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OBC DIVISION B · PART 9 · SECTION 9.35 — GARAGES AND CARPORTS

What are the foundation and floor slab requirements for garages in Ontario?

Garage foundations generally follow Section 9.15, with frost-protected walls or piers, especially where the garage is attached to the house. Floor slabs need adequate thickness on prepared granular base and should slope toward the vehicle entry or a drain.

Code reference
Garages and carports require foundations conforming to the Code's footing and foundation provisions, with limited relaxations for some small detached structures where movement will not damage attached construction. Concrete garage floors must be built on suitable compacted base material, with drainage handled by slope to the exterior or to a drain, and anchorage of the structure to its foundation.
OBC Division B, Subsections 9.35.3. and 9.35.4.

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

What this means in practice

Attachment is the pivotal question. An attached garage moves with the house, so its foundation must bear below frost depth, commonly 1.2 m or deeper depending on locality; heaving at a shallow garage foundation will crack the shared structure. Small detached garages and carports have more latitude, and thickened-edge slabs are widely used where permitted. Slope the slab about 1 to 2 percent to the door so meltwater from snow-covered vehicles drains out rather than ponding, and coordinate slab height with the required step-down and gas-proofing details at the house wall.

Plain language

An attached garage needs proper deep foundations just like the house, or frost will heave it and crack walls. The concrete floor sits on compacted gravel and should slope gently toward the garage door so snowmelt runs out.

Related questions

What gas-proofing is required between an attached garage and the house in Ontario?What is the difference between a carport and a garage under the Ontario Building Code?

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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.