UT · set by county and city amendment
How deep do deck footings and fence posts have to go in Utah? Below the frost line. Here's the number, and the winter behind it.
The requirement
30″
Footing bottoms in Utah must reach 30 inches below grade — the frost line. Across Utah it runs roughly 12″–42″ by county.
Reference: Utah Residential Code (2021 IRC as adopted in the Utah State Construction Code, Title 15A), Section R403.1.4.1; frost line depth is set by each jurisdiction in Table R301.2(1) (e.g., North Salt Lake and Herriman list 30 inches) (2021 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the Utah 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.
Wasatch Front valleys sit near 4,500 feet and see sustained subfreezing spells each winter, driving frost about two and a half feet into the ground. High mountain towns freeze deeper, while the low desert around St. George barely freezes at all.
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.
Utah adopts the IRC statewide but each city or county sets the frost number: about 30 inches across the Wasatch Front, 36-42 inches in mountain jurisdictions like Wasatch and Summit counties, and as little as 12-18 inches around St. George. Verify locally, especially at elevation.
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.
30 inches is the working number (set by county and city amendment). Across Utah it runs roughly 12″–42″ 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 Utah that means at least 30″. The post depth calculator checks both rules for you.
Below the frost line — 30″ 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.