When wet soil freezes it expands, and that expansion — frost heave — can lift a footing and crack whatever sits on it. The fix is simple in principle: put the bottom of the footing below the depth that freezes in your area.
What is frost depth
Frost depth is how deep the ground freezes in a typical winter. It varies a lot across BC — shallower on the mild coast, much deeper in the cold interior and north. Your municipality publishes the minimum footing depth for your area.
Why it matters for decks and additions
Decks, additions, fences, and any structure with a footing must reach below frost or they'll heave and settle unevenly season to season, racking the structure and cracking finishes. This is one of the most common reasons deck footings fail inspection.
Footing size
Depth keeps the footing stable; width spreads the load so it doesn't sink. Footing size depends on the load above and the soil's bearing capacity, which is why heavier structures need bigger footings.