250. Moisture in wet ground is very likely to soak up into the walls from the foundation, thus causing the building to be very unhealthy and producing rot in the woodwork. To prevent the moisture from rising through the foundation walls, a horizontal damp-proof course should be placed in all walls just below the level of the first-floor joists. This damp-proof course should be at least 6 inches above the highest level of the soil touching any part of the outer walls, and should not be broken at any point in its length. It should run at least 2 feet into all cross-walls, and where the ground is very wet, it should be continuous through all walls.

In many cases, when buildings are finished with parapet walls, it is customary to put in a damp-proof course just above the flashing of the roof or gutter, in order that the dampness produced by driving rains may not soak down into the woodwork of the roof, and from there to the walls below.

251. Damp-proof courses may be made of hot asphalt and coal tar. They should be mixed in the proportion of 9 parts of asphalt to 1 of coal tar, and put on in a 3/8 -inch layer, or the thickness of a mortar joint. The surface of brickwork that receives the asphalt should be quite dry and smooth, and the joints should be well flushed up with mortar.

Two courses of roofing slate, or very hard vitrified brick, 2-10 laid in sand-and-cement mortar with broken joints, form a cheap damp course; or a 1/2-inch layer of Portland cement mortar, mixed in the proportion of 1 part of cement to 2 of sand, is often used, but is not considered as desirable as the asphalt.