Ridge board or ridge beam
A ridge board (see Figure 83) or a ridge beam (see Figure 84) is used to ensure a level ridgeline and to facilitate placement of the rafters. Erect rafters in pairs and nail them to the ridge board or beam. Toe-nail the lower ends to the wall plate. Locate each pair of rafters directly opposite each other or offset at the ridge by not more than their thickness. Offsetting is required to maintain vertical alignment of the rafters when the lower ends are tied to ceiling joists that have been lapped (rather than butted together) at the centre load-bearing wall (see Figure 84).
A roof that slopes less than 1:3 should be vertically supported at the peak. This can be accomplished by providing a 38 × 140 mm (2 × 6 in. nominal) ridge beam supported at 1.2 m (4 ft.) intervals by 38 × 89 mm (2 × 4 in. nominal) vertical struts (see Figure 84). A load-bearing wall may be used instead of the ridge beam. Since the ridge support does not result in outward thrust of the roof members, continuous ties between the lower ends of opposing rafters are not necessary. Ridge beams are also required for steeper pitched roofs when the outside ends of the rafters cannot be tied together to resist thrust.
Intermediate support is generally provided between the ridge and exterior walls to reduce the span of the rafters. This reduces the size of the rafters that is required, as the span is measured from this intermediate point to the ridge or eave support.
For rafters in roofs that slope 1:3 or more, intermediate support is usually provided by 38 × 89 mm (2 × 4 in. nominal) collar ties nailed to the side of each pair of rafters. Since these ties are in compression and subject to buckling, ties more than 2.4 m (8 ft.) long should be supported against lateral deflection. This can be done by nailing a 19 × 89 mm (1 × 4 in. nominal) continuous member at right angles to the collar ties near their centre with three, 76 mm (3 in.) nails at each end (see Figure 83).
Intermediate support for rafters in roofs that slope less than 1:3 is usually provided by a dwarf-bearing wall (see Figure 84) built in the same way as a load-bearing partition, except that a single top plate may be used where the rafters are positioned directly over the studs.
Struts may also be used as intermediate supports for rafters in pitched roofs. A 38 × 89 mm (2 × 4 in. nominal) strut (see Figure 86) is nailed to the side of each rafter and supported on a load-bearing partition. The angle of any struts should not be less than 45° from the horizontal.
Rafters at right angles to the ceiling joists may be supported at an intermediate point by a dwarf wall sitting on a beam placed between the ceiling joists. The underside of the beam is raised at least 25 mm (1 in.) above the ceiling finish by blocks inserted under the ends of the beam at the exterior walls and intermediate load-bearing partition. The space prevents the beam from damaging the ceiling finish when deflected by the roof loads.
A beam may also be used as intermediate support for hip and valley rafters. A roof strut is used to transfer the load from the hip or valley rafter to the beam. Where intermediate support is required for a few rafters in the end section on a hip roof, a simple support can be provided by a beam (sometimes called a “strongback”) consisting of two, 38 × 89 mm (2 × 4 in. nominal) members nailed together, set on edge and nailed to the bottom of the rafters. This beam is supported at points along its length by 3 8 × 89 mm (2 × 4 in. nominal) struts radiating from a common point of support on the intermediate load-bearing wall. The ends are cut to fit the selected angle and securely nailed in place.
Source : Canada Mortgage and Housing Corporation (CMHC)