AZ · set by local jurisdictions
How deep do deck footings and fence posts have to go in Arizona? Below the frost line. Here's the number, and the winter behind it.
The requirement
12″
Footing bottoms in Arizona must reach 12 inches below grade — the frost line. Across Arizona it runs roughly 6″–30″ by county.
Reference: 2018 Phoenix Residential Code (IRC-based), Section R403.1.4 and Table R301.2(1) (2018 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the Arizona frost line. The hole bottom has to reach it — a few inches deeper seats the base cleanly below.
| Bag size | Exact | Buy (with spare) |
|---|
The buy column adds a 10% spare margin and rounds up. Running out mid-pour is the one mistake you can't fix.
Desert lowlands rarely see sustained freezes, so frost barely penetrates the soil around Phoenix. The high plateau country above 5,000 feet gets real winters, and frost reaches roughly two and a half feet at Flagstaff.
The depth tracks how much freezing a winter actually delivers. NOAA's Air Freezing Index keeps a running score of how far and how long air temperatures sit below freezing; deeper scores mean deeper frost, and building codes translate that climate record into the footing depth above. Water expands about 9% when it freezes — when that happens under a footing, the ground lifts it. That's frost heave, and it cracks foundations, tilts fence lines, and pops deck piers. Getting below the frost line puts your concrete where the ground never freezes.
Arizona is a home-rule state where each city or county adopts and amends its own code. Frost is negligible in the Phoenix and Tucson lowlands, but footings still need the 12-inch code minimum below grade (often 18 inches where expansive soils govern), while high-elevation Flagstaff requires about 30 inches.
Treat this page as the starting number, not the permit. County and city amendments are where the real precision lives, and inspectors enforce the local figure — one phone call to the building department settles it.
12 inches is the working number (set by local jurisdictions). Across Arizona it runs roughly 6″–30″ by county. Confirm the exact figure with your local building department — county amendments are common.
Bury one third of the above-grade height, never less than 24″, and never shallower than the frost line — in Arizona that means at least 24″. The post depth calculator checks both rules for you.
Below the frost line — 12″ minimum, bearing on undisturbed soil beneath it. Size the diameter with the deck footing calculator, then figure bags with the depth pre-filled above.
Yes — 811 covers every state, and the locate is free. Call a few business days before any post hole or footing dig and crews mark the buried utilities.