In situations exposed to driving rains it is very usual to build walls in two thicknesses, to prevent water from soaking through and destroying the internal wall coverings and causing woodwork to decay. In situations near the sea these cavity walls are particularly necessary, as the salt carried by the wind from the sea is deposited on the exposed surfaces, rendering them permanently liable to attract and hold moisture.

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Fig. 149.

The method of constructing hollow walls is shown in Figs. 148 and [49, in which it will be seen that the thinner skin is placed on the outside, while the thicker or weight-carrying portion is on the inside. The outside skin should not be considered as imparting additional strength to the wall, the inner one being proportioned to carry the weight of the floors and roof by itself.

The outer wall is bonded to the inner by means of specially made ties, either of vitrified stoneware, cast iron, wrought iron or wire, of the shape shown in Fig. 150. These ties are placed in rows from 2 feet 6 inches to 3 feet apart horizontally, and from 9 inches to 1 foot 6 inches vertically, and are set chequerwise. At salient angles or at any point at all subject to rough wear extra bonding ties should be inserted.

In some parts of England it is customary to place the thicker part of the wall on the outside. The objection to this is that the joists and other timbers have to be carried through the inner to the outer wall, giving a ready means for the water to penetrate to the inner surface.

There is another distinct disadvantage in having the thicker wall outside, because its strength is greatly impaired by damp.

The cavity should be provided with ample means of ventilation, and this is done by inserting airbricks at the top and bottom of the walls (see Figs. 148 and 149). The air for ventilating floors should be taken direct from the outside air and not from the cavity. This is usually done by inserting an airbrick in the outer wall, and carrying a stoneware pipe at the back of it, bent upwards so as to throw any water that may fall upon it to the outer surface, as shown in Fig. 149.

The cavity is sometimes finished off beneath the damp-proof course at the lower end; but the better and more usual arrangement is shown in Fig. 148, where a cement fillet is formed at the bottom to throw any water into small outlets formed at intervals just above the plinth. This prevents the great danger of the cavity forming a pond in the event of a gutter overflowing; but these holes must not be large enough to admit vermin.

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Fig. 150.

Window and door heads should be protected from water by building in a strip of 4 or 5-lb. lead so as to form a gutter, which projects on each side of the opening to enable water to drop clear of the frame. This is shown in Fig. 149. A cement channel coated with asphalt is sometimes used for the same purpose.

The cavity should be closed at the top, as this ensures a more equable temperature in the building.

Great care should be taken to prevent mortar from falling down the cavity in course of building and lodging upon the ties, thus forming a ready means of access for the water. For this purpose iron pipes or battens bound round with hay-bands are laid on top of one row of bonding ties, and are removed when the wall is built up to the level of the next row of bonding ties.

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Fig. 151.

It is useful to note that solid and hollow walls of the same thickness cost practically the same amount per rod, the expense of the hay-bands, bonding ties, and the extra labour in the latter making up for the bricks saved by the cavity; but strength for strength the hollow wall is much more expensive, since the strength of the outer skin is of no account, and it is consequently entirely additional to the inner constructional wall.

3. To prevent water from entering basements in water-logged soils it is best to form an asphalt tank, 3/4 inch thick, completely round the part of the building beneath the ground, carrying it horizontally over the footing and through the floor. It is best placed on the outside of the walls, so that the pressure of the earth will keep it in position. Above the ground level the asphalt terminates as a horizontal damp-proof course, as shown in Figs. 151 and 152.

Where it is desired to render an existing basement damp proof the existing floor is covered with a layer of asphalt 3/4 inch thick, and is weighted down by a layer of concrete 6 inches thick, or a paving of bricks. The joints of the existing wall are then raked out to form a key for the vertical damp course. A board 3/4 inch thick is laid flat against the wall with one edge resting on the concrete floor, and three courses of 4 1/2-inch brickwork are built up close against this board, care being taken to have the joints as free from mortar as possible on the side nearest the board. The board is now withdrawn and the cavity filled up with melted asphalt. The board is now placed with one edge on the uppermost of these three courses, and three more courses are built, the board removed, and asphalt poured into the cavity as before. This operation is carried on until the horizontal damp-proof course is reached above ground level. The section of a basement so treated is shown in Fig. 153.

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Fig. 152.

The above method is not always considered to give sufficient bond between the outer and inner walls when the soil outside is very moist, in which case one header is removed in every square yard of surface on the outer wall to a depth of 4 1/2 inches, and headers are laid tailing into the pockets thus formed, after the latter have been thoroughly lined with asphalt.

Where any of the damp-proof sheetings now on the market are used, headers must be taken out for the insertion of bonding bricks. A bitumen pocket is inserted into the hole in the old wall from which the header has been removed, the edges of which overlap the vertical sheeting (see Fig. 154).

When the pressure of the earth or when the distance between the walls is considerable, it is best to form the concrete floor above the asphalt into a beam curved on the under side, or to form an inverted brick arch, as shown in Figs. 151 and 152.

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Fig. 153.