Pitched Roofs

Roof trusses are most often pre-assem-bled, although they can be constructed on site. Pitched roofs can also be stick-built, although this is a

Types of prefabricated roof trusses.

House Roof Types
mono-pitch f f parallel chord
Momo Ptiched Roof

time-consuming process. Of the pitched roofs, the gable roof is the simplest to construct, especially with the use of lightweight roof trusses (Fig. 48). Other configurations, such as the hip roof and L-shape roof, though more complex, can also be framed with trusses (Fig. 49).

Pre-assembled Roof Trusses

Pre-assembled roof trusses offer many advantages in that they save material and speed up the process of enclosing the house. They provide, in one step, a surface for the roof sheathing, a surface for the ceiling finish material and a space for insulation. Ventilation

Temporary bracing of roof trusses

Attic Trusses Temporary Bracing
Note: Gable ends are normally sheathed prior to installation of end braces

of the attic space is easily accomplished through the eaves or gables or both, and at or along the ridge. In most cases, trusses are designed to span from exterior wall to exterior wall with no intermediate loadbearing walls to support the roof loads (Fig. 50). Thus, the entire house may be used as one large workroom during construction. This increases the flexibility of interior planning, because partitions can be placed without regard to structural requirements. Additional flexibility and speed is gained with pre-assembled components and add-on features such as garage roof trusses, porch roofs, simulated mansards and window canopies, which can also be supplied by the truss manufacturer.

Metal-plate-connected trusses can be delivered to the construction site and placed on a flat, clean portion of the site. Trusses under a 20 ft. (6 m) span are usually installed by hand.

Trusses longer than 20 ft. (6 m) require special lifting techniques to avoid damage.

The trusses must be lifted into position with care to prevent excessive lateral bending. The first to be put in place is the gable truss, which is braced to the ground and wall. Each additional truss is lifted into position, generally 24 in. (600 mm) on centre, toenailed to the top plates and temporarily braced (Fig. 51). When all trusses are plumbed and properly positioned, they are braced permanently (Fig. 52). The stiffness of the roof is increased after the sheathing has been applied.

Site Assembly of Pitched Roofs

For on-site construction, the simplest roof is the gable roof (Fig 53A). All rafters are cut to the same length and pattern, and erection is straightforward. A variation of the gable roof may include dormers for additional light, head room and ventilation (Fig. 53B and Fig. 53C). However, openable windows and fixed skylights that can be fitted on a slope between rafters will provide ventilation and light without the complexity and cost of framing a dormer. In the hip roof design, shown in Figure 53D, common rafters are fastened to the ridge board while hip rafters supply the support for the jack rafters.

Important considerations in framing a liveable attic space are Insulation and proper air and vapour barrier sealing; these techniques are

Permanent bracing of roof trusses: (A) permanent bracing of top chord plane; (B) and (C) permanent lateral bracing to web member or bottom chord plane.

top chord ridge line diagonal brace nailed to web — repeat at approximate 20' (6 m) intervals as required by truss manufacturer k h o

2" x 3" (38 x 64 mm) or 1" x 4" (19 x 89 mm) web bracing as required by truss manufacturer

2" x 3" (38 x 64 mm) or 1" x 4" (19 x 89 mm) web bracing as required by truss manufacturer

Permanent Truss Bracing

sheathing blocking lateral brace diagonal forms braced bay — repeat at both ends and at approximate 20' (6 m) intervals as required by truss manufacturer ceiling sheathing blocking lateral brace diagonal forms braced bay — repeat at both ends and at approximate 20' (6 m) intervals as required by truss manufacturer ceiling

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See isometric below t-shape trussed roof.

See isometric below ridge valley trusses roof sheathing gable end

Hip Valley Roof Construction

girder truss common trusses double wall plates

Note: For clarity, some structural members of some of the trusses have been omitted, and roof sheathing appears continuous.

ridge valley trusses roof sheathing gable end girder truss common trusses double wall plates

Note: For clarity, some structural members of some of the trusses have been omitted, and roof sheathing appears continuous.

The dimensions of roof joists and rafters for the various grades and species of lumber and for the different live loads encountered are given in Tables 29 to 32.

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Responses

  • ghenet russom
    How to build a ridge roof and valley jack rafters?
    8 years ago

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