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OBC DIVISION B · PART 9 · SECTION 9.3 — MATERIALS, SYSTEMS AND EQUIPMENT

What concrete strength is required for house foundations in Ontario?

Unreinforced footings and foundation walls generally need at least 15 MPa (2,175 psi) concrete. Garage and carport floors and exterior steps need 32 MPa air-entrained concrete for freeze-thaw exposure.

Code reference
Concrete for footings, foundation walls and similar unexposed elements must reach a minimum specified compressive strength at 28 days, commonly 15 MPa. Elements exposed to freeze-thaw and de-icing conditions, such as garage floors, carport floors and exterior steps, require higher strength (32 MPa) with air entrainment.
OBC Division B, Article 9.3.1.6.

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

What this means in practice

In practice, most Ontario suppliers deliver 20-25 MPa for foundations even though 15 MPa is the floor, because the cost difference is small and stripping is faster. The critical trap is exterior flatwork: garage slabs, porches, and steps exposed to freezing and salt must be 32 MPa with 5-8% air entrainment, or they will scale and spall within a few winters. Order by specified strength and exposure, keep site-added water in check, and get delivery tickets for the inspector.

Plain language

Foundation concrete must meet minimum strength numbers: roughly 15 MPa for footings and basement walls, and a much tougher 32 MPa air-entrained mix for garage floors and outdoor steps so winter salt and freezing don't crumble them.

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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.