Frost line depth by state

From nothing in Florida and Hawaii to 60 inches in Minnesota, 72 in Maine and North Dakota and 100 in parts of Alaska. Anything that carries a load — a deck or porch footing, a foundation, a fence or pole post — is founded below it, and never less than 12 inches down, so the ground freezing and swelling underneath cannot lift it.

What the frost line is

The depth to which the ground is expected to freeze in a design winter. Water in the soil above it turns to ice and expands, and a footing sitting in that layer is lifted — unevenly, a little more each year — which is frost heave. Below it, the ground stays put. So the building code puts the bottom of every footing below the frost line, and at least 12 inches below undisturbed ground even where it never freezes.

Why your town’s figure wins

Frost depth is adopted locally, in the table every jurisdiction fills in when it adopts the code, and it varies inside a state with elevation and soil — deeper in the mountains than on the coast, county by county. The figures here take the deep end of each state’s range, because a footing that is too shallow cannot be fixed afterwards. Call the building department before digging; it is one question.

What it decides

Deck and porch footings — the bottom of each concrete footing at or below the line; the deck calculator counts them. Fence posts — the deeper of a third of the post and the frost line plus a few inches, worked out state by state on the fence post depth table. Water lines — buried below it so they do not freeze. And foundations, unless they are built as frost-protected shallow foundations, which use rigid insulation around the edge to keep the ground beneath from freezing at all.