Wood Roof Trusses
(1) Roof trusses that are not designed in accordance with Part 4 shall,
(a) be capable of supporting a total ceiling load (dead load plus live load) of 0.35 kPa plus two and two-thirds times the specified live roof load for 24 h, and
(b) not exceed the deflections shown in Table 9.23.13.11. when loaded with the ceiling load plus one and one-third times the specified roof snow load for 1 h.
(2) The joint connections used in trusses described in Sentence (1) shall be designed in conformance with the requirements in Subsection 4.3.1.
(3) Where the length of compression web members in roof trusses described in Sentence (1) exceeds 1.83 m, such web members shall be provided with continuous bracing to prevent buckling.
(4) Bracing required in Sentence (3) shall consist of not less than 19 mm by 89 mm lumber nailed at right angles to the web members near their centres with at least two 63 mm nails for each member.
(5) Where the ability of a truss design to satisfy the requirements of Sentence (1) is demonstrated by testing, it shall consist of a full scale load test carried out in conformance with CSA S307-M, “Load Test Procedure for Wood Trusses for Houses and Small Buildings”.
(6) Where the ability of a truss design to satisfy the requirements of Sentence (1) is demonstrated by analysis, it shall be carried out in accordance with good engineering practice such as described in TPIC, “Truss Design Procedures and Specifications for Light Metal Plate Connected Wood Trusses”.
Article 9.23.13.11. Ontario Regulation 332/12 Building Code, Information published by oncodes.ca for educational purposes only.
Prefabricated Roof Trusses
Prefabricated roof trusses offer many advantages. They are designed to handle snow loads that the trusses will be subjected to, are manufactured 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.
Figure 78
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.
Figure 79
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 the ground and wall. Each additional truss is lifted into position, generally 600 mm (24 in.) on centre, toe-nailed to the top plates and temporarily braced (Figure 80). When all trusses are plumbed and properly positioned, they are braced permanently (Figure 81); a) permanent bracing of top chord plane, b) lateral web bracing and c) permanent bracing to web member or bottom chord plane. The stiffness of the roof is increased when the sheathing has been applied. Trusses must be installed and braced according to the manufacturer’s instructions and must not be cut or altered.
Figure 80
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