• Head of openings in solid walls

    Solid brickwork over the head of openings has to be supported by either a lintel or an arch. The brickwork which the lintel or arch has to support is an isosceles triangle with 60° angles, formed by the bonding of bricks, as illustrated in Fig. 89. The triangle is formed by the vertical joints between bricks which overlap ±B. In a bonded wall if the solid brickwork inside the triangle were taken out the load of the wall above the triangle would be transferred to the bricks of each side of the opening in what is termed ‘the arching effect’.

    Lintel is the name given to any single solid length of timber, stone, steel or concrete built in over an opening to support the wall over it, as shown in Fig. 89. The ends of the lintel must be built into the brick or blockwork over the jambs to convey the weight carried by the lintel to the jambs. The area of wall on which the end of a lintel bears is termed its bearing at ends. The wider the opening the more weight the lintel has to support and the greater its bearing at ends must be to transmit the load it carries to an area capable of supporting it. For convenience its depth is usually made a multiple of brick course height, that is about 75 mm, and the lintels are not usually less than 150 mm deep.

    Fig. 89 Head of openings

  • External weathering to walls of brick and block

    In exposed positions such as high ground, on the coast and where there is little shelter from trees, high ground or surrounding buildings it may well be advisable to employ a system of weathering on the outer face of both solid and cavity walling to provide protection against wind driven rain. The two systems used are external rendering and slate or tile hanging.


    The word rendering is used in the sense of rendering the coarse texture

    of a brick or block wall smooth by the application of a wet mix of lime, cement and sand over the face of the wall, to alter the appear­ance of the wall or improve its resistance to rain penetration, or both. The wet mix is spread over the external wall face in one, two or three coats and finished with either a smooth, coarse or textured finish while wet. The rendering dries and hardens to a decorative or protective coating that varies from dense and smooth to a coarse and open texture.

    Stucco is a term, less used than it was, for external plaster or rendering that was applied as a wet mix of lime and sand, in one or two coats, and finished with a fine mix of lime or lime and sand, generally in the form imitating stone joints and mouldings formed around projecting brick courses as a background for imitation cornices and other architectural decorations. To protect the com­paratively porous lime and sand coating, the surface was usually painted.

    The materials of an external rendering should have roughly the same density and therefore permeability to water as the material of the wall to which it is applied. There are many instances of the application of a dense rendering to the outside face of a wall that is permeable to water, in the anticipation of protecting the wall from rain penetration. The result is usually a disaster.

    A dense sand and cement rendering, for example, applied to the face of a wall of porous bricks, will, on drying, shrink fiercely, pull away from the brick face or tear off the face of the soft bricks, and the rendering will craze with many fine hair cracks over its surface. Wind driven rain will then penetrate the many hair cracks through which water will be unable to evaporate to outside air during dry spells and the consequence is that the wall behind will become more water logged than before and the rendering will have a far from agreeable appearance.

    Slate and tile hanging

    Fig. 85 Slate hangingIn positions of very severe exposure to wind driven rain, as on high open ground facing the prevailing wind and on the coast facing open sea, it is necessary to protect both solid and cavity walls with an external cladding. The traditional wall cladding is slate or tile hanging in the form of slates or tiles hung double lap on timber battens nailed to counter battens. Slate hanging has generally been used in the north and tile in the south of Great Britain. Either natural or manufactured slates and tiles can be used.

    As a fixing for slating or tiling battens, 50 x 25 mm timber counter battens are nailed at 300 mm centres up the face of the wall to which timber slating or tiling battens are nailed at centres suited to the gauge (centres) necessary for double lap slates or tiles, as illustrated in Fig. 85.

    As protection against decay, pressure impregnated softwood timber battens should be used and secured with non-ferrous fixings to avoid the deterioration and failure of steel fixings by rusting.

    Where slate or tile hanging is used as cladding to a solid wall of buildings normally heated, then the necessary insulation can be fixed to the wall behind the counter battens. Rigid insulation boards of organic or inorganic insulation are fixed with a mechanically oper­ated hammer gun that drives nails through both the counter battens, a breather paper and the insulation boards into the wall.

    For vertically hung slating it is usual to use one of the smaller slates such as 405 x 205 mm slate which is headnailed to 50 x 25 mm bat­tens and is less likely to be lifted and dislodged in high wind than longer slates would be. Each slate is nailed with non-ferrous nails to overlap two slates below, as illustrated in Fig. 85, and double lapped by overlapping the head of slates two courses below.The continuous layer of breather paper, that is fixed between the counter battens and the insulation, is resistant to the penetration of water in liquid form but will allow water vapour to pass through it. Its purpose is to protect the outer surface of the insulation from cold air and any rain that might penetrate the hanging and to allow movement of vapour through it.

    At angles and the sides of openings a slate one and a half the width of slates is used to complete the overlap. This width of slate is specifically used to avoid the use of a half width slate that might easily be displaced in wind.

    Fig. 86 Tile hanging

    Internal and external angles are weathered by lead soakers – hung over the head of slates – to overlap and make the joint weathertight. Slate hanging is fixed either to overlap or butt to the side of window and door frames with ‘exposed edges of slates pointed with cement mortar or weathered with lead flashings.

    At lower edges of slate hanging a projection is formed on or in the wall face by means of blocks, battens or brick corbel courses on to which the lower courses of slates and tiles bell outwards slightly to throw water clear of the wall below.

    Tile hanging is hung and nailed to 40 x 20 mm tiling battens fixed at centres to counter battens to suit the gauge of plain tiles. Each tile is hung to battens and also nailed, as security against wind, as illustrated in Fig. 86.

    At internal and external angles special angle tiles may be used to continue the bond around the corner, as illustrated in Fig. 86. As an alternative and also at the sides of openings tile and a half width tiles may be used with lead soakers to angles and pointing to exposed edges or weathering to the sides of the openings.

    As weather protection to the solid walls of buildings with low or little heat requirements the hanging is fixed directly to walling and to those buildings that are heated the hanging may be fixed to external or internal insulation for solid walling and directly to cavity walling with cavity insulation.



    Up to the early part of the twentieth century walls were generally built as solid brickwork of adequate thickness to resist the penetration of rain to the inside face and to safely support the loads common to buildings both large and small.

    At the time it was accepted that the interior of buildings would be cold during winter months when heating was provided by open fires and stoves, fired by coal or wood, to individual rooms. The people of northern Europe accepted the inevitability of a degree of indoor cold and dressed accordingly in thick clothing both during day and night  time. There was an adequate supply of coal and wood to meet the expectations for some indoor heating for the majority.

    The loss of heat through walls, windows and roofs was not a concern at the time. Thick curtains drawn across windows and external doors provided some appreciable degree of insulation against loss of heat.

    From the middle of the twentieth century it became practical to heat the interior of whole buildings, with boilers fired by oil or gas. It is now considered a necessity to be able to heat the whole of the interior of dwellings so that the commonplace of icy cold bathrooms and corridors is an experience of the past.

    In recent years an industry of scare stories has developed. Ill con­sidered and unscientific claims by ‘experts’ that natural resources of fossil fuels such as oil and gas will soon be exhausted have been broadcast. These dire predictions have prompted the implementation of regulations to conserve fuel and power by introducing insulating materials to the envelope of all new buildings that are usually heated.

    This ‘bolting the stable door after the horse has gone’ action will for very many years to come only affect new buildings, a minority of all buildings.

    A consequence is that the cavity in external walls of buildings, originally proposed to exclude rain, has been converted to function as a prime position for lightweight insulating materials with exclusion of rain a largely ignored function of a cavity wall.

    Resistance to weather

    A solid wall of brick will resist the penetration of rain to its inside face

    by absorbing rainwater that subsequently, in dry periods, evaporates to outside air. The penetration of rainwater into the thickness of a solid wall depends on the exposure of the wall to driving rain and the permeability of the bricks and mortar to water.

    The permeability of bricks to water varies widely and depends largely on the density of the brick. Dense engineering bricks absorb rainwater less readily than many of the less dense facing bricks. It would seem logical, therefore, to use dense bricks in the construction of walls to resist rain penetration.

    In practice, a wall of facing bricks will generally resist the penetration of rainwater better than a wall of dense bricks. The reason for this is that a wall of dense bricks may absorb water through fine cracks between dense bricks and dense mortar, to a considerable depth of the thickness of a wall, and this water will not readily evaporate through the fine cracks to outside air in dry periods, whereas a wall of less dense bricks and mortar will absorb water to some depth of the thickness of the wall and this water will sub­stantially evaporate to outside air. It is not unknown for a wall of dense bricks and mortar to show an outline of damp stains on its

    inside face through persistent wetting, corresponding to the mortar joints.

    The general rule is that to resist the penetration of rain to its inside face a wall should be constructed of sound, well burned bricks of moderate density, laid in a mortar of similar density and of adequate thickness to prevent the penetration of rain to the inside face.

    A solid 1 B thick wall may well be sufficiently thick to prevent the penetration of rainwater to its inside face in the sheltered positions common to urban settlements on low lying land. In positions of moderate exposure a solid wall \\ B thick will be effective in resisting the penetration of rainwater to its inside face.

    In exposed positions such as high ground and near the coast a wall 2 B thick may be needed to resist penetration to inside faces. A wall 2 B thick is more than adequate to support the loads of all but heavily loaded structures and for resistance to rain penetration a less thick wall protected with rendering or slate or tile hanging is a more sensible option.


  • Cavity wall insulation

    Partial fill


    The purpose of the air space in a cavity wall is as a barrier to the
    penetration of rainwater to the inside face of the wall. If the clear air space is to be effective as a barrier to rain penetration it should not be
    bridged by anything other than cavity ties. If the cavity is then filled with some insulating material, no matter how impermeable to water the material is, there will inevitably be narrow capillary paths around wall ties and between edges of insulation boards or slabs across which water may penetrate. As a clear air space is considered necessary as a barrier to rain penetration there is good reason to fix insulation material inside a cavity so that it only partly fills the cavity and a cavity is maintained between the outer leaf and the insulating mate¬rial. This construction, which is described as partial fill insulation of cavity, requires the use of some insulating material in the form of boards that are sufficiently rigid to be secured against the inner leaf of the cavity.
    In theory a 25 mm wide air space between the outer leaf and the cavity insulation should be adequate to resist the penetration of rain providing the air space is clear of all mortar droppings and other building debris that might serve as a path for water. In practice, it is difficult to maintain a clear 25 mm wide air gap because of protrusion of mortar from joints in the outer leaf and the difficulty of keeping so narrow a space clear of mortar droppings. Good practice, therefore, is to use a 50 mm wide air space between the outer leaf and the partial fill insulation.
    To meet insulation requirements and the use of a 100 mm cavity with partial fill insulation it may be economic to use a lightweight block inner leaf to augment the cavity insulation to bring the wall to the required U value.
    Usual practice is to build the inner leaf of the cavity wall first, up to the first horizontal row of wall ties, then place the insulation boards in position against the inner leaf. Then as the outer leaf is built, a batten may be suspended in the cavity air space and raised to the level of the first row of wall ties and the batten is then withdrawn and cleared of droppings. Insulation retaining wall ties are then bedded across the cavity to tie the leaves and retain the insulation in position and the sequence of operations is repeated at each level of wall ties.
    The suspension of a batten in the air space and its withdrawal and cleaning at each level of ties does considerably slow the process of brick and block laying.
    Insulation retaining ties are usually standard galvanised steel or stainless steel wall ties to which a plastic disc is clipped to retain the edges of the insulation, as illustrated in Fig. 83. The ties may be set in line one over the other at the edges of boards, so that the retaining clips retain the corners of four insulation boards.
    The materials used for partial fill insulation should be of boards, slabs or batts that are sufficiently rigid for ease of handling and to be retained in a vertical position against the inner leaf inside the cavity without sagging or losing shape, so that the edges of the boards remain close butted throughout the useful life of the building. For small dwellings the Building Regulations do not limit the use of combustible materials as partial fill insulation in a cavity in a cavity wall.
    To provide a clear air space of 50 mm inside the cavity as a barrier to rain penetration and to provide sufficient space to keep the cavity clear during building, an insulant with a low U value is of advantage if a nominal 75 mm wide cavity is formed between the outer and inner leaves.

    Fig. 83 Partial fill cavity insulation

    Insulation materials

    The materials used as insulation for the fabric of buildings may be

    grouped as inorganic and organic insulants.

    Inorganic insulants are made from naturally occurring materials that are formed into fibre, powder or cellular structures that have a high void content, as for example, glass fibre, mineral fibre (rock-wool), cellular glass beads, vermiculite, calcium silicate and magnesia or as compressed cork.

    Inorganic insulants are generally incombustible, do not support spread of flame, are rot and vermin proof and generally have a higher U value than organic insulants.

    The inorganic insulants most used in the fabric of buildings are glass fibre and rockwool in the form of loose fibres, mats and rolls of felted fibres and semi-rigid and rigid boards, batts and slabs of compressed fibres, cellular glass beads fused together as rigid boards, compressed cork boards and vermiculite grains.

    Organic insulants are based on hydrdocarbon polymers in the form of thermosetting or thermoplastic resins to form structures with a high void content, as for example polystyrene, polyurethane, iso-cyanurate and phenolic. Organic insulants generally have a lower U value than inorganic insulants, are combustible, support spread of flame more readily than inorganic insulants and have a comparatively low melting point.

    The organic insulants most used for the fabric of buildings are expanded polystyrene in the form of beads or boards, extruded polystyrene in the form of boards and polyurethane, isocyanurate and phenolic foams in the form of preformed boards or spray coatings.

    The materials that are cheapest, most readily available and used for cavity insulation are glass fibre, rockwool and EPS (expanded poly­styrene), in the form of slabs or boards, in sizes to suit cavity tie spacing. With the recent increase in requirements for the insulation of walls it may well be advantageous to use one of the somewhat more expensive organic insulants such as XPS (extruded polystyrene), PIR (polyisocyanurate) or PUR (polyurethane) because of their lower U value, where a 50 mm clear air space is to be maintained in the cavity, without greatly increasing the overall width of the cavity.

    Table 4 gives details of insulants made for use as partial fill to cavity walls.

    Insulation thickness

    A rough guide to determine the required thickness of insulation for a wall to achieve a U value of 0.45 W/m2K is to assume the insulant provides the whole or a major part of the insulation by using 30 mm thickness with a U value of 0.02, 46 with 0.03, 61 with 0.04, 76 with 0.05 and 92 with 0.06W/m2K.

    Table 4 Insulating materials Insulation thickness

    Total fill

    The thermal insulation of external walls by totally filling the cavity

    has been in use for many years. There have been remarkably few reported incidents of penetration of water through the total fill of cavities to the inside face of walls and the system of total fill has become an accepted method of insulating cavity walls.

    The method of totally filling cavities with an insulant was developed after the steep increase in the price of oil and other fuels in the mid-1960s, as being the most practical way to improve the thermal insulation of existing cavity walls. Small particles of glass or rock wool fibre or foaming organic materials were blown through holes drilled in the outer leaf of existing walls to completely fill the cavity.

    This system of totally filling the cavity of existing walls has been very extensively and successfully used. The few reported failures due to penetration of rainwater to the inside face were due to poor workmanship in the construction of the walls. Water penetrated across wall ties sloping down into the inside face of the wall, across mortar droppings bridging the cavity or from mortar protruding into the cavity from the outer leaf.

    From the few failures due to rain penetration it would seem likely that the cavity in existing walls that have been totally filled was of little, if any, critical importance in resisting rain penetration in the position of exposure in which the walls were situated. None the less it is wise to provide a clear air space in a cavity wherever practical, against the possibility of rain penetration.

    Where insulation is used to fill totally a nominal 50 mm wide cavity there is no need to use insulation retaining wall ties.

    With a brick outer and block inner leaf it is preferable to raise the outer brick leaf first so that mortar protrusions from the joints, sometimes called snots, can be cleaned off before the insulation is placed in position and the inner block leaf, with its more widely spaced joints is built, to minimise the number of mortar snots that may stick into the cavity. This sequence of opera­tions will require scaffolding on both sides of the wall and so add to the cost.

    Insulation that is built in as the cavity walls are raised, to fill the cavity totally, will to an extent be held in position by the wall ties and the two leaves of the cavity wall. Rolls or mats of loosely felted glass fibre or rockwool are often used. There is some likelihood that these materials may sink inside the cavity and gaps may open up in the insulation and so form cold bridges across the wall. To maintain a continuous, vertical layer of insulation inside the cavity one of the mineral fibre semi-rigid batts or slabs should be used. Fibre glass and rockwool semi-rigid batts or slabs in sizes suited to cavity tie spacing are made specifically for this purpose.

    As the materials are made in widths to suit vertical wall tie spacing there is no need to push them down into the cavity after the wall is built, as is often the procedure with loose fibre rolls and mats, and so displace freshly laid brick or blockwork. There is no advantage in using one of the more expensive organic insulants such as XPS, PIR or PUR that have a lower U value than mineral fibre materials for the total cavity fill, as the width of the cavity can be adjusted to suit the required thickness of insulation.

    The most effective way of insulating an existing cavity wall is to fill the cavity with some insulating material that can be blown into the cavity through small holes drilled in the outer leaf of the wall. The injection of the cavity fill is a comparatively simple job. The complication arises in forming sleeves around air vents penetrating the wall and sealing gaps around openings.

    When filling the cavity of existing walls became common practice, a foamed organic insulant, ureaformaldehyde, was extensively used. The advantage of this material was that it could be blown, under pressure, through small holes in the outer leaf and as the con­stituents mixed they foamed and filled the cavity with an effective insulant. This material was extensively used, often by operatives ill trained in the sensible use of the material. The consequence was that through careless mixing of the components of the insulant and careless workmanship, the material gave off irritant fumes when used and later, when it was in place, these entered buildings and caused considerable distress to the occupants. Approved Document D of the Building Regulations details provisions for the use of this material in relation to the construction of the wall and its suitability, the composition of the materials, and control of those carrying out the work. As a result of past failures this material is less used than it was.

    Glass fibre, granulated rockwool of EPS beads are used for the injection of insulation for existing cavity walls. These materials can also be used for blowing into the cavity of newly built walls.

    Table 5 gives details of insulants for total cavity fill.

    The required thickness of insulation can be taken from the two methods suggested for partial fill. In a calculation for total fill, the thermal resistance of the cavity is omitted.

    Thermal bridge

    A thermal bridge, more commonly known as a cold bridge in cold climates, is caused by appreciably greater thermal conductivity through one part of a wall than the rest of the wall. Where the cavity in a wall is partially or totally filled with insulation and the cavity is bridged with solid filling at the head, jambs or cill of an opening, there will be considerably greater transfer of heat through the solid filling than through the rest of the wall. Because of the greater transfer of

    heat through the solid filling illustrated in Fig. 84, the inside face of the wall will be appreciably colder in winter than the rest of the wall and cause some loss of heat and encourage warm moist air to con­dense on the inside face of the wall on the inside of the cold bridge. This condensation water may cause unsightly stains around openings and encourage mould growth.

    Thermal bridges around openings can be minimised by continuing cavity insulation to the head of windows and doors and to the sides and bottom of doors and windows.

    Of late an inordinate fuss has been made about ‘cold bridges’ as though a cold bridge was some virulent disease or a heinous crime.

    Solid filling of cavities around openings will allow greater transfer of heat than the surrounding insulated wall and so will window glass, both single and double, and window frames. To minimise heat transfer, cavity insulation should continue up to the back of window and door frames.

    Where solid filling of cavities around openings is used the area of the solid filling should be included with that of the window and its frame for heat loss calculation.

    Fig. 84 Thermal bridge Table 5 Insulating materials



  • Head of openings in cavity walls

    The brickwork and blockwork over the head of openings in cavity walls has to be supported. Because of the bonding of brickwork and blockwork over the opening it is necessary to provide support for the weight of the brickwork or blockwork within a 45° isosceles triangle formed by the stretcher bond and the weight of floors and roofs carried by the wall over the opening.

    The comparatively small loads over small openings are carried by a lintel or arch. With the adoption of cavity insulation as a principal method of enhancing the resistance of walls to the transfer of heat and the need to continue cavity insulation up to the back of window and door frames to minimise cold bridges, it is practice today to use lintels to support the inner and outer leaves over openings.

  • Cills and thresholds of openings

    A cill is the horizontal finish to the wall below the lower edge of a window opening on to which wind driven rain will run from the hard, smooth, impermeable surface of window glass. The function of a cill is to protect the wall below a window. Cills are formed below the edge of a window and shaped or formed to slope out and project beyond the external face of the wall, so that water runs off. The cill should project at least 45 mm beyond the face of the wall below and have a drip on the underside of the projection.
    The cavity insulation shown in Fig. 77 is carried up behind the stone cill to avoid a cold bridge effect and a dpc is fixed behind the cill as a barrier to moisture penetration.
    A variety of materials may be used as a cill such as natural stone, cast stone, concrete, tile, brick and non-ferrous metals. The choice of a particular material for a cill depends on cost, availability and to a large extent on appearance. Details of the materials used and the construction of cills are given in Volume 2.
    As a barrier to the penetration of rain to the inside face of a cavity wall it is good practice to continue the cavity up and behind the cills as illustrated in Volume 2. Where cills of stone, cast stone and con¬crete are used the cill may extend across the cavity. As a barrier to rain penetration it has been practice to bed a dpc below these cills and extend it up behind the cill, as illustrated in Volume 2. Providing the cill has no joints in its length, its ends are built in at jambs and the material of the cill is sufficiently dense to cause most of the rainwater to run off, there seems little purpose in these under sill dpcs or trays.
    The threshold to door openings serves as a finish to protect a wall or concrete floor slab below the door, as illustrated in Volume 2. Thresholds are commonly formed as part of a step up to external doors as part of the concrete floor slab with the top surface of the threshold sloping out. Alternatively, a natural stone or cast stone threshold may be formed.

    Fig. 77 Jamb lining to wide cavity

  • Openings in walls

    The practical guidance in Approved Document A in regard to openings in walls states that the number, size and position of openings should not impair the stability of a wall to the extent that the com­bined width of openings in walls between the centre line of buttressing walls or piers should not exceed two-thirds of the length of that wall together with more detailed requirements limiting the size of opening and recesses. There is a requirement that the bearing end of lintels with a clear span of 1200 mm or less may be 100 mm and above that span, 150 mm.

    Figure 73 is an illustration of a window opening in a brick wall with the terms used to describe the parts noted.

    For strength and stability the brickwork in the jambs of openings has to be strengthened with more closely spaced ties and the wall over the head of the opening supported by an arch, lintels or beams. The term jamb derives from the French word jambe, meaning leg. From Fig. 73, it will be seen that the brickwork on either side of the opening acts like legs which support brickwork over the head of the opening. The term jamb is not used to describe a particular width either side of openings and is merely a general term for the brickwork for full height of opening either side of the window. The word ‘reveal’ is used more definitely to describe the thickness of the wall revealed by cutting the opening and the reveal is a surface of brickwork as long as the height of the opening. The lower part of the opening is a cill for windows or a threshold for doors.

    Fig. 74 Solid closing of cavity at jambs

    The jambs of openings may be plain or square into which the door or window frames are built or fixed or they may be rebated with a recess, behind which the door or window frame is built or fixed.

    The cavity in a cavity wall serves to prevent penetration of water to the inner leaf. In the construction of the conventional cavity wall, before the adoption of cavity insulation, it was considered wise to close the cavity at the jambs of openings to maintain comparatively still air in the cavity as insulation. It was practice to build in cut bricks or blocks as cavity closers. To prevent penetration of water through the solid closing of cavity walls at jambs, a vertical dpc was built in as illustrated in Fig. 74. Strips of bitumen felt or lead were nailed to the back of wood frames and bedded between the solid filling and the outer leaf as shown
    As an alternative to solidly filling the cavity at jambs with cavity
    closers, window or door frames were used to cover and seal the cavity.
    Pressed metal subframes to windows were specifically designed for
    jamb of steel this purpose, as illustrated in Fig. 75. With mastic pointing between
    su rame ^ metai subframe and the outer reveal, this is a satisfactory way of sealing cavities.
    At the time when it first became common practice to fill the cavity solidly at jambs, there was no requirement for the insulation of walls. When insulation first became a requirement it was met by the use of lightweight, insulating concrete blocks as the inner leaf and the practice of solid filling of cavity at jambs, with a vertical dpc continued.
    With the increasing requirement for insulation it has become practice to use cavity insulation as the most practical position for a layer of lightweight material. If the cavity insulation is to be effective for the whole of the wall it must be continued up to the back of window and door frames, as a solid filling of cavity at jambs would be a less effective insulator and act as a thermal or cold bridge.
    With the revision of the requirement of the Building Regulations for enhanced insulation down to a standard U value of 0.45 W/m2K window for the walls of dwellings it has become practice to use cavity insu-frame lation continued up to the frames of openings, as illustrated in Fig. 76, to avoid the cold bridge effect caused by solid filling. Door and window frames are set in position to overlap the outer leaf with a frame bedded resilient mastic pointing as a barrier to rain penetration between the m mo ar frame ancj the jamb. With a cavity 100 mm wide and cavity insulation as partial fill, it is necessary to cover that part of the cavity at jambs of openings, that is not covered by the frame. This can be effected by covering the cavity with plaster on metal lath or by the use of jamb linings of wood, as illustrated in Fig. 77.

    Fig. 75 Cavity closed with frame

    Fig. 76 Cavity fill insulation

    With this form of construction at the jambs of openings there is no purpose in forming a vertical dpc at jambs.
    The advantages of the wide cavity is that the benefit of the use of the cavity insulation can be combined with the cavity air space as resistance to the penetration of water to the inside face of the wall.

  • The length of wall ties

    The spacing of wall ties built across the cavity of a cavity wall is usually 900 mm horizontally and 450 mm vertically, or 2.47 ties per square metre, and staggered, as illustrated in Fig. 72, for the conventional 50 mm wide cavity, with the spacing reduced to 300 mm around the sides of openings. In Approved Document A to the Building Regulations, the practical guidance for the spacing of ties is given as 900 and 450 mm horizontally and vertically for 50 to 75 mm cavities, 750 and 450 mm horizontally and vertically for cavities from 76 to 100 mm wide and 300 mm vertically at unbonded jambs of all openings in cavity walls within 150 mm of openings to all widths of cavities.

    The length of wall ties

  • Wall ties

    Iron ties which were used to tie the leaves of the early cavity walls were later replaced by mild steel ties that became standard for many years.

    In contact with moisture, mild steel progressively corrodes by the formation of oxide of iron, called rust, which expands fiercely to the extent that brickwork around ties may become rust stained and dis­integrate. Standard mild steel ties are coated with zinc to inhibit rust corrosion. The original zinc coating for ties, which was comparatively thin, has been increased in thickness in the current British Standard Specification, for improved resistance to corrosion. As added protection, the range of standard wall ties can be coated with plastic on a galvanised undercoating.

    On the majority of building sites wall ties are not commonly protected during delivery, storage, handling and use against the inevitable knocks that may perforate the toughest coating to mild steel and the consequent probability of rust occurring. There are, on the market, a range of standard and non-standard section wall ties made from stainless steel that will not suffer corrosion rusting during the useful life of buildings. It seems worthwhile to make the comparatively small additional expenditure on stainless steel ties as a precaution against staining and spalling of brickwork or blockwork around rusting mild steel ties.

    The standard section wall ties, illustrated in Fig. 70, are the vertical twist strip, the butterfly and double triangle wire ties. As a check to moisture that may pass across the tie, the butterfly type is laid with the twisted wire ends hanging down into the cavity to act as a drip. The double triangle tie may have a bend in the middle of its length and the strip tie has a twist as a barrier to moisture passing across the tie. Of the three standard types the butterfly is more likely to collect mortar droppings than the others.

    The wall tie illustrated in Fig. 71 is made from corrosion resistant Austenitic stainless steel. The ridge at the centre of the length of the tie is designed for strength and to provide as small as possible a surface for the collection of mortar droppings. The perforations are to improve bond to mortar.

    The length of wall ties varies to accommodate different widths of cavity and the thickness of the leaves of cavity walls. For a 50 mm cavity with brick leaves, a 191 mm or 200 mm long tie is made. For a 100 mm cavity with brick leaves, a 220 mm long tie is used.


    Resistance to weather

    Between 1920 and 1940 it became more usual for external walls of
    small buildings to be constructed as cavity walls with an outer leaf of
    Resistance to weather brick or block, an open cavity and an inner leaf of brick or block. The
    outer leaf and the cavity serve to resist the penetration of rain to the inside face and the inner leaf to support floors, provide a solid internal wall surface and to some extent act as insulation against transfer of heat.
    The idea of forming a vertical cavity in brick walls was first pro-posed early in the nineteenth century and developed through the century. Various widths of cavity were proposed from the first 6 inch cavity, a later 2 inch cavity followed by proposals for 3, 4 or 5 inch wide cavities. The early cavity walls were first constructed with bonding bricks laid across the cavity at intervals, to tie the two leaves together. Either whole bricks with end closers or bricks specially made to size and shape for the purpose were used. Later on, during the middle of the century, iron ties were used instead of bond bricks and accepted as being adequate to tie the two leaves of cavity walls.
    From the middle of the twentieth century it became common practice to construct the external walls of houses as a cavity wall with a 2 inch wide cavity and metal wall ties. It seems that the 2 inch width of cavity was adopted for the convenience of determining the cavity width, by placing a brick on edge inside the cavity so that the course height of a brick, about 65 mm, determined cavity width rather than any consideration of the width required to resist rain penetration. This was adapted to the 2 inch (50 mm) wide cavity for walls that became common until recent years.
    In constructing the early open cavity walls it was considered good practice to suspend a batten of wood in the cavity to collect mortar droppings. The batten was removed from time to time, cleaned of mortar, and put back in the cavity as the work progressed. The practice, which was largely ignored by bricklayers as it impeded work, has since been abandoned in favour of care in workmanship to avoid mortar droppings becoming lodged inside the cavity.
    With the increase in the price of fuels and expectations of thermal comfort, building regulations have of recent years made requirements for the thermal insulation of external walls that can best be met by the introduction of materials with high thermal resistance. The most convenient position for these lightweight materials in a cavity wall is inside the cavity, which is either fully or partially filled with insula¬tion. A filled or partially filled cavity may well no longer be an efficient barrier to rain penetration so that, with the recent increase in requirement for the thermal insulation of walls it has now been accepted that the width of the cavity may be increased from the traditional 50 to 100 mm to accommodate increased thickness of insulation and still maintain a cavity against rain penetration.

    Strength and stability

    The practical guidance in Approved Document A to the Building Regulations accepts a cavity of from 50 to 100 mm for cavity walls having leaves at least 90 mm thick, built of coursed brickwork or blockwork with wall ties spaced at 450 mm vertically and from 900 to 750 mm horizontally for cavities of 50 to 100 mm wide respectively. As the limiting conditions for the thickness of walls related to height and length are the same for a solid bonded wall 190 mm thick as they are for a cavity wall of two leaves each 90 mm thick, it is accepted that the wall ties give the same strength and stability to two separate leaves of brickwork that the bond in solid walls does.