Construction Technology Walls

8/2/2019 Construction Technology Walls

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WALLS

23.27 FUNCTION OF WALLS

A building enclosure includes the roof, walls, openings, foundation, and below ground materials that separate the building

from its surrounding environment. It adds a thermal barrier, a vapor barrier, drainage planes, and an air barrier.

Walls function to control, support, distribute utilities and other services, and beautify (or "uglify") our homes.

The control function of the walls is quite important. They actually control:

Rain and snow penetration Pollution, dust, other contaminants or environmental hazards Heat flow Airflow, including wind Water vapor Groundwater Light Solar radiation Fire Noise Entrance of animals, bugs, vermin Entrance of unwanted humans

Introduction

The principle of the cavity wall is quite simple. The cavity prevents moisture passing

through the wall. As long as the cavity is kept clean and the wall ties are correctly

positioned the house should remain dry even if the external leaf becomes

saturated. Water is free to run down the inner face of the external leaf (and this is

quite likely in severe exposures) and is either ejected via weep-holes or drips safely

below the DPC.

Early walls were usually brick in both leaves. During the 1930s blockwork became

more popular for the internal leaf. Blocks contained all manner of aggregates

depending on what was available locally. Early cavity walls are usually 250mm thick

( 10 inches) with the cavity 50 mm (2 inches). In modern construction cavities are

often 75mm or more wide to accommodate insulation and allow a clear space

between insulation and outer leaf.

Of course, if cavities are bridged damp penetration can occur. The most common

cause of bridging is debris in the cavity but it can also be caused by ties which slope

towards the inner leaf or ties (some designs only) being laid upside down. This

endoscope picture clearly shows mortar on the tie. The purpose of the ties is to

bind the two halves of the wall together. Many early ties have failed prematurely.

This is usually because they had insufficient protection (usually in the form of


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galvanising).

Sometimes, where walls were rendered the wall could be built from 2 block leaves

or 2 leaves of common bricks. Spotting whether early walls are cavity or solid can

be quite difficult if properties are rendered. The depth of the reveals is one clue;

another can be found in the roof space at eaves level.

2 Early Cavity Walls

By the end of the Victorian period cavity walls were

not uncommon although most external walls were still

built as solid walls. London Building Regulations (and

many local by-laws) insisted that either the inner or

outer leaf of a cavity wall should be 1 brick thick. The

two leaves were held together by cast iron or wrought

iron ties (left), or, in some cases, special cavity bricks

(right). Clickherefor a drawing showing a Victorian

cavity wall (taken from an 1898 textbook).
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To see a short video clip

explaining the problems

and demise of the solid

wall, and the

introduction of the

cavity wallclick here

By the 1920s the Regulations had been relaxed and

most new houses were built with cavity walls, but with

both leaves half-brick thick. The wall was typically

250mm thick (10inches). However, solid walls

continued in some parts of the country for many years.

Many Victorian houses, for example, those re-built

after the Second World War (following bomb damage)

were built with solid walls.

The most common form of an early cavity wall is

shown on the left. An outer leaf of brickwork would be

built in facing bricks and usually in stretcher bond

(sometimes Flemish bond was used with 'snapped'

headers, ie, headers broken in two). The inner leaf was

usually formed in common bricks, ie, bricks intended to

be plastered or kept out of sight. As with solid walls

the internal plaster was usually lime based and applied

in two or, preferably, three coats. In the 1930s and 40s

this slowly gave way to gypsum plaster.

early cavity walls

In a solid wall headers bind the wall together. In a

cavity wall this is not possible and the two leaves are

tied together by wall ties. Early ties were sometimes

formed in wrought iron or mild steel. They weresometimes unprotected or possibly coated in bitumen

or zinc (galvanising). The ties were typically positioned

every sixth course vertically and about 900mm apart

horizontally. In practice these centres were often

'stretched' to save money. The tie on the right is from

a 1920s house. You can see some deterioration at the

bottom of the tie. The galvanised protection has

disappeared leaving the steel free to rust. The rusty

part was in the external leaf.
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Click hereto see an

example of modern

bricklayers in action.

The basic technique is

the same nowadays as itwas a hundred years

ago.

By the 1930s some developers were using concrete

blocks for the inner leaf. These were often made from

locally available aggregates, often industrial wastes.

However, the use of blockwork was slow to catch on

and even as late as the 1950s bricks were still used forinternal leaves of cavity walls and internal partitions.

By the 1920s most walls included DPCs. Nowadays two

separate DPCs are used (see left), one for each leaf. In

some early cavity walls large pieces of slate were used

which actually bridged the cavity. Other materials

included lead, copper, asphalt and lead cored bitumen

felt. Today, most DPCs are made from polythene.

The inner and outer leaf

of a cavity wall should

never touch - they

should always be

separated by a DPC.

3 Early Cavity Walls - Head, Sill and Jamb

Openings in early cavity walls could take many forms.

This page shows one or two of them for cavity walls

built in the first half of the 20th century. Many aspects

of good practice were ignored and the consequences

of this sometimes manifested themselves in problems

of damp penetration. However, many of these walls

are still functioning quite adequately.
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As shown in the top right photo the lintel was often formed in precast or insitu

concrete. It could be in two halves, one for each leaf, or as a single deep lintel as

shown in the left-hand examples. It should be clear that with a solid lintel damp

penetration is a possibility due to mortar droppings bridging the cavity, or to water

running across the top of the lintel. The example on the right of the graphic

includes a cavity tray - designed to prevent the above problems. However,

condensation is still a risk and is easily confused with damp penetration. This is

explained in more detail below.

Cavity trays were not always used at the heads of openings. On many older houses

they have been added subsequently. Look for tell tale signs, usually a few new

courses of brickwork over the lintel. In this photo the cavity tray appears to be

made from copper - you can just see the front 'strip' of the tray projecting from the

wall.

The construction on the left, known as a boot lintel, alleviated the problems shown

above. The top of the lintel was usually coated in tar (from coal) or bitumen (from

oil) to prevent water soaking into the lintel itself. However, even where these exist

there is still another potential problem; condensation. This can occur on the inside

face of the lintel because it is cold. The phenomenon is sometimes referred to as

cold bridging. Moist air in a room comes into contact with the cold inner face of the

lintel and condenses. The problem is often confused with damp penetration and, as

a result, the diagnosis is often wrong. This can lead to expensive repairs which do

nothing to alleviate the underlying problem.

Some windows have soldier arches above them. This is

a row of bricks on end, usually only in the outer leaf.

The inner leaf often comprises a concrete lintel. In

narrow openings the soldier arch stays in place due to

the adhesive affects of the mortar. In wider openings

you can sometimes find a steel or wrought iron bar as

shown on the left. In a few cases the bricks may have

steel reinforcing rods running through the holes in the

bricks.
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At the jambs, ie, the sides of the opening, cavities were

often closed to provide a good fixing for the frame and

a good base for the internal plaster. The example on

the left shows a cavity wall with the inner leaf returned

to the outer leaf. This creates a path for damp

penetration but there are still lots of examples of this

construction, some of them damp free. Sometimes the

cavity was left open as shown on the right. This does

not provide a very good fixing for the window.

These examples both show a cavity closed but with the

addition of a DPC. In the left hand example the DPC is a

sheet material, possibly bitumen felt or even lead. The

right hand example shows a cavity closed with plain

tiles bedded in mortar. This principle, ie of providing a

DPC, remains today.

There were a variety of sill details. Two examples are

shown here. In the left hand example the timber sill is

bedded in mortar on the external leaf. A drip at the

end of the sill prevents water from running back under

the sill. In the right hand example a smaller sill section

sits deeper in the jamb. A sub sill formed from plain

tiles is bedded in mortar on a DPC. In this example thecavity is closed with a three quarter brick.

4 Modern Cavity Walls - Generally

Below ground level it is common to find blockwork.

Dense blocks and most aerated blocks are suitable for

use below the ground. Holes in the blockwork, with

lintels over for top for support, can be left for building

services (water will be lower).
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Modern brick/block cavity walls vary in thickness

depending on the nature of the cavity and the nature

of the inner leaf. The cavity will normally be 50mm,

75mm or even 100mm wide. The thickness of the inner

leaf depends on the type of blocks; 125mm aerated

blocks are quite common. The use of thicker

lightweight internal leaves or special aerated blocks

can preclude the need for additional insulation. See

the section on Building Regulations or Insulation for

current requirements. Click here for another example

of a modern cavity wall.

If bricks are used in the

outer leaf its thickness is

normally 102.5mm;

often referred to as

100mm or half-brick.

Where insulation is

required it can either be

in the cavity or in the

form of dry lining.

External insulation is

rare in new cavity walls.

Where cavities are filled

or partially filled goodworkmanship is vital to

prevent rain

penetration. Follow the

tips in the bullet points

below.

Ensure the cavity is kept clean Make sure all joints are well filled with mortar Use a tooled joint which compresses mortar

and forms a good seal with the brick

Ensure wall ties are level (or slope outwards)and keep ties clean Provide cavity trays (with stop ends if

necessary) over lintels etc. which bridge the

cavity

Make sure DPC details are correct Make sure trapped water can escape through

weep-holes

Ensure cavity insulation is fitted in accordancewith manufacturer's recommendations.

Although cavity walls are formed in two leaves they

should be regarded as a single structural unit. Neither

leaf should normally be built over 1.35metres high

(about 6 courses of blockwork) on its own; single

leaves are more likely to suffer wind damage. Where

rigid ties are used this difference should not exceed 2
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ties are usually only suitable for cavities up to 75mm

wide. Vertical twist ties are suitable for cavities from

50mm to 175mm (but cavities greater than 100mm are

still rare).

The spacing of the ties should be 900mm horizontally and 450mm vertically (for

cavity widths of 50-75mm). Where cavities are 76-100mm wide the spacing should

be 750 horizontally and 450 vertically. Additional ties are required within 225mm

(Code of Practice - B. Regs requires 150mm but will soon be altered) of the jambs

of any openings. They should be no more than 300mm apart vertically which

effectively means every three courses of brickwork (1 course of blockwork). Extra

ties may be required to support insulation boards.

The ties should ideally slope slightly outwards to help

prevent water reaching the inner leaf. Where partial

cavity fill is specified a special tie incorporating a large

plastic retaining washer should be used. It is preferable

to leave a cavity of 50mm although 25mm may be

acceptable in sheltered situations. The tie should be

bedded at least 50mm in each leaf.

Remember: ties sloping inwards may encourage water

penetration, and mortar droppings may also be a

problem. Good practice should avoid both these

potential defects but they are not uncommon. Poor

work can be expensive to resolve.

These photos show partial insulation. On the left the

ties has been positioned but not the retaining washer -

this can be seen on the right. When using partial

insulation extra ties are required to support the

boards.
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6 Movement Joints

Most materials expand and contract throughout their

lives depending on their temperature and moisture

content. In addition, new clay brickwork will expand

slightly for several months as it slowly absorbsmoisture in the air until equilibrium is reached.

Calcium silicate and concrete bricks will shrink slightly

(like all cement or hydraulic-lime based materials).

Long runs of brickwork therefore need to be divided

into shorter panels to prevent this movement from

causing unsightly cracking.

Movement joints are generally every 12 to 15 metres

although they should be more frequent if concrete or

calcium silicate bricks are used. In free-standing walls

the spacing should be about half the above figures (ie

about 6 to 8 metres). The joints can be hidden in a

variety of ways or may even form part of the overall

design. The one on the right is quite noticeable, the

one on the left less so. It is quite common to find them

hidden behind down pipes (below right) although the

pipe clips must not be fixed across the joint.

Movement joints are filled with a soft, compressible

material and then sealed against water penetration.
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NB. The sealer should

not stick to the filler -

this would reduce its

ability to flex.

There are a few key factors to ensure success:

Keep movement joints free from mortar Make sure brick face against movement joint

has properly filled joints.

Keep the joint width constant Keep the joint vertical Keep the bricks either side of the joint to the

same course heights.

Joint filler must be compressible The joint width in mm should be at least the

joint spacing in metres plus 30%. So joints at

12m equals a joint width of16mm.

Spacing from a corner of a building to the firstmovement joint should be 50% of the normal

joint spacing.

One method is to build-in a joint filler as work proceeds. The joint filler must be

semi-rigid material with the right amount of 'compressibility'. Hemp, fibre board

and cork will not allow enough movement. The filler should be the full depth of the

wall and should be cut back when the wall is complete to allow space for the

flexible sealant which will weatherproof the joint. The width of the gap depends on

a number of factors including the nature of the bricks, the size of the panel and the

qualities of the sealer. Sealants are usually polysulphides or silicones. They are

usually applied by 'gun' and 'tooled' to compact the sealant and ensure it adheres

to the sides of the bricks. The sealer should not stick to the filler - this would

reduce its ability to flex.

Cautionary Note

Experience shows that using this method can lead to

problems. As mortar is squeezed out of the brick joints

either side of the filler it compresses the filler and may

reduce the width of the joint. This obviously limits the

ability of the panels to expand. The picture on the right

shows how mortar, squeezed against the filler, has

actually reduced the effective width of the movement

joint.
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Alternative Method

Another method uses a piece of timber temporarily

built into the wall. The timber should be the full depth

of the wall. The timber can be raised with the

brickwork as shown on the right. When the wall is

complete the timber can be removed, the gap cleaned,

out and the filler inserted. It should be inserted so that

the applied sealant will be of the correct depth. Again,

the sealant should not stick to the filler. In both

methods care must be taken when applying the

sealant. Masking tape can sometimes be fixed to the

brick face either side of the joint to keep it clean. If

sealant spreads onto the bricks it can be difficult to

remove.

7 Cavity Wall Tie Failure

clickherefor video clip

Perhaps the most significant defect associated with

cavity walls is wall tie failure. Failure usually occurs

through rusting of the ties. Rust can fracture the ties

and this can result in redistributed loading.

Unrestrained brickwork can bow or bulge. Rusting ties

can also cause expansion of the joints which further

encourages damp penetration. Expanding joints can

ultimately lift the edge of the roof. Houses most at risk

are those built in the 1920s and 1930s and those builtduring the 1970s (when standards of galvanising were

temporarily reduced).

In the 1920s mild steel ties were generally

unprotected, bitumen coated or galvanised. This

galvanised, vertical twist tie has corroded leaving the

steel free to rust. When the ties corrode they are likely

to expand and force the bed joints apart. However,

wire ties (below right), because of their small cross

section, can sometimes fail without the associated 'tell

tale' expansion of the horizontal bed joints.
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Modern galvanised ties are based on a British Standard

set in 1981. Ties manufactured to this standard should

have at least a 60 year life. Galvanised ties are coated

in zinc. The thicker the zinc the better the protection.

In damp conditions, even if the coating is scratched,

sacrificial corrosion of the zinc protects the steel. Over

time, the area of exposed steel increases to a point

where it can't be protected and it starts to rust. As rust

forms the volume of the tie increases. The speed at

which rust occurs depends on the type of tie,

saturation of the wall, nature of the mortar and

proximity to marine climates.

Corrosion of the galvanised layer is aggravated by the

use of black ash mortars with a high sulfur content.

When wet for long periods a weak sulfuric acid is

created which attacks the zinc coating. Once the zinc

has worn away the tie can fail quite quickly,

particularly the end in the outer leaf of brickwork. The

need to remove the existing ties will depend on the

nature of the mortar, the level of corrosion, and the

type of tie. On these houses the use of thick twist ties

and aggressive ash mortar dictates complete removal.

Wire ties can often be left in place.

clickherefor video clip

of wall tie repair

There are several types of replacement tie. A stainless

steel tie with expanding metal sleeves is shown on the

right. Expanding neoprene sleeves are also common.

The one on the left has a twisted inner section for resin

fixing in the inner leaf (expanding ties can damage light

weight blocks). Tie choice, therefore, depends on a

number of factors including nature of the inner and

outer leaf, tie strength and stiffness, experience, and

cost.
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On this housing estate the inner leaf is a soft brick,

often unsuitable for expanding ties, so the inner part

of the tie is designed for resin fixing (see photo above

left). Once drilled the hole in the inner leaf needs

cleaning out to ensure a good bond for the resin. A two

part resin is then applied to the inner leaf and the tie

inserted. The outer, expanding part, of the tie will be

tightened several hours later.

Typical cost for a 3 bed

house is between 3,000and 6,000.

Where wire ties have corroded without damaging the external leaf they can

sometimes be left in place. In these cases simpler solutions are possible. One

method developed by Halfen Unistrut involves drilling the bed joints and inserting

stainless steel Spiro ties (right). When the hole has been drilled and cleaned out

resin is applied to the inner leaf. The tie is then pushed home and resin is applied

to the outer leaf.

This wall tie system involves minimum disruption to the occupiers and

causes minimum damage to the wall. In addition, repointing the drilled holes

is a simple exercise.

Once the house has been re-pointed the work is

completely hidden. It's not quite as easy to disguise a

rendered wall. Painting the render will help but it will

not completely hide the repairs - and painting render,

particularly rough-cast, is a life sentence!

8 Modern Cavity Walls -Jambs

At the jambs, ie the sides of the

opening, cavities are usually closed to

provide a good fixing for the frame

and a good base for the internal

plaster or dry lining. The example on

the left shows a cavity wall with the

inner leaf of dense blockwork

returned to the outer leaf of brick.

This detail was common until the

1980s - a better detail is to extend the

vertical DPC by 25mm into the cavity

(right). Note that by the late 1980s
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most internal leaves were being built

in lightweight rather than dense

blocks.

The DPC mentioned above is the mostcommon way to prevent water

crossing the cavity at the jamb. It's

normal practice to return the

blockwork to the brickwork rather

than the other way round - it's

cheaper and provides better weather

protection; water dripping off the side

of the DPC cannot touch the inner

leaf. The blocks can be 'specials' or

off-cuts (shown on the right). The

jamb DPC should be positioned to

overlap any horizontal DPC at the sill

and be overlapped by any cavity tray

at the head. Clickhereto see an

example of Ruberoid's DPC system for

openings in cavity walls.

The top examples are similar in that

the window is set forward in thefacework, this means that the external

reveal is quite narrow. This has

aesthetic, maintenance, and

environmental implications. Narrow

reveals make the walls look thin,

there are increased chances of rain

penetration and potentially a greater

likelihood of condensation caused by

cold bridging. These pictures also

show narrow external reveals; thewindows on the left are new

standard, metric units, those on the

left are replacement PVC.
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The risk of cold bridging should not be

ignored and a range of simple

precautions can prevent its

manifestation. The problem occurs

because of the cold bridge formed just

behind the frame (arrow on the left).

This may lower the blockwork

temperature below the dewpoint and

create a cold spot for condensation. If

the blocks are lightweight it is less

likely to occur. The temperature of

the inside surface can usually be

maintained above the dew point by

insulating the inner reveal with 25mm

or so of insulation board or by placinga strip of insulation between the

vertical DPC and the blockwork (both

shown right).

Another option is to position the

frame deeper inside the jamb. Just re-

locating it 25mm or so may eliminate

any cold bridging. Setting the frame

deeper inside the jamb will also

provide improved weather protection.

However, many windows have small

sill sections and moving the window

may require sub sills. Sub sills can be

quite expensive and are obviously

unlikely to be popular with the

volume house builders. However, long

term they may be cost effective. Look

at the sill pages for more information.

It is important to prevent any airleakage around the window. This

may require external sealants and

expanding foam or mineral woolpacking where the frame meets the

jamb (right).
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An even more radical approach (in

England - it's pretty much standard

practice in Scotland!) is to use check

reveals. In a check reveal a small

rebate is formed by running the

brickwork beyond the blockwork at

the jamb. This rebate protects the

frame and at the same time

eliminates cold bridging. Traditionally,

sash windows were always set in

rebates although, of course, they

would originally have been fixed to

solid, not cavity, walls.

An entirely different approach is use

extruded plastic sections. These avoid

the need for a separate vertical DPC

and preclude the need for messy

block cutting. 'Dacatie' (shown here) is

a common product and it's available

in a wide range of patterns, some of

which contain insulation. The sections

are nailed or screwed to the frame

and built in as work proceeds. These

could be used as an alternative to

check reveals for those parts of

England and Wales subject to severe

exposure.

A fairly recent product is shown here.

It's a foil backed 'bubble wrap'. It can

be used as a cavity insulation and as a

vertical DPC around jambs. It requires

special wall ties to hold the foilagainst the inner leaf and to make

sure that the upper sheets overlap

those below.
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The picture on the left shows another

method of forming the opening. The

white plastic extrusion helps the

bricklayers get the jambs plumb. The

extrusion is kept rigid by steel bar(s0.

These are eventually removed and the

window slotted into place inside the

extrusion. Click here for another

example.

9 Modern Cavity Walls - Lintels

Nowadays most lintels are made from galvanised steel

(there may be additional protection systems in the

form of polyesters coatings and separate cavity trays).There are two basic patterns generally available for

cavity walls - box sections (left) and open backed (right

below). Lintels generally are available in lengths of up

to 4500mm and should normally have at least 150mm

bearing on the wall either side. They are designed to

carry the distributed load of brickwork - not point

loads. In fact, the load is not as high as one might

imagine because of the bonding nature of the

brickwork. Look at the graphic (right). The lintel is only

carrying a small triangle of brickwork directly as therest of the brickwork is supported by the corbelling

action of the bonding.

These two images show modern lintel details. Note

that modern lintels contain insulation, partly to

keep energy wastage low but also to eliminate coldbridging and condensation. This is explained at the

bottom of the page. Some lintels are made from

light-weight concrete. These are usually positioned

on the inner leaf and have a separate metal traycarrying the outer leaf. Some lintels require

separate cavity trays - the need for this depends on

manufacturing techniques and/or exposure.
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The lintel type shown in the top left picture was

popular throughout the 1960s and 1970s. It's a Catnic

box section lintel - made in a variety of lengths and

sizes to suit most openings. Catnic still make lintels

and, nowadays, most of them contain insulation. These

lintels are light, easy to carry and don't normally

require a separate cavity tray (these are normally only

required in areas of severe or very severe exposure -

west of Bristol, Cumbria, most of Scotland). Catnic also

produce other lintel patterns. If the cavity work

includes an arch slightly different construction is

required. The right-hand image shows a solution from

Ruberoid. It uses a pre-formed cavity tray to prevent

damp penetration - this is explained below.

In modern construction a cavity tray is required to

direct any water running down the cavity away

through the external leaf - via weepholes (left).

Weepholes, usually every 450mm or so, can be plastic

inserts or open perp joints. As stated above some

lintels need a separate tray - this is usually formed

from polythene. The cavity tray usually sits on top of

the lintel although some companies produce details

showing the cavity tray a few courses above it (left).

Stop ends to lintels (or the cavity tray over it) have

always been recommended if cavities are filled with

insulation. The latest Approved Document C (2004)

does not specifically mention stop ends (or weep holes

for that matter) over window openings. However, the

appropriate Code of Practice recommends their use.

Although lintels only generally require 150mm

bearing either side of the opening they should be

supported on a full concrete block (it's the inner

leaf which carries most load). Failure to do thismay result in unsightly cracking in the plaster

either side of the window. The lintel on the right issupported by a small piece of block.

Click here to see a simple graphic showing DPCdetail around window.
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The cold bridge (left) can often be avoided by using an

insulated lintel as shown on the right or by using a

lintel without a continuous lower web. Another option

is to move the window frame deeper into the opening.

If the frame sits directly under the cavity the cold

bridging effect is reduced although there will be

implications for the window sill - in most cases a

separate subsill will be required. Cold bridges can be

very difficult to identify and resolve. Condensation,

due to cold bridging, is often mistaken for damp

penetration - 'solutions' may be expensive and wholly

inappropriate if diagnosis is incorrect. Note that many

manufacturers of lintels show cavity insulation

stopping at the top of the lintel (below examples). The

BRE, however, in Thermal Insulation - Avoiding Risks,2002, suggest it should be cut to fit the lintel profile as

in the left and right-hand examples.

If the window cannot be moved deeper into the reveal

another option is to insulate the soffit. In this example

insulation board is fixed to the bottom of the lintel

thus eliminating the cold bridge. This is probably

cheaper than moving the window frame deeper into

the reveal although the extra protection offered to the

frame by moving it inwards may pay-off in the long

term. Most modern, box-section lintels are available

with an insulation fill - they will help avoid cold

bridging although consideration should still be given to

insulating the soffit. In addition, where lintels have a

continuous lower web, it is often perforated to help its

thermal efficiency. Note that lightweight concrete

lintels (which sit over the internal leaf) are also

available. These often have a steel tray to support the

outer leaf.

It's also important to note that in modern construction

steps must be taken to avoid air leakage. This may

require sealants around the window frame, inside and

out. Click here for a modern example of a window

opening showing suitable details at jambs, head and


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sill.

10 Modern Cavity Walls - Sills

Sill details depend on a number of factors. Perhaps themost common detail in housing is the integral sill -

where the sill forms part of the window frame. The sill,

which can be hardwood or softwood, is bedded in

mortar onto the external leaf of brickwork. A groove in

the sill helps secure it in position. It is not normally

fixed (windows are normally fixed at the jambs only). A

typical detail from the 70s and 80s is shown on the

right. This form of construction is not regarded as good

practice today because of the risk of cold bridging (see

below)

Sections through windows look, on first sight, fairly

complex. Consider the design of the on the left. The

groove in the back (far left) is to take the window

board, the groove in the bottom is the bedding groove

and the groove at the front is a drip - to help prevent

water running back under the sill. The drip encourages

a build-up of water. The weight of the drip eventually

overcomes the water's surface tension and it drops to

the ground. The drip must therefore be positioned well

clear of the wall. In addition, sills often have additional,

small, inverted 'V' shaped grooves underneath to help

prevent the timber from twisting.

To provide better support for tiling (kitchens andbathrooms) it was common practice in the 1960s to1980s to close the cavity by using a cut block

(right). This obviously bridges the cavity and, to

prevent damp penetration, a DPC was required.

This construction could also be used below awindow board.
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In modern construction, with well insulated cavities,

there are risks of cold bridging where cavities are

closed. The graphics (left) show a range of modern

options. A vertical DPC, ie between the brick and block,

is required if the cavity is closed; some authorities say

that a DPC under the window only needs to be used

where sills are permeable or composed of sections.

A sub sill may be required with the windowsshown on the left if they are set deeper into the

jamb (the drip will not clear the wall). Sub sills can

be formed from special bricks, stone, concrete or

plain tiles (see photos below).

In exposed situations, particularly where cavities

are filled, it is nowadays recognised as goodpractice to provide stop ends to sill DPCs and

cavity trays. Where there is a sill DPC it should belapped with the jamb DPC as shown here. Where

there is no sill DPC the jamb DPCs should be

continued 150mm below sill level.

Mastic beading is not normally necessary around

the window for weather protection. The NHBC

recommend it in very exposed situations. However,it is important to prevent any air leakage around

the window. This may require internal and externalsealants and expanding foam or mineral woolpacking where the frame meets the surround.

Clickhere for a modern window opening showing

jambs, sill and head.

These two photographs show examples of sub sills.

They are typical of some of the speculative

construction that took place in the mid 1980s. The

work is not a very high standard but the nature of the

construction is clear enough. Click here to see a

modern window with a concrete sub sill.

Clickhereto see an example of Ruberoid's DPC

system for windows in cavity walls.
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11 Modern Cavity Walls - Parapets

Parapet walls require careful detailing. At the top of the wall it is good

practice to provide a weathered coping. The coping can be once or twice

weathered - in other words it can slope in one direction as shown in the

picture (below left) or in both directions. A once weathered copingnormally directs the water onto the roof to avoid water running down the

external face. In some designs brick copings are used although careful

detailing may be required if the copings don't have drips (sometimes

called throatings).

A good coping stone will overhang the wall either side and will incorporate

small drips to prevent water running back under the coping. A full-width

DPC should be bedded in mortar to prevent water penetrating the coping

through the coping joints. Unlike the example on the left the DPC shouldbe laid on a rigid support to prevent it sagging into the cavity and allowing

water to pond where it may freeze and expand in cold weather. Any

sagging may also form a trough and allow water to penetrate the cavity

where the DPC is lapped.

Because parapet walls are exposed on both sides a cavity tray is required

to prevent water running down the cavity face of the inner leaf and

penetrating the building. The example on the left shows the tray stepping

down to the outer face; water escapes through weepholes. Although this

may cause minor staining of the wall it is sometimes preferable to sloping

the tray inwards. This is because sloping the tray inwards may allow

rainwater to run along the underside of the tray and reach the internal

leaf. Click on the picture for an example. In moderate or sheltered

exposures this is not normally an issue. However, if the cavity contains

cavity batts the tray should always slope outwards to protect the top of

the insulation. If it slopes inwards there is the risk of water running down

the cavity face of the external leaf and crossing the cavity on top of the

batts.
https://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/window3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/parapet1.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/parapet3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/parapet1.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/parapet3.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/parapet1.jpghttps://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/parapet3.jpg


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Flashings should be in the same joint as the cavity tray (DPC) and always

under them to prevent the risk of water ingress.
https://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/parapet2.jpg

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