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with a high level of quality control, reduce lumber waste and speed up the process of enclosing the house. They provide a surface for the roof sheathing, a surface for the ceiling finish material and a space for insulation. Raised heel trusses (Figure 78) are used in most new houses because they provide a deeper space at the eaves to accommodate greater amounts of insulation. The energy efficiency requirements in most building codes lead to the use of trusses, rafters and roof joists with a vertical dimension at the outer face of the exterior wall framing of 300 mm (12 in.) or more. Ventilation of the attic space is easily provided through the eaves and at the ridge. In most cases, trusses are designed to span from exterior wall to exterior wall with no intermediate load-bearing walls to support the roof loads (Figure 79). This gives flexibility to interior planning because partitions can be placed without regard to structural requirements. A continuous air barrier and vapour retarder can be applied to the underside of the trusses before the interior partitions are erected. 78 Raised heel roof truss provides depth for insulation
Prefabricated trusses should be stored on a flat, clean area of the construction site. Trusses shorter than a 6 m (20 ft.) span are usually installed by hand. Trusses longer than this require special lifting techniques to avoid damage. Trusses must be lifted into a vertical position carefully to prevent excessive lateral bending. The gable truss is placed first and braced to 79 Types of prefabricated roof trusses
king-post
Howe
Fink or W
mono-pitch
parallel chord
scissor truss
provide truss anchors as requested by manufacturer or by code
mansard