Frost line depth in Ketchikan Gateway Borough, Alaska
Frost reaches about 23 inches in typical soil across Ketchikan Gateway Borough. Codes in this climate commonly set footing depth at 18 to 30 inches.
Moderate frostFrost depth by soil type
| Soil type | Est. frost depth |
|---|---|
| Sand / gravel (drains well) | 30″ |
| Silt / loam (typical yard soil) | 23″ |
| Clay (holds water) | 20″ |
County average. Methodology.
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 Ketchikan Gateway Borough building or permitting office in Alaska and call 811 before digging.
Frost depth by ZIP code in Ketchikan Gateway Borough
| ZIP | City | Est. frost | Code range |
|---|---|---|---|
| 99901 | Ketchikan | 23″ | 18–30″ |
What this means for your project in Ketchikan Gateway Borough
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 30″ 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 23″ 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; add a margin below the estimate above for unusually cold winters.
Slabs & frost-protected foundations
Frost-protected shallow foundations (IRC R403.3) use the 100-year air freezing index — 568 °F-days here — to size perimeter insulation, letting footings sit shallower than the frost line.
Weather stations used
| NOAA weather station | Distance | 100-yr AFI |
|---|---|---|
| Ketchikan, AK | 2 mi | 561 |
| Beaver Falls, AK | 9 mi | 709 |
| Annette Airport, AK | 24 mi | 509 |
Nearby counties
Common questions
What is the frost line depth in Ketchikan Gateway Borough?
About 23 inches in typical soil, based on NOAA's 100-year air freezing index. Across the county's ZIP codes the estimate runs from 23 to 23 inches.
How deep should footings be in Ketchikan Gateway Borough, Alaska?
Codes in climates like this commonly require 18 to 30 inches. The Ketchikan Gateway Borough building department publishes the required frost depth in its adopted code (IRC Table R301.2).
Does frost depth vary within Ketchikan Gateway Borough?
Very little. Estimates across the county stay within 1 inches. Soil type matters more than location here.