The _Slate_ used for roofs is an altered form of clay, possessing a laminated structure. The ease with which it splits along the planes, renders it peculiarly suitable for this purpose. The Welsh slates are considered the best.
=Terra-cotta= is made from certain kinds of clay, mixed with glass, pottery or sand; then ground up, strained, and kneaded; and lastly thrown into moulds and baked in a kiln.
=Iron and Wood= have occasionally been employed alone in building houses. The former, owing to its good conducting powers for heat, is cold in winter and hot in summer; while the latter becomes rotten from exposure to wet, and is also very combustible. Corrugated iron buildings lined with wood are also employed, but are not very satisfactory.
For roofs, slates or tiles are the materials most frequently employed; but occasionally lead and corrugated iron are used, also thatch in country places, and tarred felt for temporary buildings.
=Lead= is the most suitable metallic covering for roofs, as it is durable and easily worked. It is, however, heavy and demands considerable strength in the timbers by which it is supported. =Galvanized iron= has also been largely used. It is cheaper and lighter than lead. Both lead and zinc require very careful laying if they are to be weather-tight.
=Thatch= protects the interior of a house well from extremes of heat and cold.
In preparing to build a house, or in entering into a house already built, the following requisites should each receive careful attention:—
1. The site of the house should be healthy, and its relation to surrounding objects in accordance with the laws of health. (See page 201).
2. The house should be warm in winter, and cool in summer.
4. There should be an abundant and uninterrupted supply of air.
5. The water supply should be abundant, conveniently arranged, and pure.
6. The excreta and waste-water should be immediately removed from the house and its annexa.
The three last requisites have already received consideration. Of those still to be considered, =dryness= is the most important. A damp house is certain to be an unhealthy one. It is this for two reasons:—1st, it is a cold house, as damp walls, like damp clothes, conduct the heat of the body away much more rapidly than dry walls; 2nd, if the pores of the bricks are occupied by water, air cannot pass through, and thus the ventilation and purification of the house are greatly impeded. Damp may arise from the ground on which a house stands, or from the rain beating against the walls, or from a defective roof. Unless special means are taken to prevent it, moisture rises by capillary attraction through brick after brick.