Roof Truss

A roof truss is a rigid frame of members arranged in triangles that spans from one supporting wall to another and carries the roof. Because a triangle cannot change shape once the lengths of its sides are fixed, a triangulated frame can span a long distance using relatively light members, with loads carried mainly as tension and compression along each member.

In most modern houses in North America, the UK, Australia and New Zealand, roofs are framed with factory-made timber trusses whose joints are connected with toothed metal connector plates (often called gang-nail or nail plates). These metal-plate-connected wood trusses are delivered to site and lifted onto the walls, a method that is faster than cutting rafters by hand.

Parts of a truss

• Top chords: the sloping members that form the roof planes, doing the job of rafters.
• Bottom chord: the horizontal tie that resists outward spread and usually carries the ceiling.
• Webs: the internal diagonal and vertical members that complete the triangulation.
• Heel: where top and bottom chords meet over the wall. A raised-heel (energy-heel) truss lifts the top chord to leave room for full-depth insulation at the eaves.

Common configurations

• King post truss: a central vertical member from apex to tie; one of the simplest forms.
• Queen post truss: two verticals with a horizontal member between them, suited to modest spans.
• Fink (W) truss: a common house truss with webs arranged in a W pattern.
• Attic, scissor and mono trusses: variants that create living space, vaulted ceilings or single-slope roofs.

Historic timber trusses, such as king-post, queen-post, crown-post and hammerbeam roofs, were framed with carpentry joints and pegs rather than metal plates.

Practical notes

Trusses are engineered as a system, so cutting or removing webs or chords, for example during a loft conversion, can compromise the whole roof and generally needs an engineer's design. Bracing between trusses is part of the design and keeps them upright and stable against wind.