Vermont is one of the few states that names a number: a foundation must reach below the local frost line and never less than five feet below finished grade, though that rule rides on the Fire and Building Safety Code, which covers public buildings and not an owner occupied single family house.
5 feet
Vermont's residential code sets one frost line depth for the whole state: 5 feet.
-delete & replace- IBC section 1809.5 Frost Protection. Except where otherwise protected from frost, foundations and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods: (1) Extending below the frost line of the locality, but not less than 5'-0" below finished grade. (2) Constructing in accordance with ASCE 32. (3) Erecting on solid rock.2025 Vermont Fire & Building Safety Code, Division of Fire Safety, Vermont Department of Public Safety, 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. Vermont writes one of them out.
A foundation has to sit below the local frost line and in no case less than 60 inches below finished grade, unless it is built to ASCE 32 or set on solid rock.
-delete & replace- IBC section 1809.5 Frost Protection. Except where otherwise protected from frost, foundations and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods: (1) Extending below the frost line of the locality, but not less than 5'-0" below finished grade. (2) Constructing in accordance with ASCE 32. (3) Erecting on solid rock.2025 Vermont Fire & Building Safety Code, Division of Fire Safety, Vermont Department of Public Safety, 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. St. Johnsbury is the coldest of the 2 cities on the record in Vermont.
1,175degree days
average freezing index a winter
Saint Johnsbury
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| St. Johnsbury | Vermont | 1,175 | 2,184 | 166 |
| Burlington | Vermont | 1,120 | 2,314 | 150 |
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 Vermont marked at 1,148 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 Wisconsin at 4 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 Vermont: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Vermont code read September 22, 2026 from 2025 Vermont Fire & Building Safety Code, Division of Fire Safety, Vermont Department of Public Safety. 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.