Missoula County, MT Frost Line Depth: 36 in

Verified

36in

91 cm · 3 ft

Set in the building code Missoula County has adopted.

Source and how we checked

Document: missoulacounty.build

Climate: 163 nights a year at or below 32 °F (NOAA 1991–2020, Missoula 2 Ne, 9.6 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

163nights a year at or below 32 °F
99%of nearby soil rated “moderate” for frost action (USDA)

How deep is the frost line in Missoula County?

Missoula County's building authority sets the frost line depth at 36 in (91 cm). Footings, piers and other permanent supports must reach at least this depth (IRC R403.1.4.1).

Who sets the frost line depth in Missoula County?

Montana leaves the frost line depth to each city or county, and Missoula County fills it in when it adopts the residential code. The value above comes from Missoula County’s own document on missoulacounty.build. See how Montana sets frost line depth.

How deep do footings, decks and water lines need to be in Missoula County?

House or garage footing Deck footing Water service line Fence post

House or garage footing in Missoula County: dig at least 36 in (91 cm). Below the frost line and at least 12 in deep (IRC R403.1.4).

Minimum depths in Missoula County with a 36 in frost line
Project	Depth	Rule
House or garage footing	36 in	Below the frost line and at least 12 in deep (IRC R403.1.4)
Deck footing (attached)	36 in	Below the frost line and at least 12 in deep (IRC R507.3)
Water service line	42 in	6 in below the frost line and at least 12 in deep (IRC P2603.5)
Fence post bottom	36 in	No code rule; common practice is below the frost line

Work out posts, concrete and footing sizes with the fence post depth calculator or the deck footing calculator.

How cold does it get in Missoula County?

Average lows are at or below freezing from Nov to Mar. 163 nights a year reach 32 °F or colder; Dec is coldest, with an average low of 19.6 °F.

NOAA 1991–2020 climate normals, Missoula 2 Ne station, 9.6 mi away, elevation 3,420 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	19.8	21.9	27.4	33	40.8	46.9	52.2	51	43.6	33.7	26.2	19.6
High	34.1	39	49.1	56.8	66.5	73.4	85.1	84	73.1	56.5	41.2	32.9

Missoula County has fewer freezing nights than 4 of the 7 other Montana places we track. The Montana median is 167. Coldest: Yellowstone County; mildest: Great Falls.

How likely is the soil in Missoula County to heave?

Moderate 99%

USDA rates the sampled soil around Missoula County as 1% low, 99% moderate for frost action (main soils: Waldbillig, Coerock, Holloway).

USDA NRCS soil survey, 9 sample points within 2 mi. Higher frost action means more risk of heave; see frost heave.

How do you confirm the frost line before you dig in Missoula County?

Call 811 at least a few working days before you dig, so buried lines are marked.
Confirm the frost line depth with the Missoula County building department.
Ask whether your project needs a permit (decks, larger sheds and footings usually do).
Dig below 36 in unless the building department tells you otherwise.
Book the footing inspection before you pour concrete.

Missoula County at a glance

Frost line depth: 36 in
Population (2023 est.): 121,849
Mean annual temperature: 46.2 °F
ZIP codes (17): 59801 · 59802 · 59803 · 59804 · 59808 · 59812 · 59820 · 59821 · 59823 · 59825 · 59826 · 59833 · 59834 · 59846 · 59847 · 59851 · 59868