What has to go under a basement floor slab in Ontario?
Under a basement slab you need at least 100 mm (4 in.) of coarse, clean granular material such as crushed stone or gravel, plus a sealed soil gas barrier, typically polyethylene, where required for soil gas control, all placed on undisturbed or properly compacted soil with topsoil and organics removed.
Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.
The granular layer does three jobs: it provides uniform bearing, breaks capillary rise of moisture, and forms the permeable layer that makes a radon depressurization rough-in effective. Sand is not a substitute for coarse clean stone under the soil gas provisions. Sequence matters on site: poly goes over the stone with laps and penetrations sealed, and anything cast into the slab, floor drains, sump pits, plumbing stubs, must be sealed to the barrier. Insulation under the slab is optional for comfort in most cases but required in some energy compliance packages for heated slabs.
Strip the topsoil, level the ground, spread at least 100 mm of clean crushed stone, then lay sealed plastic sheeting before the concrete goes down. The stone gives even support and the plastic keeps ground moisture and soil gases like radon out of your basement.
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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.