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OBC DIVISION B · PART 9 · SECTION 9.30 — FLOORING

How thick does the subfloor need to be in an Ontario house?

With joists at 400 mm (16 in) on centre, plywood or OSB subflooring is typically 15.5 mm (5/8 in) minimum with edge support. Thin panel underlay of at least 6 mm may be added under resilient finishes.

Code reference
Structural subflooring thickness is set by joist spacing and panel type, with common lumber, plywood and OSB options tabulated and tongue-and-groove or blocked edges required where panel edges are unsupported. Where finish flooring needs a smoother base, panel-type underlay of limited minimum thickness must be fastened over the subfloor with joints offset from those below.
OBC Division B, Subsections 9.23.15. and 9.30.2.

Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.

What this means in practice

Two layers do two jobs: the structural subfloor carries loads between joists, while underlay only smooths the surface for vinyl, tile or other thin finishes. The 15.5 mm panel at 400 mm joist spacing is the workhorse spec; wider joist spacing or ceramic tile assemblies push you to thicker panels or an added layer, and tile installation standards frequently demand stiffer floors than the bare code minimum. Offset underlay joints from subfloor joints and fasten tightly, since squeaks and cracked tile almost always trace back to panel movement.

Plain language

Under your finished floor sits a structural wood panel, usually 5/8-inch plywood or OSB on joists spaced 16 inches apart. A thin second smoothing layer is added under vinyl or tile so every bump does not telegraph through.

Related questions

What kind of finish flooring is required in bathrooms and laundry rooms in Ontario?

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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.