Iowa adopts the 2024 International Residential Code as its residential building code and leaves the frost line depth in Table R301.2 to be determined by where the project sits, so the number a builder needs comes from the city or county building department rather than from the state.
Iowa leaves the frost line depth to the local building department, which is where the number for an address comes from.
Note: The values for Table R301.2 are determined by the location of the project and referenced footnotes from Table R301.2.Iowa Administrative Code, Inspections and Appeals [481] Chapter 301, State Building Code, subrule 301.8(3), read September 23, 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 Iowa 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. Decorah is the coldest of the 3 cities on the record in Iowa.
1,211degree days
average freezing index a winter
Decorah
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Decorah | Iowa | 1,211 | 2,557 | 154 |
| Dubuque | Iowa | 1,116 | 2,328 | 147 |
| Des Moines | Iowa | 806 | 1,896 | 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 Iowa marked at 1,044 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 Iowa: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Iowa code read September 23, 2026 from Iowa Administrative Code, Inspections and Appeals [481] Chapter 301, State Building Code, subrule 301.8(3). 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.