Another source of dampness in dwellings, as pointed out by Eassie, is to be found in the practice of building the house walls close against the earth, without taking the precaution to erect a blind area-wall between the house wall and the earth excavation. Fig. 2 exhibits both these important improvements--the damp-course and the area-wall--applied to the same dwelling: _a_ represents the main wall of the house, and _b_ the area-wall, which is built against the excavated subsoil, leaving the space _c_ between the two walls; the thick black line underneath the floor-joist represents the damp-proof course, which interposes between the subsoil _d_, with the foundations built upon it, and the main wall of the house. This damp-proof course usually consists of a layer of pitch or asphalte, or slates bedded in cement, or specially glazed tiles, known as Taylor’s or Doulton’s manufactures. By the use of this impervious course, the upward passage of the ground water is effectually arrested. The intervening area _c_ it is also well to drain, but this water should never drain into the soil drain, if avoidable, and certainly not until it has been thoroughly disconnected. There should always, also, be a current of air introduced from the outer air, by way of ventilators put at the top of the blind area _c_, and an air brick placed above or below the damp-proof course--preferably above--in order that the space between the ground and the joists or stone flooring of the basement may be thoroughly ventilated. This ventilation is shown by the arrows between _e_ and _e_. Such air currents should always be provided under floors, whether there be a basement or not, and also always between the joists of the upper floors, and in the roof, in order to ward off dry-rot and ensure a constant circulation of air. (Eassie.) _Roof._--The first detail to be decided on is the “pitch” or slope to be given to the roof, and this will depend both on the nature of the covering material and the character of the climate. In the tropics, where rain falls in torrents, a flat pitch helps to counteract the rush of water; in colder regions the pitch must be such as to readily admit of snow sliding off as it accumulates, to prevent injury to the framework by the increased weight. The pitches ordinarily observed, stated in “height of roof in parts of the span,” are as follows:--Lead, 1/40; galvanized iron or zinc, ⅕; slates, ¼; stone, slate, and plain tiles, 2/7; pantiles, 2/9; thatch, felt, and wooden shingles, ⅓ to ½.