Site Assembly of Pitched Roofs
For on-site construction, the gable roof is the simplest roof constructed with rafters or trusses (Figure 82), which are the same length and pattern and can be erected rather easily. Dormers may be added to provide additional light, headroom and ventilation (see Figure 88). The hip roof design uses common rafters fastened to a ridge board, with hip rafters providing the support for the jack rafters (Figure 83b).
For pitched roofs framed with dimension lumber, nail the ceiling joists in place after the interior and exterior wall framing is complete but before the rafters are erected, because otherwise the thrust of the rafters will tend to push out the exterior walls. Ceiling joists are generally used to tie the lower ends of the rafters in pitched roofs that slope 1:3 or more. To prevent the rafter ends from moving outward, nail the ceiling joist to the side of each pair of rafters (Figure 83). For hip roofs where jack rafters run perpendicular to the ceiling joists, a stub joist can tie the jack rafter back to the ceiling joist (Figure 85). Lap and nail the joists together or splice them at the centre load-bearing wall to provide a continuous tie across opposing rafters. The number of nails required in the connections depends upon the roof slope, rafter spacing, snow load and the width of the house. The additional roof load imposed by dwarf walls that run at right angles to ceiling joists (Figure 84) must be considered when choosing the size of the joists. An increase in the depth of the joists to the next standard depth will usually provide the additional strength required where the roof slope is more than 1:4. When the slope is 1:4 or less, determine the size of the ceiling joists from the span tables for roof joists.
Since hip rafters are usually about 51 mm (2 in.) deeper than common or jack rafters, they reduce the space along the end walls to the extent that in low-slope roofs, there may not be enough room to install the outside ceiling joist at normal spacing from the wall. In this case, doubled joists are used and positioned to suit the available space (Figure 85). Tail joists are then added and toenailed to the outside wall plate and end-nailed Since hip rafters are usually about 51 mm (2 in.) deeper than common or jack rafters, they reduce the space along the end walls to the extent that in low-slope roofs, there may not be enough room to install the outside ceiling joist at normal spacing from the wall. In this case, doubled joists are used and positioned to suit the available space (Figure 85). Tail joists are then added and toenailed to the outside wall plate and end-nailed to the doubled joist. The spacing of these tail joists is usually the same as that of the main ceiling joists.
Cut rafters to length with the proper angle at the ridge and notches (known as bird’s mouth joints) on the eaves. The heel or lower part of the rafters should bear directly over the exterior wall. Depending on the design of the roof and the shape of the outside walls, place the rafters:
The last method is used where a portion of the outside wall is inset. The ceiling joists extend beyond the exterior wall and are nailed to the side of the rafters. This provides lateral support for the bearing wall and stability for the ends of the rafters.