Wisconsin is one of the few states that simply states the number: its own Uniform Dwelling Code puts footings 48 inches down everywhere in the state, or deeper where frost goes deeper, and the state plumbing code adds a predicted frost depth table running from two and a half to ten feet depending on the zone and the backfill soil.
4 feet
Wisconsin's residential code sets one frost line depth for the whole state: 4 feet.
Footings and foundations, including those for landings and stoops, shall be placed below the frost penetration level or at least 48 inches below adjacent grade, whichever is deeper, except as allowed under sub. (2).Chapter SPS 321, Construction Standards, SPS 321.16 Frost protection, Wisconsin Administrative Code, read September 22, 2026. Read it at the source
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. Wisconsin writes both of them out.
Wisconsin writes one number into its own dwelling code: footings and foundations go 48 inches below adjacent grade, or deeper if frost penetrates deeper, and may never be placed on frozen material.
Footings may not be placed on frozen material.Chapter SPS 321, Construction Standards, SPS 321.16 Frost protection, Wisconsin Administrative Code, read September 22, 2026. Read it at the source
The state plumbing code sets no single burial number; it publishes a predicted frost depth read off a four zone state map and the backfill soil type, from two and a half feet in clay in the mildest zone to ten feet in gravelly sand in the coldest, and requires buried piping to sit below that depth or be insulated.
Frost protection for a building sewer shall not be required where the predicted depth of frost as determined from Figure 382.30-1 and Table 382.30-6 does not extend below the top of the building sewer.Chapter SPS 382, Design, Construction, Installation, Supervision and Inspection of Plumbing, Table 382.30-6, Wisconsin Administrative Code, 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. Wausau is the coldest of the 3 cities on the record in Wisconsin.
1,591degree days
average freezing index a winter
Wausau ASOS
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Wausau | Wisconsin | 1,591 | 2,960 | 166 |
| Madison | Wisconsin | 1,099 | 2,304 | 155 |
| Milwaukee | Wisconsin | 797 | 1,900 | 132 |
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 Wisconsin marked at 1,162 degree days for comparison alongside 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 Vermont at 5 feet, Michigan at 42 inches and Connecticut at 42 inches. That is a reading of other states’ codes rather than a depth for an address in Wisconsin: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Wisconsin code read September 22, 2026 from Chapter SPS 321, Construction Standards, SPS 321.16 Frost protection, Wisconsin Administrative Code. 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.