• 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.


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    Strength and Stability

    Up to the middle of the twentieth century the design and construction of small buildings, such as houses, was based on tried, traditional forms of construction. There were generally accepted rule of thumb methods for determining the necessary thickness for the walls of small buildings. By and large, the acceptance of tried and tested methods of construction, allied to the experience of local builders using traditional materials in traditional forms of construction, worked well. The advantage was that from a simple set of drawings an experienced builder could give a reasonable estimate of cost and build and complete small buildings, such as houses, without delay.
    With the increasing use of unfamiliar materials, such as steel and concrete, in hitherto unused forms, it became necessary to make calculations to determine the least size of elements of structure for strength and stability in use. The practicability of constructing large multi-storey buildings provoked the need for standards of safety in case of fire and rising expectations of comfort and the need for the control of insulation, ventilation, daylight and hygiene.
    During the last 50 years there has been a considerable increase in building control, that initially was the province of local authorities through building bylaws, later replaced by national building regula¬tions. The Building Regulations 1985 set out functional requirements for buildings and health and safety requirements that may be met through the practical guidance given in 11 Approved Documents that in turn refer to British Standards and Codes of Practice.
    In theory it is only necessary to satisfy the requirements of the Building Regulations, which are short and include no technical details of means of satisfying the requirements. The 11 Approved
    Documents give practical guidance to meeting the requirements, but there is no obligation to adopt any particular solution in the documents if the requirements can be met in some other way.
    The stated aim of the current Building Regulations is to allow freedom of choice of building form and construction so long as the stated requirements are satisfied. In practice the likelihood is that the practical guidance given in the Approved Documents will be accepted as if the guidance were statutory as the easier approach to building, rather than proposing some other form of building that would involve calculation and reference to a bewildering array of British Standards and Codes and Agrement Certificates.
    In Approved Document A there is practical guidance to meeting the requirements of the Building Regulations for the walls of small buildings of the following three types:
    (1) residential buildings of not more than three storeys
    (2) small single storey non-residential buildings, and
    (3) small buildings forming annexes to residential buildings (including garages and outbuildings).
    Limitations as to the size of the building types included in the guidance are given in a disjointed and often confusing manner.


    The maximum height of residential buildings is given as 15 m from the lowest ground level to the highest point of any wall or roof, whereas the maximum allowable thickness of wall is limited to walls not more than 12 m. Height is separately defined, for example, as from the base of a gable and external wall to half the height of the gable. The height of single storey, non-residential buildings is given as 3 m from the ground to the top of the roof, which limits the guidance to very small buildings. The maximum height of an annexe is similarly given as 3 m, yet there is no definition of what is meant by annexe except that it includes garages and outbuildings.


    The least width of residential buildings is limited to not less than half

    the height. A diagram limits the dimensions of the wing of a residential building without defining the meaning of the term ‘wing’, which in the diagram looks more like an annexe than a wing. Whether the arms of a building which is ‘L’ or ‘LP shaped on plan are wings or not is entirely a matter of conjecture. How the dimensions apply to semi-detached buildings or terraces of houses is open to speculation. In seeking to give practical guidance to meeting functional requirements for strength and stability and at the same time impose limiting dimensions, the Approved Document has caused confusion. One further limitation is that no floor enclosed by structural walls on all sides should exceed 70 m2 and a floor without a structural wall on one side, 30 m2. The floor referred to is presumably a suspended floor, though it does not say so. As the maximum allowable length of wall between buttressing walls, piers or chimneys is given as 12 m and the maximum span for floors as 6 m, the limitation is in effect a floor some 12 x 6 m on plan. It is difficult to understand the need for the limitation of floor area for certain ‘small’ buildings.


  • Concrete blocks

    These are used extensively for both loadbearing and non-loadbearing walls, externally and internally. A concrete block wall can be laid in less time and may cost up to half as much as a similar brick wall. Lightweight aggregate concrete blocks have good insulating properties against transfer of heat and have been much used for the inner leaf of cavity walls with either a brick outer leaf or a concrete block outer leaf.

    A disadvantage of some concrete blocks, particularly lightweight aggregate blocks, as a wall unit is that they may suffer moisture movement which causes cracking of applied finishes such as plaster. To minimise cracking due to shrinkage by loss of water, vertical movement joints should be built into long block walls, subject to moisture movement, at intervals of up to twice the height of the wall. These movement joints may be either a continuous vertical joint filled with mastic or they may be formed in the bonding of the blocks.

    Because the block units are comparatively large, any settlement movement in a wall will show more pronounced cracking in mortar joints than is the case with the smaller brick wall unit.

    For some years it was fashionable to use concrete blocks as a fairface external wall finish. The blocks were accurately moulded to uniform sizes and made from aggregates to provide a variety of colours and textures. Blocks made to give an appearance of natural stone with plain or rugged exposed aggregate finish were used.

    These special blocks are less used that they were, particularly because of the fairly rapid deterioration in the appearance of the blocks due to irregular weather staining of smooth faced blocks and the patchy dirt staining of coarse textured blocks.

    Concrete blocks are manufactured from cement and either dense or lightweight aggregates as solid, cellular or hollow blocks as illustrated in Fig. 60. A cellular block has one or more holes or cavities that do not pass wholly through the block and a hollow block is one in which the holes pass through the block. The thicker blocks are made with cavities or holes to reduce weight and drying shrinkage.

    The most commonly used size of both dense and lightweight concrete blocks is 440 mm long x 215 mm high. The height of the block is chosen to coincide with three courses of brick for the convenience of building in wall ties and also bonding to brickwork. The length of the block is chosen for laying in stretcher bond.

    For the leaves of cavity walls and internal loadbearing walls 100 mm thick blocks are used. For non-loadbearing partition walls 60 or 75 mm thick lightweight aggregate blocks are used. Either 440 mm x 215 mm or 390 x 190 mm blocks may be used.

    Concrete blocks may be specified by their minimum average compressive strength for:

    (1) all blocks not less than 75 mm thick and
    (2) a maximum average transverse strength for blocks less than 75 mm thick, which are used for non-loadbearing partitions.
    The usual compressive strengths for blocks are 2.8, 3.5, 5.0, 7.0. 10.0, 15.0, 20.0 and 35.0 N/mm2. The compressive strength of blocks used for the walls of small buildings of up to three storeys, recommended in Approved Document A to the Building Regulations, is 2.8 and 7N/mm2, depending on the loads carried.

    Concrete blocks may also be classified in accordance with the aggregate used in making the block and some common uses.

  • Properties of bricks

    This is a somewhat vague term commonly used in the description of
    bricks. By general agreement it is recognised that a brick which is to have a moderately good compressive strength, reasonable resistance to saturation by rainwater and sufficient resistance to the disruptive action of frost should be hard burned. Without some experience in the handling, and of the behaviour, of bricks in general it is very difficult to determine whether or not a particular brick is hard burned.

    A method of testing for hardness is to hold the brick in one hand and give it a light tap with a hammer. The sound caused by the blow should be a dull ringing tone and not a dull thud. Obviously different types of brick will, when tapped, give off different sorts of sound and a brick that gives off a dull sound when struck may possibly be hard burned.

    Bricks Compressive strength
    Bricks Absorption
    Frost resistance of bricks
    Efflorescence of bricks
    Sulphate attack on mortars and renderings of bricks

  • Types of brick

    These are bricks which are sufficiently hard to safely carry the loads
    normally supported by brickwork, but because they have a dull texture or poor colour they are not in demand for use as facing bricks
    which show on the outside when built and affect the appearance of buildings. These ‘common’ bricks are used for internal walls and for rear walls which are not usually exposed to view. Any brick which is sufficiently hard and of reasonably good shape and of moderate price may be used as a ‘common’ brick. The type of brick most used as a common brick is the Fletton brick.


    This is by far the widest range of bricks as it includes any brick which
    is sufficiently hard burned to carry normal loads, is capable of withstanding the effects of rain, wind, soot and frost without breaking up and which is thought to have a pleasant appearance. As there are as many different ideas of what is a pleasant looking brick as there are bricks produced, this is a somewhat vague classification.

    Engineering bricks

    These are bricks which have been made from selected clay, which have been carefully prepared by crushing, have been very heavily moulded and carefully burned so that the finished brick is very solid and hard and is capable of safely carrying much heavier loads than other types of brick. These bricks are mainly used for walls carrying exceptionally heavy loads, for brick piers and general engineering works. The two best known engineering bricks are the red Southwater brick and the blue Staffordshire brick. Both are very hard, dense and do not readily absorb water. The ultimate crushing resistance of engineering bricks is greater than 50 N/mm2.

    Semi-engineering bricks

    These are bricks which, whilst harder than most ordinary bricks, are not so hard as engineering bricks. It is a very vague classification without much meaning, more particularly as a so-called semi-engineering brick is not necessarily half the price of an engineering brick.

    Composition of clay

    Clays suitable for brick making are composed mainly of silica in the form of grains of sand and alumina, which is the soft plastic part of clay which readily absorbs water and makes the clay plastic and which melts when burned. Present in all clays are materials other than the two mentioned above such as lime, iron, manganese, sulphur and phosphates. The proportions of these materials vary widely and the following is a description of the composition, nature and uses of some of the most commonly used bricks classified according to the types of clay from which they are produced.


    There are extensive areas of what is known as Oxford clay. The clay is
    composed of just under half silica, or sand, about one-sixth alumina, one-tenth lime and small measures of other materials such as iron, potash and sulphur. The clay lies in thick beds which are economical to excavate. In the clay, in its natural state, is a small amount of mineral oil which, when the bricks are burned, ignites and assists in the burning.

    Because there are extensive thick beds of the clay, which are economical to excavate, and because it contains some oil, the cheapest of all clay bricks can be produced from it. The name Fletton given to these bricks derives from the name of a suburb of Peterborough around which the clay is extensively dug for brickmaking. Flettons are cheap and many hundreds of millions of them are used in building every year. The bricks are machine moulded and burned and the finished brick is uniform in shape with sharp square edges or arises. The bricks are dense and hard and have moderately good strength; the average pressure at which these bricks fail, that is crumble, is around 21 N/mm2

    The bricks are light creamy pink to dull red in colour and because of the smooth face of the brick what are known as ‘kiss marks’ are quite distinct on the long faces. These ‘kiss marks’ take the form of three different colours, as illustrated in Fig. 46.

    The surface is quite hard and smooth and if the brick is to be used for wall surfaces to be plastered, two faces are usually indented with grooves to give the surface a better grip or key for plaster. The bricks are then described as ‘keyed Flettons’. Figure 47 is an illustration of a keyed Fletton.


    By origin the word marl denotes a clay containing a high proportion
    of lime (calcium carbonate), but by usage the word marl is taken to denote any sandy clay. This derives from the use of sandy clays, containing some lime, as a top dressing to some soils to increase fertility. In most of the counties of England there are sandy clays, known today as marls, which are suitable for brick making. Most of the marl clays used for brick making contain little or no lime. Many of the popular facing bricks produced in the Midlands are made from this type of clay and they have a good shape, a rough sandy finish and vary in colour from a very light pink to dark mottled red.


    The gault clay does in fact contain a high proportion of lime and the
    burned brick is usually white or pale pink in colour. These bricks are of good shape and texture and make good facing bricks, and are more than averagely strong. The gault clay beds are not extensive in this country and lie around limestone and chalk hills in Sussex and Hampshire.

    Clay shale bricks

    Some clay beds have been so heavily compressed over the centuries by the weight of earth above them that the clay in its natural state is quite firm and has a compressed flaky nature. In the coal mining districts of this country a considerable quantity of clay shale has to be dug out to reach coal seams and in those districts the extracted shale is used extensively for brick making. The bricks produced from this shale are usually uniform in shape with smooth faces and the bricks are hard and durable. The colour of the bricks is usually dull buff, grey, brown or red. These bricks are used as facings, commons and semi-engineering, depending on their quality.

    Calcium Silicate bricks

    Calcium silicate bricks are generally known as sand-lime bricks. The
    output of these bricks has increased over the past few years, principally because the output of Fletton bricks could not keep pace with the demand for a cheap common brick and sand-lime bricks have been mainly used as commons. The bricks are made from a carefully controlled mixture of clean sand and hydrated lime which is mixed together with water, heavily moulded to brick shape and then the moulded brick is hardened in a steam oven. The resulting bricks are very uniform in shape and colour and are normally a dull white. Coloured sand-lime bricks are made by adding a colouring matter during manufacture. These bricks are somewhat more expensive than Flettons and because of their uniformity in shape and colour they are not generally thought of as being a good facing brick. The advantage of them however is that the material from which they are made can be carefully selected and accurately proportioned to ensure a uniform hardness, shape and durability quite impossible with the clay used for most bricks.

    Flint-lime bricks
    Special bricks

  • Stock brick

    The term ‘stock brick’ is generally used in the south-east counties of England to describe the London stock brick. This is a brick manufactured in Essex and Kent from clay composed of sand and alumina to which some chalk is added. Some combustible material is added to the clay to assist burning. The London stock is usually predominantly yellow after burning with shades of brown and purple. The manufacturers grade the bricks as 1st Hard, 2nd Hard and Mild, depending on how burned they are. The bricks are usually irregular in shape and have a fine sandy texture. Because of their colour they are sometimes called ‘yellow stocks’. 1st Hard and 2nd Hard London stocks were much used in and around London as facings as they weather well and were of reasonable price. In other parts of England the term stock bricks describes the stock output of any given brick field.

  • Brick and Block Walls

    The majority of the walls of small buildings in this country are built of brick or block. The external walls of heated buildings, such as houses,
    are built as a cavity wall with an outer leaf of brick, a cavity and an inner leaf of concrete blocks. Internal walls and partitions are built, in the main, of concrete blocks.

    Block Walls

  • Brick damp-proof courses

    Two or three courses of dense, semi-engineering or engineering bricks were laid in hydraulic lime and later cement mortar. There is little likelihood of these dense bricks fracturing under moderate settlement. Because of the dissimilar colour and texture of these bricks to that of facing bricks and the cost of the material this form of dpc is little used.

  • Polythene sheet

    Polythene sheet for use as a dpc should be black, low density polythene sheet of single thickness not less than 0.46 mm, weighing approximately 0.48 kg/m2. Polythene sheet is flexible, can withstand distortion due to moderate settlement in a wall without damage and is an effective barrier against moisture. It is laid on an even bed of mortar and lapped at least the width of the dpc at running joints and intersections. Being a thin sheet material, polythene makes a thinner mortar joint than bitumen dpc, and is sometimes preferred for that reason.

    Its disadvantage as a dpc is that it is fairly readily damaged by sharp particles in mortar or the coarse edges of brick.