The preparation of the foundation is comparatively easy where the ground is solid, but if the site be water-logged, marshy, quicksand, or entirely under water, its preparation then becomes a problem that may baffle the best engineers. Artificial foundation beds may have to be resorted to. These are accomplished in several ways. One very common method is to drive wooden piles into the ground, saw off the tops and construct the foundation on these piles. Another method is that of the cofferdam. The foundation having been properly prepared, care must be taken that the footings or base course of the walls are of sufficient strength to bear the weight of the superstructure. The footings should be of stone or concrete and should be a little thicker than the walls. Precautions for a dry basement must also be considered. This subject will be discussed under House Dampness. The air of the basement must be kept pure by proper ventilation, since it is being constantly drawn up into the house. This will receive some attention in the chapter on Ventilation. Walls The walls of the house may be of brick, stone, timber or concrete. The method of construction will depend upon the material used. If timber is utilized the security of the walls will be governed by the timber posts, sills and plates. The materials used should be as little absorbent of moisture as possible. All wood should be well seasoned. Floors In order to meet all sanitary requirements the “floors must be secure and proof against air, dust, sound, vermin, fire and water.” But it is not often that the floors meet all these sanitary demands. Ordinary floors are made of hard boards tongued and grooved, tightly fitted and laid upon joists placed about one foot apart; the under surface is lathed and plastered to form the ceiling of the room below, or some sort of patent ceiling may be used. However, there is great objection to the space thus left between the floor and the ceiling, as it becomes a receptacle for dust and greatly endangers the spread of fire. It is better, according to sanitary demands, to have the floor of reinforced concrete, or the space within filled with mineral wool or asbestos; or the floor may be made of steel beams encased in terra-cotta with interspaces made of concrete. Roof For roofing some non-absorbent material is best. Wood shingles are possibly the most common in this country, although they are rapidly being replaced with composition shingles. The wood shingles are somewhat absorbent and are inflammable. Slate and tile are also used. The slate is a good conductor of heat, and is hot in summer and cold in winter. Tiles are heavy but are warm in winter and cool in summer. Lead, zinc and copper have all been used, but they are good conductors of heat and for various other reasons are not exactly desirable. The best method of preventing too rapid conduction of heat is to have an air space between the roof and the ceiling of the top floor. House Dampness