ID · set by county and city amendment
How deep do deck footings and fence posts have to go in Idaho? Below the frost line. Here's the number, and the winter behind it.
The requirement
24″
Footing bottoms in Idaho must reach 24 inches below grade — the frost line. Across Idaho it runs roughly 24″–48″ by county.
Reference: 2018 Idaho Residential Code (IDAPA 24.39.30 Rules of Building Safety, adopting the 2018 IRC statewide), Table R301.2(1) and Section R403.1.4.1 (2018 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the Idaho 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.
Cold, dry continental winters push frost roughly two feet into the southern valley soils around Boise. Long subzero stretches in the panhandle, the eastern highlands, and mountain towns drive freezing much deeper; depth tracks elevation more than latitude.
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.
Treasure Valley cities (Boise, Meridian, Nampa) require 24 inches; eastern Idaho jurisdictions commonly use 30-36 inches, and high-elevation mountain towns require up to 48 inches. Each jurisdiction fills in its own frost depth, so verify with the local building department.
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.
24 inches is the working number (set by county and city amendment). Across Idaho it runs roughly 24″–48″ 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 Idaho that means at least 24″. The post depth calculator checks both rules for you.
Below the frost line — 24″ 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.