In the UK, a concrete tiled roof would normally have rafters at 600mm centers, roof battens at 300mm centers and ceiling joists at 400mm centers. The United States still uses imperial units of measurement and framing members are typically spaced sixteen or twenty-four inches apart.
With continual improvements in steel girders, these became the major structural support for large roofs, and eventually for ordinary houses as well. Another form of girder is the reinforced concrete beam, in which metal rods are encased in concrete, giving it greater strength under tension.
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.
When the warm, moist air from below rises into the cold roof space; condensation begins as the air temperature drops to the 'dew point' or as the warm air comes into contact with any of the cold surfaces in the roof.