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. CHAPTER XXXII. CONSTRUCTION OF THE HOUSE. 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. 3. It should be always dry. 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. The =Foundation= requires to be solid and substantial, otherwise sinking occurs, with cracking of the walls, resulting in an unsafe condition, and an exposure to rain and wind.