The importance of keeping moisture out of walls as far as possible need hardly be dilated upon.

In addition to the great importance of a dry building for sanitary reasons, it is also most necessary for good construction; dampness in the masonry soon communicates itself to the woodwork, and causes rot throughout the building, besides which, the masonry itself is not sound, the mortar, unless of good hydraulic lime, or cement, does not set, and is always liable to the attacks of frost.

To give some idea of the quantity of water that the walls of an improperly protected building may contain, and of the evil effects caused by damp, the following remarks are quoted from an official report.1

"In England the common bricks absorb as much as a pint or pound of water. Supposing the external walls of an ordinary cottage to be one brick thick, and to consist of 12,000 bricks, they will be capable of holding 1500 gallons or 6 tons of water when saturated. To evaporate this amount of water would require nearly a ton of coal, well applied. The softer and more workable stones are of various degrees of absorbency, and are often more retentive of moisture than common brick. Professor Ansted states that the facility with which sandstone absorbs water is illustrated by the quantity it contains both in its ordinary state and when saturated. He states that even granite always contains a certain percentage of water, and in the dry state is rarely without a pint and a half in every cubic foot. Sandstone, however, even that deemed fit for building purposes, may contain half a gallon per cubic foot, and loose sand at least two gallons. When water presents itself in any part of such material it readily diffuses itself by the power of capillary attraction, by which, it is observed on some walls in Paris, it ascends 32 feet from the foundations. Walls of such absorbent constructions are subject to rising wet by capillary attraction, as well as the driving wet of rain or storm. To guard against the driving wet on the coast, expensive external coverings, 'weather slates,' are used. But these do not stay the interior rising wet This wet having to be evaporated lowers temperature. Damp walls or houses cause rheumatism, lower strength, and expose the system to other passing causes of disease."

It is a wise precaution to cover the whole surface of the ground under a dwelling with a layer of concrete, or asphalte, in order to prevent the damp and bad air out of the ground from rising into the building.2

1Report on Dwellings in the Paris Exhibition, by Edwin Chad wick, Esq., C. B.

2This is enjoined by the Model Bye-Laws of the Local Government Board.

This precaution is, however, generally omitted because it involves expense; but measures to keep the walls dry are or should be adopted in nearly all buildings intended for occupation by human beings.

The walls of a building are liable to be charged with moisture -

1. By wet rising in them from the damp earth.

2. By rain falling upon the exterior of the walls.

3. By water from the roofs or leaking gutters soaking into the tops of the walls.

Of these evils the first may be prevented by the construction of dry areas or "air-drains " and by the introduction of damp-proof courses; the second may be counteracted by impervious outer coatings or by the use of hollow walls; and the third avoided by the use of projecting eaves with proper gutters - or where parapet walls are used, by an upper damp course.

Air-Drains are narrow dry areas, 9 inches or more in width, formed around such parts of the walls of a building as are below the ground.

They prevent the earth from resting against the walls and imparting to the masonry its moisture, which, rising by capillary attraction, might cause the evils already referred to.

The outer wall of the area should rise slightly above the surrounding ground, so as to prevent the water from the surface from entering the air-drain. Arrangements should be made for keeping the area clear of vermin, for ventilating it, and also for draining off any moisture that may accumulate at the bottom.

In the section Fig. 61 is shown an air - drain 12 inches wide, having a rubble retaining wall, and being covered by flagstones built into the wall and weathered on the upper surface; of these, one here and there is removable in order to give access to the drain. The air-holes shown in the figure ensure the thorough ventilation of the drain and of the space below the floor of the building.

There are several forms of air-drains; the width of the area is often much less than that shown in the figure, and sometimes is so reduced that the arrangement simply amounts to providing a hollow wall. In other examples the outer retaining wall is curved in plan, between the piers, being concave on the inside, by which additional strength is gained and thinner walls may be used. The area is frequently covered by a small quadrant arch turned against the wall, instead of by paving.

In some cases, to avoid the expense of air-drains, the outer surface of the portion of wall below ground is rendered with cement, asphalted, or covered with a layer of slates attached to the wall.

Substitutes for properly built air-drains may be cheaply formed by placing a flagstone in an inclined position against the outside of the wall to be protected.

Wide and open areas are much more expensive, but allow a freer circulation of air, exclude damp more thoroughly, and are, on the whole, superior to air-drains.

Horizontal Damp-Proof Course

Even where air-drains are provided, a damp-proof course should be inserted in all walls, to prevent the moisture out of the soil from rising in the masonry.

The. damp-proof course should be 6 inches or more above the level of the external ground, but under the wall-plate carrying the floor-joists.

There are several forms in which a damp-proof course may be provided.

It may be of glazed pottery slabs built into the wall, as shown at D D in Fig. 6. The joints between the slabs must be left empty, or the damp will rise through them.1

A layer of tough asphalte about 3/8 inch thick is often used instead, as at A in Fig. 7.

In buildings finished with a parapet wall, a damp-proof course should be inserted just above the flashing of the gutter, so as to prevent the wet which falls upon the top of the parapet from soaking down into the woodwork of the roof and into the walls below.

In some localities damp-proof courses are formed of asphalted felt, or with slates set in cement; these latter are rather liable to crack, and thin impervious stones, or courses of Staffordshire bricks in cement, are better. Sheet lead has been used for the same purpose, and is most efficacious, but very expensive.

Arches over vaults, or cellars under footpaths, are frequently rendered all over the extrados with asphalte or cement to prevent the penetration of wet.

Vertical Damp-Proof Course

In addition to the precautions adopted to prevent damp out of the ground from rising in walls, it is necessary (especially when using inferior bricks or porous stones) to prevent moisture falling upon the outer face from penetrating to the interior of the wall.

The wet may be kept out of the interior of the wall by rendering the exterior surface with cement, covering it with slates fixed on battens, or with glazed tiles set in cement. Taylor's pottery facing bricks answer the same purpose.

1 To prevent wet which comes into the hollow space, through the outer portion of the wall, from finding its way along the top of the damp-proof course to the interior of the wall, a cement fillet may be run along the angle at the bottom of the hollow space between the top of the damp-proof course and the inner portion of the wall, and an exit should be afforded - in any case temporarily - for the water at various points by-leaving openings in the brickwork. If these openings are left permanently they should be protected by gratings.

Another plan patented by Mr. Taylor consists of overlapping slates placed vertically in the middle of the wall - the two portions of which are united by peculiar iron ties.

The Hygeian rock impervious wait-lining, patented by Mr. White of Abergavenny, consists of a vertical sheet of waterproof composition introduced into the thickness of the wall.

The wall is built up, two or three courses at a time, in two vertical slices, with about -inch opening between them, the inner parts of the horizontal joints next to this opening being left empty. The melted composition being run in, fills all the openings thus left, and not only prevents the penetration of moisture but adds to the strength of the wall.

It is stated that a 9-inch wall built with the lining is stronger than an 18-inch wall built in the ordinary way.

Fig. 4, from Mr. White's circular, shows the application of his system to a water-tight tank.

This system may often be useful for parts of buildings in very damp places, but it must be remembered that walls perfectly impervious to air are, for sanitary reasons, undesirable for inhabited rooms.

Hollow Walls not only exclude the damp, but the layer of air they contain being a non-conductor of heat tends to keep the building warm. Such walls are formed in two separate portions, standing vertically parallel to one another, and divided by a space of about 2 or 3 inches, sometimes 4- inches.

These two portions are generally united either by special bonding bricks or by iron cramps. There are several ways of arranging the thickness of the portions of the wall, and the consequent position of the air space.

In some cases the two portions are of equal thickness, the air space being in the centre.

Very frequently one of the portions is only 4 inches thick, built in brickwork in stretching bond; the other is of such thickness as may be necessary to give the whole stability.

In such a case the thin 4 portion is sometimes placed on the outer, and sometimes on the inner side of the wall.