UWE

Cavity Walls

Contents

1 Introduction

2 Early Cavity Walls

3 Early Cavity Walls - Head, Sill and Jamb

4 Modern Cavity Walls - Generally

5 Modern Cavity Walls - Wall Ties

6 Movement Joints

7 Cavity Wall Tie Failure

8 Modern Cavity Walls - Jambs

9 Modern Cavity Walls - Lintels

10 Modern Cavity Walls - Sills

11 Modern Cavity Walls - Parapets

1 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 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). Click here for a drawing showing a Victorian cavity wall (taken from  an 1898 textbook).
To see a short video clip explaining the problems and demise of the solid wall, and the introduction of the cavity wall click 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 were sometimes 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.
Click here to see an example of modern bricklayers in action. The basic technique is the same nowadays as it was 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 for internal 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.
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.
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 the cavity 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). 
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 good workmanship is vital to prevent rain penetration. Follow the tips in the bullet points below.

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  • 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 accordance with 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 blocks (rigid ties are less tolerant of difference in bed joint thickness and trying to bend them will dislodge masonry.) Details on the spacing of ties can be found elsewhere on this site. The picture on the right shows an internal leaf which has been 'lifted' a bit high.
In most houses the external leaf is laid in stretcher bond. This is the part of the wall which is on view but what about the wall behind? Blockwork is by far the most common method of forming the internal leaf. Like brickwork blocks have to be laid in a bond to ensure its stability. At quoins the construction can vary. There are three examples shown here of forming quoins to ensure that there is a good bond in the 'run' of the wall. In no case should the bond, ie, the lap, be less than one quarter of a block's length. If the internal leaf is built in lightweight blocks, bricks should not be used to 'make-up' the bond. Cracking may occur at the interface of the two materials and the differing insulation values can lead to pattern staining and possible condensation.     
Where window openings occur the cavity is normally closed. This usually requires a vertical DPC. More information is contained in the section on Jambs. Where internal walls connect with the inner leaf of external walls they can either be bonded as shown here (right) or connected with ties.

5 Modern Cavity Walls - Wall Ties

There are a variety of wall ties on the market. In recent years ties have been made from steel, either galvanised or stainless, or plastic (right). All ties incorporate drips to prevent water crossing the cavity. They are available in various lengths to suit different cavity widths. In the 2004 Regs (Approved Document A) there is a requirement to use stainless steel ties (others may be acceptable if they have British Board of Agrement approval). Butterfly and double triangular 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.

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 absorbs moisture 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.
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 first movement joint should be 50% of the normal joint spacing.

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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.

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

click here for 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 built during 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.
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.
tief9.jpg (67775 bytes) 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.
click here for 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. 
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 3,000 and 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 most internal leaves were being built in lightweight rather than dense  blocks.
The DPC mentioned above is the most common 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. Click here to 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 the facework, 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; the windows on the left are new standard, metric units, those on the left are replacement PVC.
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 placing a 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 air leakage around the window. This may require external sealants and expanding foam or mineral wool packing where the frame meets the jamb (right).

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 foil against the inner leaf and to make sure that the upper sheets overlap those below. 
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 the rest 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 cold bridging 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 tray carrying the outer leaf. Some lintels require separate cavity trays - the need for this depends on manufacturing techniques and/or exposure.

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 this may result in unsightly cracking in the plaster either side of the window. The lintel on the right is supported by a small piece of block. 

Click here to see a simple graphic showing DPC detail around window.

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 sill.  

10 Modern Cavity Walls - Sills

Sill details depend on a number of factors. Perhaps the most 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 and bathrooms) it was common practice in the 1960s to 1980s 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 a window board.

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 windows shown 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 good practice to provide stop ends to sill DPCs and cavity trays. Where there is a sill DPC it should be lapped 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 external sealants and expanding foam or mineral wool packing where the frame meets the surround. Click here 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.

Click here to see an example of Ruberoid's DPC system for windows in cavity walls.

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 coping normally 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 should be 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. 
Flashings should be in the same joint as the cavity tray (DPC) and always under them to prevent the risk of water ingress.

©2006 University of the West of England, Bristol
except where acknowledged