Frost line depth in Wisconsin

Wisconsin Administrative Code SPS 321.16, the frost-protection rule in the Uniform Dwelling Code, is unusually careful about wording. It does not say footings go 48 inches down. It says they go below frost penetration or at least 48 inches below adjacent grade, whichever is deeper. In the northern counties, where frost regularly drives past four feet, the first half of that sentence is the one doing the work.

That distinction is worth carrying into any quote you get. A contractor who treats 48 inches as the requirement everywhere is reading half the rule, and in Vilas or Douglas County that is the half that leaves a footing inside the frost zone.

Wisconsin has not amended the water service burial rule itself, so the model-code language applies: at least six inches below the frost line and never less than twelve inches down. Given the 48-inch floor, that puts a Wisconsin service line four and a half feet or more below the yard almost everywhere in the state.

Across all 204 Wisconsin stations with published normals, the median first 32°F night falls around October 6 and the median station records 86 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 NORTH PELICAN, where the median first freeze is September 18, to MILWAUKEE MITCHELL AP at October 29.

The median Wisconsin station sees about 86 nights a year below 20°F and its first freeze around 6 October, which is a fortnight earlier than Illinois immediately to the south. The internal range is wide: North Pelican in the northwoods reaches its median first freeze on 18 September, while Milwaukee’s lakefront airport holds off until 29 October. Lake Michigan buys the eastern shore roughly six weeks, which is the single biggest geographic factor in the state’s winter plumbing risk.

Where these numbers come from

The Wisconsin frost figure was researched against the body that publishes it, and it’s recorded with a confidence note of “” 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.
• Wisconsin Legislature — Wis. Admin. Code SPS 321.16, "Frost protection" (Uniform Dwelling Code): footings/foundations placed below frost penetration or at least 48 inches below adjacent grade, whichever is deeper The Wisconsin 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.