How deep do foundations have to be for frost protection in Ontario?
Foundations in frost-susceptible soil must bear below the depth of frost penetration. Across much of southern Ontario that is commonly taken as about 1.2 m (4 ft) below finished grade, and deeper in northern regions; local municipalities set the accepted value.
Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.
Frost heave happens when wet, fine-grained soil freezes and expands, lifting anything bearing above the frost line, so decks, porches, additions and garage footings are the usual victims rather than deep basements, which normally exceed frost depth anyway. Confirm the local frost depth with the municipality since 1.2 m is a common southern Ontario figure but 1.5 to 1.8 m applies in colder zones. Alternatives such as insulated shallow foundations are possible but must follow recognized design methods. Sonotubes for decks need the same frost-depth bearing.
Footings must sit below the level where the ground freezes, because freezing soil swells and can lift a structure. In much of southern Ontario that means roughly 1.2 m (4 ft) down; your local building department can tell you the exact depth for your area.
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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.