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OBC DIVISION B · PART 9 · SECTION 9.16 — FLOORS-ON-GROUND

What is the minimum concrete thickness for a basement floor slab in Ontario?

A basement floor slab that is not structural must be at least 75 mm (3 in.) of concrete, placed over a prepared granular base. Slabs that carry structural loads, such as supporting walls or acting as rafts, must be designed and are typically thicker.

Code reference
Floors-on-ground that do not support structural loads must be not less than 75 mm thick, cast over a base prepared in accordance with the floors-on-ground provisions. The concrete must meet the minimum strength requirements for interior floors, and the surface finished to suit the intended use; structurally loaded slabs fall outside these prescriptive minimums.
OBC Division B, Subsection 9.16.4.

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

What this means in practice

Seventy-five millimetres is the code floor, but many builders pour 90 to 100 mm for durability, saw-cut crack control and future finishing. The slab thickness rule only works together with the base requirements: granular fill, a soil gas barrier where required, and consistent subgrade compaction. Thin spots often occur where plumbing trenches were backfilled poorly, and slabs cast over soft or organic soil will crack regardless of thickness. If the slab supports posts, bearing walls or a garage vehicle load, it needs specific design rather than the 75 mm default.

Plain language

Your basement floor needs at least 75 mm (about 3 in.) of concrete. It is a wearing surface sitting on gravel, not a structural element, so what is underneath it, clean compacted stone and a sealed plastic layer, matters just as much as the thickness.

Related questions

What has to go under a basement floor slab 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.