The design and construction of roofs is governed to a large extent by climatic conditions, but no matter what the climate the roof structure must be watertight and as durable as possible.
There are a great many kinds and forms of roofing materials and nearly all are intended to be laid over a layer of boards or sheathing supported by roof rafters, light trusses, or joists.
The sheathing usually consists of nominal 1-inch boards laid tight, but if wood shingles are to be used, the boards may be spaced a few inches apart.
Other types of sheathing material are precast metal-bound gypsum boards, precast reinforced lightweight concrete plank, formed sheet-metal panels, and plywood. Fiberboards are sometimes laid over the sheathing to provide thermal insulation. The principal roofing materials are asphalt-prepared, built-up, rigid, and metal roofings, and wood shingles.