Framing for Floor Openings where Double Headers and Double Trimmers are used
When framing for large openings such as stairwells or fireplaces, the trimmer joists must be doubled if they support header joists (joists into which other joists are framed) more than 800 mm (32 in.) long. Header joists longer than 1.2 m (4 ft.) must also be doubled. Where unusually large openings occur, trimmer joists that support header joists more than 2 m (6 ft. 6 in.) long and header joists that are more than 3.2 m (10 ft. 6 in.) long must be designed according to accepted engineering practice.
Nailing and assembly methods generally used in the framing of floor openings are shown in Figure 64.
Joist hangers are often used to support long joist headers and tail joists.
Joist twisting can be reduced and load sharing between joists improved by cross bridging, blocking, strapping or a ceiling finish fastened to the underside of the joists. Where a ceiling finish is not used, restraint should be provided at intermediate locations between the supports and at distances not greater than 2.1 m (6 ft. 10 in.).
Intermediate restraint may be provided by the following methods: 19 × 64 mm (1 × 3 in. nominal) or 38 × 38 mm (2 × 2 in. nominal) cross-bridging; or continuous 19 × 64 mm (1 × 3 in. nominal) wood strapping nailed to the bottoms of the joists or both cross-bridging and strapping. Continuous wood strapping is not required when a ceiling finish is provided.
Figure 64
Source : Canada Mortgage and Housing Corporation (CMHC)
