How thick does a foundation wall need to be in Ontario?
Minimum thickness comes from OBC tables based on wall material, backfill depth and lateral support. Solid concrete walls can be as thin as 150 mm (6 in.) with shallow backfill, but 200 mm (8 in.) is the common choice for full-height basements; deeper backfill can require more or reinforcement.
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The practical driver is backfill height: a walkout basement side with little soil against it needs less wall than the full-depth side, but the wall is normally poured one thickness throughout. Lateral support at the top, meaning the floor framing anchored to the wall, is baked into the table assumptions, so backfilling before the floor deck is installed and braced is both a table violation and a classic cause of cracked or bowed walls. Large openings, tall pours over about 3 m, and surcharge loads from driveways are the usual triggers for engineered design.
Most Ontario basement walls are 200 mm (8 in.) of poured concrete, which handles a typical full-depth basement. The exact minimum depends on how much soil pushes against the wall, and the wall should not be backfilled until the floor above is holding its top in place.
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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.