Frost line depth in Minnesota

Minnesota Rule 1303.1600 does what most state codes decline to do. Rather than leaving the frost-line cell for each city to fill in, it assigns every county to a zone and gives that zone a depth. Zone I, the thirty-odd counties across the north from Kittson to Cook, gets 60 inches. Zone II, which covers the Twin Cities and everything south, gets 42.

Five feet is a serious excavation, and it is why footing and service-line work in northern Minnesota is priced differently from the same job three hundred miles south. It also explains a piece of local plumbing folklore that is actually true: a properly installed Minnesota water service is deep enough that it effectively never freezes, and the freeze calls the state generates come from the parts of the system that are above ground in unheated space.

The plumbing code adds its own layer. Minnesota Rule 4714.0609 requires building supply piping to be buried below the maximum local frost depth rather than merely below the frost line, which pushes the service line deeper than the footing figure alone would imply.

Across all 179 Minnesota stations with published normals, the median first 32°F night falls around October 2 and the median station records 104 nights a year below 20°F, which is the coldest threshold NOAA counts and a decent proxy for a hard freeze rather than a light frost. The state’s spread runs from HIBBING CHISHOLM HIBBING AP, where the median first freeze is September 8, to LA CRESCENT DAM 7 at October 28.

Minnesota has the coldest median in the country among the states we cover: the typical station sees more than a hundred nights a year below 20°F, and the first one arrives before Halloween nearly everywhere. The internal spread is nearly two months, from Hibbing in the Iron Range in early September to the Mississippi bluffs at La Crescent at the end of October. A statewide rule of thumb is close to useless here, which is part of why the code splits the state in two.

Where these numbers come from

The Minnesota frost figure was researched against the body that publishes it, and it’s recorded with a confidence note of “state code, verified” in our source ledger. Freeze dates are NOAA normals for stations in the state, picked by a fixed rule rather than by hand: first-order stations, spread west to east. The full method is on our methodology page.

• Freeze dates and cold-night counts: NOAA NCEI U.S. Climate Normals, 1991 to 2020, annual and seasonal by station. Retrieved 2026-09-09. Field definitions are in the by-variable readme.
• ZIP code locations: U.S. Census Bureau 2023 ZIP Code Tabulation Area gazetteer, matched to the nearest station by great-circle distance.
• Minnesota Rule 1303.1600, Footing Depth for Frost Protection (Office of the Revisor of Statutes) The Minnesota frost depth figure and basis.
• Minnesota Rule 4714.0609, Minnesota Plumbing Code, building supply/water service depth (Office of the Revisor of Statutes) The Minnesota frost depth figure and basis.
• 2021 International Residential Code, Section P2603.5 (Freezing), ICC free code viewer The water service line burial rule.

The full method, including how we pick a station and what we do when there is not one close enough, is on our methodology page.