Frost depth in Alaska
Alaska has two code depths: 42 inches in 72 inches — and which one applies depends on the county you build in. Add 6 inches of bearing and that becomes a dig depth of 48" / 78". The numbers below are the code values an inspector measures, not climate averages — the local building department always has the final word.
| Area / zone | Frost depth | Dig depth | Set by |
|---|---|---|---|
| Southcentral & coastal (Anchorage) | 42" | 48" | Anchorage Municipal Code Title 23 |
| Interior (Fairbanks, North Slope) | 72" | 78" | Set by borough; local amendment |
Two rows, two numbers: the county you build in decides which one governs your permit. Never take the shallower value to a counter in a deeper zone.
Why Alaska's number looks the way it does
Alaska has the deepest footing requirements in the country. Anchorage and the southern coast sit at 42 inches thanks to the moderating ocean, but Fairbanks and the North Slope — where the ground stays frozen for months — set the national ceiling at 72 inches, and boroughs can require more.
From frost depth to dig depth in Alaska
Footings bear on undisturbed soil below the frost line, so the hole goes 6 inches deeper than the code number. In Alaska that means:
| Code frost depth | Dig depth | Shortest Sonotube |
|---|---|---|
| 42" | 48" | 4 ft tube |
| 72" | 78" | 8 ft tube |
A 4-inch gravel base at the bottom of the hole is common practice for drainage, and if you use one, add it to the tube length rather than the dig depth — the footing still has to reach the full 42–72 inches below grade.
How much concrete a footing takes in Alaska
Bags of 80-lb mix per footing, for a solid pier with no post in the hole, at Alaska's code depth:
| Frost depth | 10" hole | 12" hole | 16" hole | 20" hole |
|---|---|---|---|---|
| 42" (48" dig) | 4 bags | 6 bags | 10 bags | 15 bags |
| 72" (78" dig) | 6 bags | 9 bags | 16 bags | 24 bags |
The bags calculator converts any total into 80, 60, 50 or 40-pound bags, and the concrete calculator handles slab and footing volumes.
Alaska frost depth FAQ
How deep is the frost line in Alaska?
Alaska has two code depths, and the county you build in decides which one an inspector measures: 42 inches in Southcentral & coastal (Anchorage), and 72 inches in Interior (Fairbanks, North Slope). These are code values, not the average depth the ground actually froze last winter.
How deep do footings have to go in Alaska?
Bearing sits below the frost depth, so dig to the code depth plus 6 inches: 42 inches of frost means a 48-inch dig, and 72 inches of frost means a 78-inch dig in Alaska. The extra 6 inches is the bearing layer that keeps the footing from sitting on frost-susceptible soil, and it is what Anchorage Municipal Code Title 23 expects to see on a permit drawing.
How do I know which Alaska zone my county is in?
The citation in the table above names the authority that drew the lines — Anchorage Municipal Code Title 23; Set by borough; local amendment. Your county or city publishes its own amendment to IRC Table R301.2(1) (or its own zone map), and that local document, not a state table, is what a permit reviewer checks. One call to the building department settles it before you rent an auger.
Plan a pour in Alaska
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Last updated: 2026-11-02