Frost line depth by state

The depth the ground freezes in winter. Water expands about 9% when it freezes — if that happens under a footing, the ground lifts it. Footings, deck piers, and fence posts must reach below the frost line so they sit on ground that never freezes.

Winter severity. NOAA's Air Freezing Index tracks how far and how long air temperatures sit below freezing; long, hard winters push frost five feet down in Minnesota while Florida's ground never freezes at all. Building codes turn that climate record into a local digging depth.

No — it's the starting number. Counties and cities amend their codes, and inspectors enforce the local figure. Confirm with your local building department before you pour.

Footings still go at least 12″ below grade on undisturbed soil — that minimum is about bearing, not frost. It applies everywhere, Florida included.