Plywood, OSB or tongue-and-grooved lumber no wider than 184 mm (8 in. nominal) is typically used for subflooring. Minimum thicknesses for subflooring are shown in Table 22.

Plywood and OSB are often used as subflooring under wood-strip flooring or as a combination subfloor and underlay for resilient flooring or ceramic tile. When used as a combination subfloor and underlay, the side joints must be supported on blocking at least 38 × 38 mm (2 × 2 in. nominal) fitted between the joists unless the panels have tongue-and-groove edges.

Install subfloors with the direction of the panels or planks at right angles to the floor joists an
with the end joints staggered and nailed along the edges at 150 mm (6 in.) on centre and 300 mm (12 in.) at intermediate supports. Use ringed underlay nails, which are designed to resist withdrawal and “nail popping,” or approved staples where the panels provide a combination subfloor and underlay. See Table 23 (p. 287) for sheathing and subflooring fastening requirements.

Floor stiffness can be increased and floor squeaks minimized by applying elastomeric glue between
the floor joists and the subfloor. The subfloor, adhesive and joists act together as a strengthened floor frame, thus reducing deflection between adjacent joists.

All subfloor panels that do not have tongue and groove joints should be covered with an underlay where a vinyl floor covering or ceramic tile is used. An underlay is not required where the edges of the subflooring are supported.

All plywood and OSB panels used for subflooring should be the exterior type (manufactured with waterproof adhesives).

For a lumber subfloor, 19 mm (1 in. nominal) thick boards are generally used. This thickness can be reduced to 17 mm (11⁄16 in.) where joists are spaced at no more than 400 mm (16 in.) on centre. Apply the boards so that the end joints occur over the joists and stagger end joints throughout the floor. Boards may be applied at right angles to the joists or diagonally at an angle of 45°. When the subflooring is laid at right angles to the joists, place strip flooring at right angles to the subflooring unless an underlay is used. Diagonal subflooring permits strip flooring to be laid either parallel to or across the joists. Nail the boards with at least two 51 mm (2 in.) nails at each support. Lumber subflooring must be covered with a panel-type underlay when the floor is finished with resilient flooring.

Source : Canada Mortgage and Housing Corporation (CMHC)