Modern timber roofs are mostly framed with pairs of common rafters or prefabricated wooden trusses fastened together with truss connector plates. Timber framed and historic buildings may be framed with principal rafters or timber roof trusses. Roofs are also designated as warm or cold roof depending on how they are designed and built with regard to thermal building insulation and ventilation. The steepness or roof pitch of a sloped roof is determined primarily by the roof covering material and aesthetic design. Flat roofs actually slope up to approximately ten degrees to shed water. Flat roofs on houses are primarily found in arid regions.[2]
In high-wind areas, such as where a cyclone or hurricane may make landfall, the main engineering consideration is to hold the roof down during severe storms. Every component of the roof, as of course the rest of the structure, has to withstand the uplift forces of high wind speeds. This is accomplished by using metal ties fastened to each rafter or truss. This is not normally a problem in areas not prone to high wind or extreme weather conditions.
In the UK, a concrete tiled roof would normally have rafters at 600 mm (24 in) centers, roof battens at 300 mm (12 in) centers and ceiling joists at 400 mm (16 in) centers. The United States still uses imperial units of measurement and framing members are typically spaced sixteen or twenty-four inches apart.
The roof framing may be interrupted for openings such as a chimney or skylight. Chimneys are typically built with a water diverter known as a cricket or saddle above the chimney. Flashing is used to seal the gap between the chimney and roofing material.
