![]() ![]() Principal rafters may be mixed with common rafters or carry common purlins. Heavy timber rafters typically spaced 240 cm (8 ft) to 370 cm (12 ft) apart are called principal rafters. Timber trusses also are built in a variety of styles using wood or metal joints. They are designed by the manufacturer for each specific building. Pre-manufactured roof trusses come in a wide variety of styles. The gap in the sheathing at the ridge is the space designed to allow natural ventilation. The rafters, tie beams and plates serve to transmit the weight of the roof to the walls of the buildingĪ truss roof with tongue and groove sheathing. Collar beams or collar ties may be fixed higher up between opposite rafters for extra strength. ![]() 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. Heavier under purlins or purlin plates are used to support longer rafter spans. ![]() 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. 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. Flashing is used to seal the gap between the chimney and roofing material.Ī simple rafter roof consists of rafters that the rafter foot rest on horizontal wall plates on top of each wall. Chimneys are typically built with a water diverter known as a cricket or saddle above the chimney. The roof framing may be interrupted for openings such as a chimney or skylight. The United States still uses imperial units of measurement and framing members are typically spaced sixteen or twenty-four inches apart. 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. This is not normally a problem in areas not prone to high wind or extreme weather conditions. This is accomplished by using metal ties fastened to each rafter or truss. Every component of the roof, as of course the rest of the structure, has to withstand the uplift forces of high wind speeds. 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. Flat roofs on houses are primarily found in arid regions. Flat roofs actually slope up to approximately ten degrees to shed water. The steepness or roof pitch of a sloped roof is determined primarily by the roof covering material and aesthetic design. 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. Timber framed and historic buildings may be framed with principal rafters or timber roof trusses. Modern timber roofs are mostly framed with pairs of common rafters or prefabricated wooden trusses fastened together with truss connector plates. A roof being framed in the United States circa 1955
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