Floor Sheathing and Underlayment
Plywood provides a superior floor beneath resilient flooring and carpeting because it is dimensionally stable. It remains flat to give a smooth, uniform surface that does not crack, cup or twist. T&G plywood panels reduce labour and material costs by eliminating the need for blocking at panel edges.
Select Tight Face grade is particularly suitable for use under resilient finish flooring. If Select grade is used, splits and other minor permitted characteristics in the grade should be filled with a material which, on setting, will provide a hard, firm surface.
Sheathing grade is suitable for subflooring, beneath such applications as hardwood flooring or concrete topped floors.
Plywood panels used as wall sheathing provides excellent resistance to lateral forces such as wind and earthquake loads. Also, damage to finish materials in the structure due to uneven settling of the frame is minimized due to the extra rigidity plywood adds to the building.
Plywood sheathing contributes to the over-all thermal performance of the wall because the large panels have fewer joints through which heat can escape.
A combined sheathing and siding system using one plywood layer as both a structural sheathing and a finish cladding is a sound and economical method of wall construction in which only plywood is applied over the studs. Specialty textured plywoods are manufactured to provide a good appearance for such applications.
Plywood is often used for roof sheathing. The panel size combines speed of erection with lightweight portability. The stiffness of the panels constitutes to diaphragm action when prescribed framing and nailing patterns are used.
Panels should be applied with the face grain at right angles to the supports to provide maximum strength. A 3mm gap should be left between panels to allow for expansion.
H-clips, used only for roof sheathing, are an alternative to solid blocking. In conjunction with 9.5mm (3/8″) plywood roof sheathing, spanning rafters placed 600mm (2′) apart, H-clips are subject to the following conditions:
- One H-clip shall be placed midway in the joint of abutting sheets between each pair of rafters or joists. Rafter or joist spacing shall not exceed 600mm (2′).
- H-clips must fit snugly.
- Abutting plywood sheets shall be fitted as closely as clips permit. Occasional misfit of abutting sheets may be tolerated providing gaps at maximum opening do not exceed 6mm (1/4″).
As an alternative to using H-clips, specialty tongue and grooved plywood for roofing applications provide faster installation and better structural integrity. The T&G lock together to form a better rain barrier from rain during construction.