Timber roof truss

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 such as purlins. The space between each truss is known as a bay.

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.

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. In historic carpentry the top chords are often called rafters, and the bottom chord is an often a tie beam. 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.

The terms closed truss and open truss are used in two ways to describe characteristics of truss roofs. Closed truss: 1) A truss with a tie beam; 2) a roof system with a ceiling so the framing is not visible. Open truss: 1) A truss with an interrupted tie beam or scissor truss which allow a vaulted ceiling area; 2) Roof framing open to view, not hidden by a ceiling.

A king post truss has two principal rafters, a tie beam, and a central vertical king post. The simplest of trusses, it is commonly used in conjunction with two angled struts.

The king post is normally under tension, and requires quite sophisticated joints with the tie beam and principal rafters. In a variation known as a king bolt (rod) truss the king post is replaced by a metal bolt (rod), usually of wrought iron.

A queen post truss has two principal rafters and two vertical queen posts. 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. Sometimes a palladian truss is defined as a compound truss with a queen post and king post truss in the same assembly.

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