Montana runs on the 2021 residential code with a long list of state amendments, but frost depth is not one of them, so the number you dig to comes from the city, county or town building official rather than from the state.
Montana leaves the frost line depth to the local building department, which is where the number for an address comes from.
Snow loads shall be determined by the building official. In areas of the state outside of certified city, county, or town jurisdictions, the design snow load shall be based on the ground snow loads calculated using the 2022 edition of the American Society of Civil Engineer's "Minimum Design Loads and Associated Criteria for Buildings and Other Structures, ASCE/SEI 7-22."ARM 24.301.154 Incorporation by Reference of International Residential Code, Administrative Rules of Montana, read September 22, 2026. Read it at the source
That makes the building department issuing the permit the office with the number, and it is a short call: the counter gives the depth for the address, and the same call settles whether a deck footing or a slab edge needs a footing inspection before the concrete goes in. What Montana does publish is below, and it is worth having in hand first.
A frost line depth turns into two separate requirements in most codes: how far a footing has to bear below grade, and how far a buried water service has to sit below the same line. Montana writes both of them out.
Montana keeps the model code's footing rules and adds one local escape hatch, letting the building official accept footings designed to the older 2012 edition or footings engineered by a design professional, but it sets no frost depth of its own for them to reach.
Subsection R 403.1.1 , Minimum size, is modified to add the following: "Exception: The building official may allow footings to be designed in accordance with Section R403 of the 2012 IRC or may allow footings engineered by a design professional."ARM 24.301.154 Incorporation by Reference of International Residential Code, Administrative Rules of Montana, read September 22, 2026. Read it at the source
Montana adopts the 2021 Uniform Plumbing Code with a long list of its own amendments, but none of them touches the buried water service rule, so how deep a supply line goes is still tied to the local frost depth rather than to a number Montana states.
The department adopts and incorporates by reference the Uniform Plumbing Code, 2021 edition, unless another edition is specifically stated, together with the following appendix chapters and amendments:ARM 24.301.301 Incorporation by Reference of Uniform Plumbing Code, Administrative Rules of Montana, read September 22, 2026. Read it at the source
The air freezing index is the quantity the frost provisions of the model codes are written on: add up how far each day’s mean temperature fell below freezing over a winter, in degree days, and take the deepest run the winter managed. It measures how much cold the ground had to absorb, which is why it is the honest way to set one place beside another. Bozeman is the coldest of the 3 cities on the record in Montana.
790degree days
average freezing index a winter
Bozeman Montana State Univ
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Bozeman | Montana | 790 | 1,838 | 178 |
| Missoula | Montana | 728 | 1,766 | 177 |
| Billings | Montana | 683 | 1,917 | 145 |
Computed from NOAA National Centers for Environmental Information, Global Historical Climatology Network daily temperature records for each station, from 1951 onward. These are measurements of winter, not depths.
Only a handful of states answer with one number for the whole state, and they are the only codes that can be set beside each other directly. Ranked by the freezing index at their own reference cities, with Montana marked at 734 degree days for comparison against what those codes settled on:
Left to right, the mildest winters to the hardest. The figure on the right is what that state wrote into its code.
The nearest by winter are Connecticut at 42 inches, Massachusetts at 4 feet and Michigan at 42 inches. That is a reading of other states’ codes rather than a depth for an address in Montana: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Montana code read September 22, 2026 from ARM 24.301.154 Incorporation by Reference of International Residential Code, Administrative Rules of Montana. Codes are amended and a local amendment can be deeper than the state figure, so the building department issuing the permit has the number that binds. Every state is on the frost line depth hub.