A simple rafter roof consists of rafters that the rafter foot rest on horizontal wall plates on top of each wall.The top ends of the rafters often meet at a ridge beam, but may butt directly to another rafter to form a pair of rafters called a couple. Depending on the roof covering material, either horizontal laths, battens, or purlins are fixed to the rafters; or boards, plywood, or oriented strand board form the roof deck (also called the sheeting or sheathing) to support the roof covering. Heavier under purlins or purlin plates are used to support longer rafter spans. Tie beams, which may also serve as ceiling joists, are typically connected between the lower ends of opposite rafters to prevent them from spreading and forcing the walls apart. Collar beams or collar ties may be fixed higher up between opposite rafters for extra strength. The rafters, tie beams and plates serve to transmit the weight of the roof to the walls of the building.
Stone lintels have been used to support roofs since prehistoric times, but cannot bridge large distances. The stone arch came into extensive use in the ancient Roman period and in variant forms could be used to span spaces up to 140 feet (43 m) across. The stone arch or vault, with or without ribs, dominated the roof structures of major architectural works for about 2,000 years, only giving way to iron beams with the Industrial Revolution and the designing of such buildings as Paxton's Crystal Palace, completed 1851.
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.
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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