Frost line depth in Ambler, AK 99786
Frost can reach about 88 inches deep in typical soil here (76″ in clay to 111″ in sand). Local codes in climates like this commonly set footing depth at 72 to 108 inches.
Severe frostFrost depth by soil type
| Soil type | Est. frost depth |
|---|---|
| Sand / gravel (drains well) | 111″ |
| Silt / loam (typical yard soil) | 88″ |
| Clay (holds water) | 76″ |
Estimates use the modified Stefan equation with a turf / light-snow surface. Bare, plowed or paved ground freezes deeper; deep snow cover keeps frost shallower. How we calculate this.
Verify before you dig. These are climate-based estimates. The legally required frost depth is the figure your building department publishes in its IRC Table R301.2 (climatic and geographic design criteria). Call the Northwest Arctic Borough building or permitting office in Alaska and call 811 before digging.
What this means for your project in Ambler
Deck footings & piers
Footings must bear below the frost line and at least 12″ below undisturbed ground. For planning in this area, budget for holes around 108″ deep until your permit office confirms the exact figure.
Fence posts
Set posts at least one-third of their above-ground height. Where frost heave is a problem, carry concrete footings below the frost line (about 88″ here in typical soil) and bell or taper the bottom.
Water & irrigation lines
Bury supply lines below the frost depth. Your water utility or plumbing code sets the minimum cover; in a climate this cold, 4 feet or more is common for service lines.
Slabs & frost-protected foundations
Frost-protected shallow foundations (IRC R403.3) use the 100-year air freezing index — 7,951 °F-days here — to size perimeter insulation, letting footings sit shallower than the frost line.
Weather data behind this estimate
| NOAA weather station | Distance | 100-yr AFI |
|---|---|---|
| Kotzebue Airport, AK | 163.6 mi | 7,278 |
| Tanana, AK | 218 mi | 7,452 |
| Barrow Airport, AK | 250.1 mi | 10,179 |
99786 ranks #1 of 11 ZIP codes in Northwest Arctic Borough for frost depth (1 = deepest).
Common questions
How deep is the frost line in Ambler, AK?
Based on NOAA's 100-year air freezing index of 7,951 °F-days for 99786, frost can reach about 88 inches in typical soil, 76 inches in clay and 111 inches in sand or gravel.
How deep do deck footings need to be in 99786?
Footings must sit below the local frost line and at least 12 inches below undisturbed soil. In climates like Ambler, codes commonly set the depth at 72 to 108 inches. The Northwest Arctic Borough building department sets the exact requirement.
Why does soil type change the frost depth?
Wet clay holds a lot of water, and freezing that water releases heat that slows the frost. Dry sand and gravel hold little water and conduct cold faster, so frost goes deeper.
Where does this data come from?
Freezing-index values come from NOAA National Climatic Data Center station records (1951–80), the same data behind the air-freezing-index map in the International Residential Code. We average the three nearest stations, weighted by distance.