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.
A common method of ventilating a roof is to make openings in the soffit and ridge to allow natural air flow. This example also has ventilated exterior walls called rainscreen construction.
The elements in the design of a roof are: the material, the construction, and the durability.
Ventilation of the roof deck speeds the evaporation of water from leakage or condensation and removes heat which helps prevent ice dams and helps asphalt shingles last longer. Building codes in the U.S. specify ventilation rates as a minimum of 1 sq. ft. of opening per 150 sq. ft. (1:150) with a ratio of 1:300 in some conditions. Warm air rises so ceiling insulation is designed to have a higher r-value and the insulation is often installed between the ceiling joists or rafters. A properly insulated and ventilated roof is called a cold roof. A warm roof is a roof that is not ventilated, where the insulation is placed in line with the roof pitch. A hot roof is a roof designed not to have any ventilation and has enough air-impermeable insulation in contact with the sheathing to prevent condensation such as when spray foam insulation is applied directly to the under-side or top-side of the roof deck or in some cathedral ceilings.
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