Floor Joists

Joists are selected to meet strength, deflection and vibration requirements on which the joist selection tables are based. The spans in the tables are measured between the inside edges of the joist supports and have been calculated for standard Canadian lumber sizes. Strength requirements ensure the floor joist system is strong enough to support the anticipated loads. Deflection requirements ensure that the deformation of the floor under heavy load is within acceptable limits and will not lead to defects, such as cracking of the ceiling below the floor. Vibration requirements ensure that the floor is adequately stiff so that, for example, foot traffic will not cause dishes in cabinets to rattle.

Dimension lumber floor joists are usually 38 mm (2 in. nominal) thick and either 140, 184, 235 or 286 mm (6, 8, 10 or 12 in. nominal) deep depending on the loading, span, spacing between joists, and species and grade of lumber.

Alternatives to dimension lumber floor joists include laminated veneer lumber (LVL) joists, parallel chord trusses and wood I-joists. Wood I-joists have become common residential floor framing materials because they can span longer distances and they shrink less because they are made from dry materials. Typical residential wood I-joists are 241 mm (91⁄2 in.) and 302 mm (117⁄8 in.) deep. Wood I-joists have special installation details (Figure 61) that differ from those for dimension lumber joists.

Figure 61

Principles for wood I-joist installation

Because of their thin webs, wood I-joists are less capable than lumber joists of supporting vertical loads (for example, from load-bearing walls above). For this reason, special blocking techniques are used to transfer vertical loads around and through the wood I-joists (Figure 62). All manufacturers provide technical information that describes these requirements, and their directions must be followed.

Where a sill plate is used, joists are installed after the sill plates have been levelled, caulked and anchored to the foundation wall. The joists are located and spaced according to the design.

Joist spacing of 400 mm (16 in.) on centre is the most common. For heavy loads or when floor depth is limited, joists spaced 300 mm (12 in.) may be acceptable. Conversely, if floor depth is not a limitation, deeper joists at 600 mm (24 in.) spacing may prove more economical.

A joist with a bow edgewise should be placed with the crown on top—it will tend to straighten when the subfloor and floor loads are applied.

Figure 62

Load transfer requirements for wood I-joists

Source : Canada Mortgage and Housing Corporation (CMHC)