Guide · Footing depth
A footing must bear on undisturbed soil at least 12 inches below grade, and below the local frost line — whichever is deeper. In Florida that means about a foot; in North Dakota it can mean five. Here is why, and the depths by region.
Footing depth is governed by two minimums that both apply at once, from the International Residential Code:
Take the deeper of the two. In a warm climate with no frost, the 12-inch minimum governs. In a cold climate, the frost line governs and can push the footing to 36, 48, or 60 inches down.
Frost depth is set locally — IRC Table R301.2(1) is deliberately blank in the model code so each city or county writes in its own value. These are real adopted figures for a spread of locations; treat them as examples and confirm your own with the building department.
| Location | Frost line (in) | What sets footing depth |
|---|---|---|
| Miami, FL | ~0 | No frost — 12″ minimum governs |
| Nashville, TN | 12 | 12″ (frost = minimum here) |
| Kansas City, MO | 36 | Frost line |
| Denver, CO | 36 | Frost line |
| Boston, MA | 48 | Frost line |
| New York, NY | 48 | Frost line |
| Minneapolis, MN | 42 | Frost line |
| Northern MN / North Dakota | 60 | Frost line |
| Southern Ontario, Canada | ~48 | Frost line (≈1.2 m) |
Local frost depths from adopted municipal/state codes (e.g. Minnesota Rules 1303.1600, NYC Admin Code §27-658, 780 CMR for Massachusetts). Warm regions have a negligible frost line, so the IRC 12-inch minimum governs. Your jurisdiction’s published value always controls.
Width and thickness handle the structural load; depth is almost entirely a frost question. Water in soil expands when it freezes, and a footing sitting in soil that freezes gets lifted by frost heave — then dropped as it thaws. That seasonal movement is what cracks foundation walls and racks door frames. Put the footing below the frost line and the soil under it never freezes. The IRC also forbids footings from bearing on frozen soil at any depth.
Two code-sanctioned ways to go shallower than the frost line: a frost-protected shallow foundation (FPSF, IRC R403.3) uses rigid insulation to keep the ground under a heated building from freezing, and footings on solid rock need not go deeper. Freestanding decks not attached to a dwelling are also exempt from frost-depth footings under the IRC — though many local jurisdictions still require them.
Once you know the depth, size the concrete: whole footings on the concrete footing calculator, deck and pier depth on the sonotube size chart, and how many piers a deck needs on how many footings for a deck. For width and thickness, see how wide a footing should be.
FootingTakeoff reads a footing plan and returns total linear feet, pad count, and an invoice at your rates — automatically.
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