AK · set by local jurisdictions
How deep do deck footings and fence posts have to go in Alaska? Below the frost line. Here's the number, and the winter behind it.
The requirement
42″
Footing bottoms in Alaska must reach 42 inches below grade — the frost line. Across Alaska it runs roughly 32″–120″ by county.
Reference: Anchorage Municipal Code Title 23, Local Amendments to the IRC, Table R403.1 (Footing Depths) (2012 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the Alaska 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.
Long, severe winters with months of subfreezing air push frost several feet into the ground across most of the state, and interior and northern Alaska sit on permafrost that never fully thaws. The milder coastal southeast around Juneau sees shallower frost penetration.
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.
Alaska has no mandatory statewide residential code; cities adopt the IRC with their own amendments. Anchorage requires footings 42 inches below finished grade unless engineered, Fairbanks uses about 42 inches, Juneau about 32 inches, and permafrost regions require engineered foundations that can exceed 100 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.
42 inches is the working number (set by local jurisdictions). Across Alaska it runs roughly 32″–120″ 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 Alaska that means at least 42″. The post depth calculator checks both rules for you.
Below the frost line — 42″ 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.