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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.
Two king post trusses linked to support a roof. Key:1: ridge beam, 2: purlins, 3: common rafters. This is an example of a "double roof" with principal rafters and common rafters. A timber roof truss is a structural framework of timbers designed to bridge the space above a room and to provide support for a roof.
Greek: the ridge height is 1 ⁄ 9 to 1 ⁄ 7 the span (an angle of 12.5° to 16°); Roman: the ridge height is 2 ⁄ 9 to 1 ⁄ 3 the span (an angle of 24° to 34°); Common: the rafter length is 3 ⁄ 4 the span (about 48°); Gothic: the rafters equal the span (60°); and; Elizabethan: the rafters are longer than the span (more than 60°). [7]
For longer span rafters, building materials manufacturers have created laminated veneer lumber (LVL) rafters that can be 2–5 times longer than typical wood rafter. In the US, most wood rafters have maximum length of 20 feet (6.1 m). [5] If a longer rafter is needed, LVL is an alternative.
A hammer-beam is a form of timber roof truss, allowing a hammerbeam roof to span greater than the length of any individual piece of timber.In place of a normal tie beam spanning the entire width of the roof, short beams – the hammer beams – are supported by curved braces from the wall, and hammer posts or arch-braces are built on top to support the rafters and typically a collar beam.
A view of a roof using common purlin framing. The purlins are marked in red. This view is from the inside of the building, below the roof. The rafters are the beams of wood angled upward from the ground. They meet at the top of the gable at a ridge beam, which has extra bracing to attach it to the rafters.
The simplest form of roof framing is a common rafter roof. This roof framing has nothing but rafters and a tie beam at the bottoms of the rafters. The next step in the development of roof framing was to add a collar, called a collar beam roof. Collar beam roofs are suitable for spans up to around (4.5 meters). [5]
The chord size and web configuration are determined by span, load and spacing. The parallel chord truss, or flat truss, gets its name from its parallel top and bottom chords. It is often used for floor construction. A combination of the two is a truncated truss, used in hip roof construction.