1. Why frost line sets footing depth
Water in the soil expands as it freezes. If a footing sits above the frost line, the ground beneath it can heave upward in winter and settle unevenly in spring, which over time cracks concrete and racks the deck frame. Building codes require footings to bear below the frost line so the soil supporting them stays at a stable temperature year-round.
2. General regional bands
Required frost depth is set locally, but as a general pattern across the US:
- Little to no frost (Gulf Coast, southern Florida, parts of the Southwest): some jurisdictions require only a shallow footing, often around 12 in., mainly for bearing rather than frost.
- Moderate frost (much of the mid-latitude US): commonly somewhere in the 24–36 in. range.
- Deep frost (northern-tier states, mountain regions, most of Canada): commonly 36–48 in. or more.
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These bands are a general pattern to orient your expectations, not a lookup table. Frost depth is set by the local building department and can differ between two towns in the same state, or with elevation within the same county.
3. Finding your actual required depth
- Call or check the website of your local building department — this is the authoritative source, not a national chart.
- If you pull a permit, the required footing depth is usually stated on the permit or inspection checklist.
- A local contractor or structural engineer will know the depth used for footings in your specific area.
4. Using the footing calculator
Once you know your required depth and footing diameter, plug them into the footing concrete calculator along with your post count to get total concrete volume. Pair this with the deck planning checklist if you haven't yet confirmed post locations and count.