Key: 1: ridge board, 2: purlins, 3: common rafters.
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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. Trusses usually occur at regular intervals, linked by longitudinal timbers called purlins. The space between each truss is known as a bay.[1]
Timber roof trusses were a medieval development. Earlier roofs had been supported by coupled rafters – pairs of rafters linked by horizontal beams. But such roofs were structurally weak, and lacking any longitudinal support were prone to racking, a collapse resulting from horizontal movement.[2]
The top members of a truss are known generically as the top chord, bottom members as the bottom chord, and the interior members as webs. The top chords as often called rafters, and the bottom chords tie beams. There are two main types of timber roof trusses: closed, in which the bottom chord is horizontal and at the foot of the truss, and open, in which the bottom chords are raised to provide more open space, also known as raised bottom chord trusses.[3]
Closed trusses are usually hidden from view by a ceiling, whereas in an open truss construction the spaces between timbers are left unfilled.[4]
Closed trusses
King post truss
Key: 1: king post, 2: tie beam, 3: principal rafters, 4: struts.
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A king post truss has two principal rafters, a tie beam, and a central vertical king post.[1] The simplest of trusses, it is commonly used in conjunction with two angled struts.
Queen post truss
Key: 1: queen posts, 2: tie beam, 3: straining beam, 4: principal rafters.
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A queen post truss has two principal rafters and two vertical queen posts.[1] The queen post truss extends the span, and combined with spliced joints in the longer members extends the useful span for trusses of these types. As with a king post, the queen posts may be replaced with iron rods and thus called a queen rod truss. This truss is often known as a palladiana (Palladian truss) in Italy, as it was frequently used by the Venetian architect Andrea Palladio.[5][a]The palladiana had been used in Italy for many years before Palladio’s time, but has been given his name because of its extensive use in the buildings he designed.[5]
The queen post truss and the king post truss may be combined, by using the straining beam of the queen post truss as the tie beam for a king post truss above.[6] Such combinations are known as compound trusses.
Open trusses
Arch-braced truss
Key: 1: principal rafters, 2: collar beam, 3: arch braces.
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Lacking a tie beam, the arch-braced truss gives a more open look to the interior of the roof. The principal rafters are linked by a collar beam supported by a pair of arch braces, which stiffen the structure and help to transmit the weight of the roof down through the principal rafters to the supporting wall.[7]
Hammer-beam truss
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The hammer-beam truss can be thought of as an arch-braced truss built on a tie beam from which the middle has been removed, each end supported by braces and carrying hammer-posts that bear the weight of the open structure of the roof.[1]
The hammer-beam roof was the culmination of the development of the arch-braced truss, allowing greater spaces to be spanned. The hammer-beam roof of Westminster Hall in London, designed by Hugh Herland and installed between 1395 and 1399, was the largest timber-roofed space in medieval Europe, spanning a distance of just over 20 metres (66 ft).[8]
A false hammer-beam truss is one in which there are no hammer-posts on the hammer-beam.[1]
Scissor truss
The scissor truss has braces crossing and fixed to each other, tying pairs of rafters together. A scissor-braced truss has an additional king pendant attaching the structure to the ridge board.[1]