Montana

Statewide summary — confirm with your building department before designing

No value on this row was changed by this audit, but every number here is a statewide summary and none of them is the value for a particular lot. Montana fills in IRC Table R301.2 locally, and the state Building Codes Program directs designers to the ASCE 7 Hazard Tool for wind and to the local building official for snow. The seismic design category is the weakest entry: checked against the USGS ASCE 7-16 design maps at Site Class D and Risk Category II it runs from A in Billings and across eastern Montana to D in Bozeman, Kalispell and the Yellowstone country, with Missoula and Helena in between, so the stored C describes west-central Montana and nowhere else. No single letter works for this state. The City of Billings publishes its own adopted criteria and uses an ultimate design wind speed of 110 mph at Risk Category II with Exposure C, a 42 inch frost depth and a ground snow load of 33 psf, so the stored wind speed matches the largest city exactly and the stored 48 inch frost depth is conservative for Billings. Montana law sets a minimum design roof snow load of 30 psf, and ground snow loads in the mountain ranges are far above the 40 psf shown here. Winter design temperature is ASHRAE data and was not verified. Confirm all of this with the authority having jurisdiction before pouring a footing or framing a roof. Snow load audit, August 2026. Montana is the case-study state of this group and the answer is still that the state publishes no ground snow load. The Department of Labor and Industry adopts the state building code by rule under 50-60-203 MCA, currently the 2021 I-codes effective June 11, 2022 as amended by ARM Title 24, chapter 301, and under 50-60-205 MCA the state code applies within any county, city or town that has not adopted its own, with the state enforcing it there. The department Building Codes Program states on its Snow Load Information page, in capitals, that "PURSUANT TO MONTANA LAW THE MINIMUM DESIGN ROOF SNOW LOAD AFTER ALLOWED REDUCTIONS SHALL BE 30 PSF," and immediately after that "Snow loads shall be determined by the building official." For state-owned agencies and for areas outside certified city, county and town jurisdictions it directs the designer to the ASCE 7 Hazard Tool or to telephone the State Building Codes Bureau; inside a certified jurisdiction it directs the designer to that jurisdiction building official. The department published amendment document for the 2024 code cycle, draft MAR Notice 2026-95.2, keeps exactly this structure: it deletes IBC subsection 1608.2, Ground Snow Loads, and IRC subsection R301.6, Roof Load, and replaces both with the same building-official determination, the same 30 psf minimum design roof snow load unless justified by a Montana licensed design professional, and a pointer to ASCE 7-22 outside certified jurisdictions. Two consequences. First, the 30 psf figure is a roof snow load, not a ground snow load, and it is a floor rather than a maximum; it is not converted here because the conversion needs exposure, thermal and slope assumptions Montana does not supply. Second, because the adopted rule routes ground snow load to a private standards body tool rather than to any state table, and that tool is outside the scope of this audit, snow_load_min_psf and snow_load_max_psf are left null. Montana State University does operate a Ground Snow Load Finder that returns ground snow load in psf by latitude, longitude and elevation, derived from a 2004 civil engineering report; it was not used as the source of a number because it is a university research tool rather than the code agency and it states that local and state requirements supersede the values it provides. A related case-study fact, for wind rather than snow: the department Wind Load Information page states that the IBC identifies some areas of Montana as special wind regions, that the Building and Commercial Measurements Bureau has received no data from case studies of those regions, and that it therefore requires the minimum wind loads to be taken as if the region were not identified as a special wind region. That is the department declining to give a number for the special wind regions, recorded here as a fact. Verbatim current ARM text could not be read. rules.mt.gov is a JavaScript application whose policy-library API returns HTTP 401 to an anonymous request, and the legacy mtrules.org host now redirects to it, so the department own published pages and its published amendment document were used instead. No CAPTCHA or challenge was involved and none was circumvented. None of the four existing values was changed. Seismic design category C is not a statewide truth and is retained only because no single letter is: the USGS ASCE 7-16 design maps service at Site Class D and Risk Category II returns short-period design accelerations of about 0.077 g at Glendive and 0.138 g at Billings, category A; about 0.414 g at Missoula and 0.455 g at Helena, category C; and about 0.57 g at Bozeman, 0.685 g at Kalispell and 0.874 g at West Yellowstone, category D. Ground snow load 40 psf could be neither confirmed nor refuted against a Montana source, because the state publishes none.