Michigan states one number for the whole state: exterior footings and foundations go 42 inches below actual grade, the same 42 inches the state writes into the frost line depth column of its climatic design table, and only the building official can change it on documented local evidence.
42 inches
Michigan's residential code sets one frost line depth for the whole state: 42 inches.
All exterior footings and foundation systems shall extend 42 inches below actual grade.R 408.30522 Minimum depth, Construction Code, Part 5. Residential Code, Michigan Administrative Rules, 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. Michigan writes one of them out.
Michigan amends the International Residential Code to a single statewide number, 42 inches below actual grade for every exterior footing and foundation system, and lets the building official modify that only on evidence of freezing degree days, soil type, ground water, snow depth experience or exposure.
R403.1.4. Minimum depth. All exterior footings and foundation systems shall extend 42 inches below actual grade. Where applicable, the depth of the footings shall also conform to section R403.1.4.1 of the code.R 408.30522 Minimum depth, Construction Code, Part 5. Residential Code, Michigan Administrative Rules, 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. Sault Ste. Marie is the coldest of the 3 cities on the record in Michigan.
1,623degree days
average freezing index a winter
Sault Ste Marie Sanderson Field
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Sault Ste. Marie | Michigan | 1,623 | 3,012 | 173 |
| Marquette | Michigan | 1,172 | 2,192 | 157 |
| Detroit | Michigan | 566 | 1,455 | 127 |
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 Michigan marked at 1,120 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, Wisconsin at 4 feet and Connecticut at 42 inches. That is a reading of other states’ codes rather than a depth for an address in Michigan: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Michigan code read September 22, 2026 from R 408.30522 Minimum depth, Construction Code, Part 5. Residential Code, Michigan Administrative Rules. 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.