A difficulty often arises in bonding when facing work with bricks of a slightly different size from those used in "backing," as it is technically termed. As it is, of course, necessary to keep all brickwork in properly levelled courses, a difference has to be made in the thickness of the mortar joints. Apart from the extra labour involved, this obviously is detrimental to the stability of the wall, and is apt to produce unequal settlement and cracking. Too much care cannot be taken to obtain both facing and backing bricks of equal size.

Fig. 8.  Slate damp proof course. Fig. 8.

Dishonest bricklayers do not hesitate, when using for the face of a wall bricks of a quality superior to those used for the interior, to use "snapped headers," that is cutting the heading bricks in halves, one brick thus serving the purposes of two as regards outward appearance. This is a most pernicious practice, unworthy of adoption by any craftsman of repute, for a skin of brickwork 4½ in. thick is thus carried up with a straight mortar joint behind it, the proper bonding with the back of the wall by means of headers being destroyed.

American building acts describe the kind of bond to be used for ordinary walls, and the kind for faced walls. Tie courses also require an extra thickness where walls are perforated with over 30% of flues.

The importance for sanitary and other reasons of keeping walls dry is admitted by all who have observed the deleterious action of damp upon a building.

Walls are liable to become damp, (1) by wet rising up the wall from the earth; (2) by water soaking down from the top of the Prevention of damp. wall; (3) by rain being driven on to the face by wind. Dampness from the first cause may be prevented by the introduction of damp-proof courses or the construction of dry areas; from the second by means of a coping of stone, cement or other non-porous material; and from the third by covering the exterior with impervious materials or by the adoption of hollow walls.

Fig. 9.  Asphalt damp proof course. Fig. 9.

After the footings have been laid and the wall has been brought up to not less than 6 in. above the finished surface of the ground, and previous to fixing the plate carrying the ground floor, there should always be introduced a course of some damp-proof material to prevent the rise of moisture from the soil. There are several forms of damp-proof course. A very usual one is a double layer of roofing slates laid in neat Portland cement (fig. 8), the joints being well lapped. A course or two of Staffordshire blue bricks in cement is excellent where heavy weights have to be considered. Glazed stoneware perforated slabs about 2 in. thick are specially made for use as damp-proof courses. Asphalt (fig. 9) recently has come into great favour with architects; a layer ½ or ¾ in. thick is a good protection against damp, and not likely to crack should a settlement occur, but in hot weather it is liable to squeeze out at the joints under heavy weights. Felt covered with bitumen is an excellent substitute for asphalt, and is not liable to crack or squeeze out. Sheet lead is efficient, but very costly and also somewhat liable to squeezing. A damp-proof course has been introduced consisting of a thin sheet of lead sandwiched between layers of asphalt.

Basement storeys to be kept dry require, besides the damp-proof course horizontally in the wall, a horizontal course, usually of asphalt, in the thickness of the floor, and also a vertical damp-proof course from a level below that of the floor to about 6 in. above the level of the ground, either built in the thickness of the wall or rendered on the outside between the wall and the surrounding earth (fig. 10).

By means of dry areas or air drains (figs. 11 and 12), a hollow space 9 in. or more in width is formed around those portions of the walls situated below the ground, the object being to prevent them from coming into contact with the brickwork of the main walls and so imparting its moisture to the building. Arrangements should be made for keeping the area clear of vermin and for ventilating and draining it. Dry areas, being far from sanitary, are seldom adopted now, and are being superseded by asphalt or cement applied to the face of the wall.

Fig. 12.  Air drains. Fig. 12. Fig. 11.  Air drains. Fig. 11. Fig. 10.  Damp proof courses for basements. Fig. 10.

Moisture is prevented from soaking down from the top of the wall by using a covering of some impervious material in the form of a coping. This may consist of ordinary bricks set on edge in cement with a double course of tiles immediately below, called a "creasing," or of specially made non-porous coping bricks, or of stone, cast-iron, or cement sloped or "weathered" in order to throw the rain off.

Fig. 13.  Slates or tiles fixed on battens. Fig. 13.

The exterior of walls above the ground line may be protected by coating the surface with cement or rough cast; or covering with slates or tiles fixed on battens in a similar manner to those on a roof (fig.13).

The use of hollow walls in exposed positions has already been referred to.

The by-laws dated 1891, made by the London County Council under section 16 of the Metropolis Management and Buildings Acts Amendment Act 1878, require that "every wall of a house or building shall have a damp course composed of materials impervious to moisture approved by the district surveyor, extending throughout its whole thickness at the level of not less than 6 in. below the level of the lowest floor. Every external wall or enclosing wall of habitable rooms or their appurtenances or cellars which abuts against the earth shall be protected by materials impervious to moisture to the satisfaction of the district surveyor..." "The top of every party-wall and parapet-wall shall be finished with one course of hard, well-burnt bricks set on edge, in cement, or by a coping of any other waterproof and fire-resisting material, properly secured."