUK, using a hawk and trowel.

The scraped finish is a trowel-applied renderingwhere the mortar for the final coat is made with agritty sand chosen for its colour. When the mortar hasset, but before it becomes too hard, the surface isscraped or abraded, with a piece of expanded metalfixed to a wood-float, or with an old saw blade. Thisremoves the immediate surface and drags some of thecoarser particles from the matrix while leaving othersexposed. The resulting surface is uniformly textured,not unlike bush-hammered concrete (Figure 18).

Figure 20: A rubbed Tyrolean finish with some of the larger projections

removal by stoning.

Figure -21: Tooled rendering divided into 1.5 m by 1 in panels.

8


Figure 22: The textured rendering contrasts with the smooth rendered margins to the windows.

Tyrolean rendering has a textured finish: it is madewith a proprietary, cement-rich, pigmented, workablemortar. A small quantity at a time is placed in aportable hand-held container and projected onto thewall in the form of individual 'drops' from sprungtines on a rotating spindle. The drops of mortar,which average from about 3 mm to 8 mm accordingto the workability of the wet mix, are dispersed acrossthe face of the wall to provide a uniform coverage(Figure 19).

Rubbed Tyrolean is a modification of the above finishproduced by rubbing the hardened Tyrolean with acarborundum block to give a smooth, flat outer sur-face with a recessed texture. This gives a finish thatlooks different from plain Tyrolean rendering and itcan be useful in places such as corridors where therougher texture would be unsuitable (Figure 20).

Travertine rendering resembles the fissured surfacethat is characteristic of marble of that name. The fis-sures are produced by a small quantity of rounded5 mm aggregate mixed into the mortar, whichbecomes trapped under the pressure of the float, androlls to form grooves through the thickness of the finalcoat. The grooves can be straight or in swirls accord-ing to the method of trowelling. A similar, but perhapsless satisfactory, dragged or scored finish can be pro-duced with a wooden float that has a nail (or severalnails) protruding through.

Tooled renderings are to be found across mainlandEurope, usually in towns, where the finely texturedfinishes are similar to those on buildings faced withdressed stone. Tooled renderings are rare in the UK,perhaps because of their high labour content or a lackof experience with the particular finish (Figure 21).

A variety of patterns and textures can be produced inthe surface of a rendering with a trowel or small handtool, either during application or immediately after-wards (Figures 22 and 23).

Alternatively, impressions can be formed in thefreshly-applied rendering with suitably profiledimplements such as the base of a wine bottle or a smallsquare ribbed block. Such finishes are often used withpargeting (translated by 'throwing'), familiar in EastAnglia and in other parts of Europe (Figure 24).

Plain trowelled finishes are best produced with awooden float which gives the surface a sand-papertexture (Figure 9). This finish is not recommendedgenerally, because of its liability to colour 'shading'and the tendency to craze, as with smoother surfaces.In practice, the wood float finish cannot be guaran-teed sufficiently uniform in colour to be acceptablewithout painting, particularly where large areas areinvolved.

Many plain renderings are intended from the outset tobe painted. A masonry paint should be used over awood-float finish. The paint should be one that is per-meable to moisture vapour so as to allow the surfaceto 'breathe'.

figure 23. Painted, textured rendering to semi-detached houses.

Figure 24: Traditional white pargeted finish on a modern building.

9


4 Materials for rendering

4.1 MaterialsCement

Mortar is normally made with one of the followingcements:

Portland cement to BS 12 (4)strength class 42.5 N(grey cement, formerly known as ordinary Port-land cement).

White Portland cement to BS 12 (4), strength class62.5. The effect of the relatively higher strength ofwhite cement should be borne in mind whenbatching; to compensate for this, a proportion ofthe cement (say 25%) could be substituted with anequal volume of dry hydrated lime.

Antique white Portland cement to BS 12(4)

strength class 52.5, is an 'off-white' or ivory colour,with properties between those of white and greycements.

'Improved' Portland cement to BS 12 (4), strengthclass 32.5 R. This relatively new cement contains aplasticizer to entrain a small quantity of air (about7% to 10%) which improves the workability andcohesiveness of the mortar, and also its frost resis-tance when it has hardened. Other plasticiersshould not be added to the mortar.

Masonry cement to BS 5224 (l5) is designed to pro-duce mortar with improved workability and cohe-siveness by having Portland cement blended withabout 24% of a finely graded inert powder fillerand a plasticizer. When batching mortar, theamount of sand per batch is reduced comparedwith mixes made with other cements, in order tomaintain similar proportions of cement to sand.Neither lime nor other admixtures should beincluded in the mix.

Sulfate-resisting cement to BS 4027 (6 ) should bespecified for rendering onto clay bricks of cate-gories MN or FN. Naturally occurring soluble saltscould prove troublesome when used with otherPortland cements if these bricks become saturatedand a low permeability rendering prevents themfrom drying (Figure 5). It would also be necessaryfor sulfate-resisting cement to be used in buildingthe wall.

Lime

Dry hydrated lime to BS 890 (7) is used to improve theworkability and cohesiveness of mortar. It tends toreduce the loss of workability due to suction from thebackground and thus makes the mortar easier toapply. Shrinkage of the mortar on drying is alsoreduced and this lessens the risk of crazing. Buildinglime is often used with mixes based on Portlandcement but it should not be used with masonrycement.

A mixture of lime and sand on its own (known ascoarse stuff) is not mortar; it cannot be used without

the addition of Portland cement. Its advantage is thatit contains a specified proportion of lime thoroughlymixed with sand and therefore requires only the addi-tion of cement and water.

Sand

Sand for rendering should be clean, sharp, well-gradedand comply with the grading given in Table A ofBS 1199 (8). It is perhaps best described as a rathercoarse building sand or a fine concreting sand, beingbetween the two in terms of particle size or fineness.

One of the most common problems with renderings -namely that of cracking or crazing - is due to shrink-age of the mortar which is caused by the use of sandthat is too fine or which contains clay or silt. Suchmaterials may be popular with operatives because theymake the mortar cohesive or 'buttery' and easy to use,but fine sands need more water than a coarser sand,and so the shrinkage will be greater and the mortarwill be weaker. Sand normally used for mortar forbuilding walls is unlikely to be suitable for rendering.If a suitable sand cannot be obtained locally, it wouldbe better to use a fine concreting sand or to mix twosands thoroughly to produce a better overall grading,or to bring in a sand from another region.

Admixtures

Plasticizers designed for mortars should comply withBS 4887: Part 1 (9); they work by entraining air. Theymust be added at the prescribed dosage, strictly inaccordance with their manufacturer's published rec-ommendations. Household or industrial detergentsshould not be used as plasticizers; they have a plasti-cizing effect but they are not intended for this purposeand the volume, size and distribution of bubbles theyproduce is uncontrolled. This can seriously weaken amortar and impair its durability. Plasticizers shouldnot be added to mixes made with masonry cementor 'improved' Portland cement because they alreadycontain plasticizers.

Waterproofing admixtures are not recommended forgeneral use in renderings. This is because renderings(like brickwork) are designed to absorb some waterwhen they are exposed to driving rain; a waterproof-ing admixture in a final coat would tend to preventnormal drying out. Some plasterers use waterproofingadmixtures in undercoats to facilitate application ofcertain finishes such as dry dash (where it helps toprevent the butter-coat drying too quickly), pargetingand scraped finishes.

Water

Water for use in mortars must be of drinkable quality.

Pigments

Pigments should be to BS 1014 (10) they are mainlymetallic oxides in the form of fine, inert powder.Because the powder is difficult to disperse evenlythroughout a batch of mortar, it is recommended thatpigmented mortar is best obtained as a premixedlime : sand coarse stuff or as a ready-to-use mortar.



Lathing

Lathing or steel mesh is used to provide a carrier forrenderings over walls that are too uneven or too weakand friable to support a rendering directly. To ensuregood long-term durability, lathing, mesh, angle bead-ing and fixings should be austenitic stainless steel toBS EN 10095 ( l n , although galvanized steel may besatisfactory in sheltered positions between ground andfirst floor levels.

One-coat renderings

There are several proprietary renderings intended tobe applied as a single coat 20 mm or more in thick-ness. These generally use white cement with a light-coloured sand for white rendering and may include apigment to give a choice of colour. A mortar plasticizerin powder form is often included. The material issupplied dry, in sealed bags; it has to be stored like

bagged cement. The advantages of these materials arein the careful selection and blending of sands toachieve a uniform grading with minimum voids,which limits their shrinkage, and in their accuratebatching of the ingredients by weight.

4.2 Storage of materials on siteCement and hydrated lime should preferably be storedin a dry shed in such a way that the oldest stocks canbe used first; failing that they may be stored on pallets,clear of the ground and covered with a tarpaulin.

An area of hard-standing should be prepared, with asloping base for drainage, to store the sand in such away that it does not become mixed with soil or otherimpurities. Sand for rendering should be stored sepa-rately from sand for mortar for building walls or foruse in concrete. The stockpile should be covered all thetime when it is not actually being used.

5 Workmanship

5.1 Planning the work

Access

Scaffolding should be designed to be independent ofthe wall in order to allow clearance for uninterruptedaccess for the application of the rendering. The scaf-folding should of course comply with the provisionsof Health and Safety at Work legislation.

Programme

Ample time should be allowed in the building pro-gramme to cater for the delay of several days betweensuccessive coats of rendering that is necessary to per-mit hardening of each layer. At the same time, it isimportant to plan so as to avoid discontinuity orinterruption during the course of the application.Although straight, horizontal or vertical dayworkjoints might be acceptable in the undercoat, as theywill generally be completely masked by the final coat,unplanned joints in the final coat are not acceptable.Either the whole face has to be 'brought down' in acontinuous operation, or the work has to be dividedinto manageable sections with joints in predeterminedspecified positions.

Protection

Renderings are essentially thin layers, with a high sur-face-to-volume ratio; they are particularly prone tothe effects of heavy rain, frost and direct heat from thesun during the course of the work. Fine plastic meshsheeting attached to the scaffolding provides good

protection from these extremes. Planning and costingshould include their provision.

5.2 Preparation of walls toreceive renderings

Walls must be clean of dust and any contamination.For new work, a good brushing down is normally allthat may be required by way of preparation. Thesurface should be checked for key and suction: that is,a texture to provide a grip for the rendering, and theability to absorb, which may be checked by splashingthe wall with water. It follows that a wall that is satu-rated from heavy rain will not be in a fit condition toreceive a rendering until it has dried.

Brick walls should be built with the mortar jointsraked back squarely to a depth of 10 mm. Block wallsmay have an adequate key in the texture of the block,but smooth-faced blocks and in situ concrete shouldbe prepared by abrasive blast treatment or by applyinga stipple (Figure 25) or a spatter-dash, which shouldbe allowed to harden for a couple of days and then bechecked for adhesion before the undercoat is applied.

A stipple may also be used to reduce high suction, orvariability in suction, and is particularly useful wherethe background comprises a mixture of differentmaterials.

The stipple mix is prepared from cement and sand,usually in the proportions 1 to 1'J or 1 to 2, mixed withwater and a bonding agent, such as styrene butadienerubber (sbr) or an acrylic polymer, to form a slurry.



Figure 25: Producing a stippled key on a smooth surface.

The polymer manufacturer's instructions for their useshould be observed. The slurry should be brushed vig-orously onto the wall to wet the surface thoroughlyand then, with a freshly loaded brush, it should be

Table 1: Mortar designations, relative strengths, and comparable

given a stipple texture. The treated surface should beprotected from drying out for the first day and then itshould be allowed to harden for another day or twobefore the rendering undercoat is applied.

The alternative for surfaces that do not readily afford akey is to use metal lathing. This is the only way to pro-vide a key for rendering over unsound, friable surfaces.

5.3 Choosing the right designationof mortar

Tables 1 to 4 describe the mortar designations and themix proportions using the full range of materials com-monly employed. They are based on the use of clean,well-graded sand complying with BS 1199 (8). Table 5shows how, by changing the quantity of sand, thenormal range of mortar properties given in Tables 2to 4 may be retained when poor quality sands have tobe used.

Table 2: Nominal mortar mix portions by volumefor cement : lime : sand mixes

Designation

I

II

III

IV

V

Cement :

1

1

1

1

1

lime :

O to ¼

½

1

2

3

sand

3

4½

6

9

12

5.4 Batching and mixing mortarWhen calculating the volume of sand required for abatch, no allowance is made for bulking. The sand isassumed to be damp, but not saturated. Sand nor-mally is damp when it is delivered and its moistureshould remain sensibly constant in storage if thestockpile is covered to protect it from rain and preventit from drying out.

Sand may be measured by the bucketful, loosely filled(not tamped) and levelled, or with a purpose-madegauge box (Figures 26 and 27). Measuring by theshovelful is simply not acceptable.

Batches should be based on the use of whole bags ofcement and lime that weigh 25 kg. The volume of abag of cement is about 17.5 li, but that of a bag ofhydrated lime is 35 li!

Plasticizers should be dosed carefully to the supplier'sdirections.

Tilting, free-fall mixers are suitable for producing theworkable mortars used for rendering, providing themixer is maintained in a clean condition and notcaked with hardened mortar. The mixing time shouldbe reasonably constant: two or three minutes shouldbe satisfactory. Mortar should not be left turning over


Designation

I

II

III

IV

V

Description

Strong

Moderately strong

Medium strength

Moderately low strength

Low strength

Typical backgrounds

In situ concrete, engineering bricks,dense blocks

Some facing bricks, calcium silicatebricks

Some common bricks, lightweightaggregate blocks

Autoclaved aerated concrete, somesofter bricks

Weak bricks, weak stone

Table 3: Proportions by volume of (a) lime : sand to produce coarse stuff and(b) cement : coarse stuff to produce mortar

Designation

IIIIIIIVV

(a) Coarse stuff

Lime

1]

1

1

1

sand

12

9

6

4 ¼

3

(b) Mortar

Cement: coarse stuff

1 : 31 : 4½1 : 61 : 91 : 12

Table 4: Mortar mix proportions by volume (a) using masonry cement : sandmixes and (b) using Portland cement : sand mixes with a mortar plasticizer

Designation

IIIIIIV

(a) Masonry cement:sand

1 : 3 '/21:51 : 6½

(b) Portland cement:sand plus plasticizer

1 :41 :61 :8


trevorc

trevorc

Table 5: Reduced proportions (to cement) by volume of sand for mortars made with sand that is eitherslightly finer or slightly coarser than the recommended grading - refer to text

Designation

II

III

IV

V

Portland cement,lime, sand mixes

Table 2

4

5

8

10

Coarsestuff

Table 3 (b)

4

5

8

10

Masonry cement,sand mixes

Table 4 (a)

2'/24

5'/2

Portland cement, sand,plasticizer mixes

Table 4 (b)

3

5

7

Figure 26: Measuring sand by tin.' bucketful when making mortar.

for long in an unattended mixer because it will beliable to develop a variable (uncontrolled) air contentwhich can affect its workability, its appearance and,more seriously, its durability.

Freshly mixed mortar should be discharged from themixer and delivered to where it will be used. In aver-age weather conditions it should be used within a cou-ple of hours of mixing, but a much shorter time isappropriate in hot weather and a correspondinglylonger time may be acceptable when it is cold.

Retempering of the mix should be restricted to replac-ing any water lost by evaporation.

Retarded ready-to-use mortar from a member com-pany of the Mortar Industry Association, where thesupplier is responsible for batching, is a qualitycontrolled product (Figure 28). Pigmented mortar orlime : sand coarse stuff is best obtained ready-mixed,in order to achieve consistent results.

The workability of the mortar should be adjusted tosuit the background and the weather conditions at the

Figure 27: A bottomless gauge box for measuring a batch of sand.

time (Figure 29). Porous materials with high suctionwill need a more workable mix whilst materials suchas expanded metal which has no suction at all are bestrendered with a dryer, stiffer mortar.

5.5 Applying renderingsRendering has to be applied with firm pressure on thetrowel to force the mortar into intimate contact withthe wall so that it fills any crevices and interstices in

Figure 28: Ready-mixed, retarded mortar being delivered to skips for

temporary storage on site.



trevorc

trevorc

Figure 29: The consistence of rendering mortar {on the spot hoard) and its adhesion to the hawk.

the surface (Figure 30). Raked joints for example,should be completely filled in the process. Throwingthe rendering achieves the same effect by helping toprevent pockets of air being trapped at the interface.As noted above, the consistence of the mortar has tobe adjusted to suit the absorbency and the nature ofthe surface. Compared with ordinary mortar forbuilding walls, it is usually slightly stiffer and lessworkable.

5.6 Undercoats

The undercoat should be built up to its full thicknessin at least two consecutive applications over a periodof about half an hour, depending upon the rate ofstiffening, beginning with a very tight first layerapplied with considerable pressure on the float. It is

Figure 31: Ruling the undercoat to thickness.

Figure 32: Finishing the undercoat with a plastic float.

then ruled off with a straightedge, working to stringlines or the 'grounds' which were set in place previ-ously; any hollows identified in ruling off are filled atthis stage (Figures 31 and 32).

For the majority of final coats, the undercoat is leftwith a 'combed' keyed finish, formed when the mortarhas stiffened slightly (Figure 33). The combed groovesshould be in distinct wavy lines that penetrate at leasta third of the thickness of the layer. In addition to providing a key for the next coat, the grooves allow thelayer to shrink in small sections without creatingstresses that could lead to visible cracks and perhapsdebonding or separation from the background.

Figure 33: Combing the undercoat to form a key.

14

Figure 30: Applying undercoat between vertical 'grounds'.

Rendering - a practical handbookLicensed copy: unielon, , 10/12/2009, Uncontrolled Copy, ®The Concrete Society

The undercoat should then be allowed to harden. Thestrength will be impaired if it dries too quickly; it hasto be kept damp so that the cement and water cancontinue to react. In warm dry weather the surfaceshould be sprayed gently with water occasionally for aday or two.

When the mortar has hardened it should be allowed todry for two or three days; this further delay has to bebuilt into the programme. It is necessary for drying totake place in order to provide suction for the next orfinal coat. The loss of moisture is accompanied byshrinkage of the undercoat mortar. Some fine cracksmay develop in the surface, especially if the sand israther too fine or the mortar is rich. This should notbe a serious problem. Much of the shrinkage takesplace during the first few days of drying, and so it isbetter to delay the application of the second and finalcoats until some of the shrinkage has taken place inthe undercoat rather than to apply a subsequent coatwhen its background is still most susceptible toshrinkage.

5.7 Rendering onto metal lathingThe work should be divided into panels not exceeding10 m2 and preferably with the length of panels notmore than about 1½ times their width. The risk of

cracking in renderings on metal lathing is reduced ifsmaller panels are employed. Manufacturers of metallathing and ancillary systems (Figures 34, 35 and 36)provide guidance on the way the lathing should befixed. It is important to observe this advice particularlywith respect to tying sheets of lathing at laps withinpanels and providing horizontal and vertical jointswhere movement can occur.

The lathing should be spaced about 5 mm from thebackground to enable the mortar to surround thelathing completely.

Three-coat rendering should always be used on lath-ing and mesh. The first undercoat should be mixdesignation I or II; its main purpose is to reduce theflexibility of the lathing so as to provide a firm back-ground to receive the subsequent coats. They are cho-sen to suit the finish.

Figure36: Preformed section used in horizontal joints between rendered

panels.

Figure 34 Gulvanized bell-cast edging with plastics nosing.

Figure 35: Movement joint section with protective tape.

5.8 Final coats

With plain trowelled finishes, the plane surface of thefinal coat is produced by the skill and coordination ofthe plasterer rather than by reliance on 'grounds' orspacers. The actual thickness achieved is less criticalthan the thickness needed for undercoats which pro-vide renderings with their protective properties. It ismore important in the final coat to ensure uniformityof thickness over the whole surface. Some plasterersprefer to rule off the final coat with a timber straight-edge; others use a Darby - a long steel blade with twohandles - while some complete the application with asteel float (Figure 37). The actual application of themortar needs to be distinguished from the process offinishing. Renderings should not be finished with asteel trowel or float. This would provide a smoothdense surface that is liable to craze. It is better tofinish the rendering with a wooden float which pro-duces a characteristic sand-paper texture that is lesslikely to craze (Figure 38). A wood float finish is usu-ally considered sufficiently smooth to accept a masonrypaint where that may be required.

Other finishes, particularly the textured roughcast anddry dash, are preferred for their appearance, theirdurability and their resistance to the weather. In areas



Figure 38: Unpainted, wood. float finishFigure 37: Applying the final coat to a thickness of about 5 mm.

6 References and sources of furtherinformation

1 BSI. Code of practice for external renderings.London, BSI. 48pp. BS 5262: 1991.

2 Ashurst ). and Ashurst N. Mortars, plasters andrenders. Aldershot, Gower Technical Press, 1988.100pp. English Heritage Technical Handbooks.Practical Building Conservation Series, Vol. 3.

3 Building Research Establishment. The assessmentof wind loads. Part 3: Wind climate in the UnitedKingdom. Garston, BRE, 1989. BRK Digest 346.

4 BSI. Specification for Portland cement. London, BSI.16pp. BS 12: 1996.

5 BSI. Specification for masonry cement. London, BSI,16pp. BS 5224: 1995.

6 BSI. Specification for sulfate-resisting Portland cement.London, BSI, 18pp. BS 4027: 1996.

7 BSI. Specification for building limes. London, BSI, 24pp.BS890: 1995.

8 BSI. Specifications for building sands from naturalsources. London, BSI, 8pp. BS 1199 and 1200: 1976(1996).

9 BSI. Mortar admixtures. Part I: Specification for air-entraining (plasticizing) admixtures. London, BSI,12pp. BS 4887: Part 1: 1986.

10 BSI. Specification for pigments for Portland cementand Portland cement products. London, RSI, 16pp.BS 1014: 1975.

11 BSI. Heat resisting steels and nickel alloys. London, BSI,30pp. BS EN 10095: 1991.

12 Building Research Establishment. Choosing externalrendering. Garston, BRE, 1994. Good BuildingGuide 18. 6pp.

13 Monks W. External rendering. Crowthorne, BritishCement Association, 1992. 32pp. Appearance matters2. ref'47.102.

14 BSI. Workmanship on building sites. Code of practice forplastering and rendering. London, BSI. 26pp. BS 8000:Part 10: 1995.

Sources of further informationIt may be appreciated there is much more to renderingthan it has been possible to discuss here. For those seekingfurther technical information, all the essential points arecovered in the British Standard BS 5262:1991 Code ofpractice for external renderings(1). There is also a usefulBRE guide on Choosing external rendering(12) and a BCAhandbook External rendering(13) . BS 8000: Part 10 Work-manship on building sites. Code of practice for plastering andrendering'4', may also be consulted. The following organi-sations will also be able to provide information and advice.

British Cement Association, Century House, TelfordAvenue, Crowthorne, Berkshire RG45 6YS. Tel: 01344762676, Fax: 01344 761214, www.bca.org.uk.

Building Research Establishment, Garston, Watford,Herts WD2 7JR. Tel: 01923 664000, Fax: 01923 664010,www.bre.co.uk.

Concrete Advisory Service, 37 Cowbridge Road,Pontyclun, South Wales CF72 9EB. Tel: 01443 237210, Fax:01443 237271.

Mortar Industry Association, 156 Buckingham Palace-Road, London, SW1W 9TR. Tel: 020 7730 8194, Fax: 0207730 4355, www.mortar.org.uk.


exposed to moderate or sheltered environments, the adopted with advantage. All rendered finishes can bescraped finish, Tyrolean and patterned finishes may be produced in a range of colours.


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Bill Monks

Renderings are used to provide buildings with attractive water-resistantfinishes. The technique has been used for many years and in many partsof Europe. A wide range of effects and colours can be achieved bycareful choice of materials, attention to detail and good workmanship.Renderings which are properly designed and properly applied should be asdurable as the structure of the building.

This handbook sets out the basic principles for achieving attractive, durableand trouble-free renderings for buildings. Architects, specifiers, buildingcontractors and site staff will find it an invaluable guide from design rightthrough to work on site. The handbook is divided into four sections forconvenience: design, specification, materials and workmanship, but inpractice they are inextricably linked.

Bill Monks has recently retired as an independent building and constructionconsultant. He has worked for Ove Arup & Partners, and tor the Cement &Concrete Association. He has special expertise in the appearance andsurface finish of concrete and has written widely on these topics, and is theauthor of several of the well-known series of guides Appearance matters.

Good Concrete Guides give concise, 'best practice' guidance on materials,design and construction.

GCG 1: Guidance on specification, mix design and production of concretefor industrial floors

GCG 2: Pumping concreteGCG 3: Rendering - a practical handbook

ISBN 0 946691 62 2Order reference CS116

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