Frost line depth in Lewis and Clark County, Montana
Frost reaches about 45 inches in typical soil across Lewis and Clark County, from 39″ in the mildest ZIP code to 47″ in the coldest. Codes in this climate commonly set footing depth at 36 to 54 inches.
Deep frostFrost depth by soil type
| Soil type | Est. frost depth |
|---|---|
| Sand / gravel (drains well) | 56″ |
| Silt / loam (typical yard soil) | 45″ |
| Clay (holds water) | 39″ |
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 Lewis and Clark County building or permitting office in Montana and call 811 before digging.
Frost depth by ZIP code in Lewis and Clark County
What this means for your project in Lewis and Clark County
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 54″ 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 45″ 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 — 2,071 °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 |
|---|---|---|
| Austin 1 W, MT | 9 mi | 2,013 |
| Helena, MT | 13 mi | 2,269 |
| Holter Dam, MT | 18 mi | 1,453 |
| Lincoln Ranger Station, MT | 27 mi | 2,031 |
Nearby counties
Common questions
What is the frost line depth in Lewis and Clark County?
About 45 inches in typical soil, based on NOAA's 100-year air freezing index. Across the county's ZIP codes the estimate runs from 39 to 47 inches.
How deep should footings be in Lewis and Clark County, Montana?
Codes in climates like this commonly require 36 to 54 inches. The Lewis and Clark County building department publishes the required frost depth in its adopted code (IRC Table R301.2).
Does frost depth vary within Lewis and Clark County?
Yes. Estimates differ by 8 inches between the coldest and mildest ZIP codes, mostly from latitude and elevation. Check your own ZIP below.