New Hampshire adopts the 2024 International Residential Code by reference statewide and publishes no frost depth of its own, so the frost depth your footing has to clear is the one the local building official applies under the adopted code.
New Hampshire leaves the frost line depth to the local building department, which is where the number for an address comes from.
The state building code established in RSA 155-A shall be effective in all municipalities, as defined in RSA 155-A:1, XI, in the state, without further local modification, except as permitted by this section, and shall be enforced as provided in RSA 155-A:7.RSA 155-A:3 Enforcement mechanism, Chapter 155-A New Hampshire Building Code, New Hampshire General Court, read September 22, 2026. Read it at the source
That makes the building department issuing the permit the office with the number, and it is a short call: the counter gives the depth for the address, and the same call settles whether a deck footing or a slab edge needs a footing inspection before the concrete goes in. What New Hampshire does publish is below, and it is worth having in hand first.
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. Berlin is the coldest of the 3 cities on the record in New Hampshire.
1,407degree days
average freezing index a winter
Berlin
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Berlin | New Hampshire | 1,407 | 2,438 | 176 |
| Concord | New Hampshire | 871 | 1,815 | 168 |
| Nashua | New Hampshire | 657 | 1,488 | 159 |
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 New Hampshire marked at 978 degree days for comparison against 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 Michigan at 42 inches, Vermont at 5 feet and Wisconsin at 4 feet. That is a reading of other states’ codes rather than a depth for an address in New Hampshire: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
New Hampshire code read September 22, 2026 from RSA 155-A:3 Enforcement mechanism, Chapter 155-A New Hampshire Building Code, New Hampshire General Court. 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.