Idaho publishes no frost depth of its own for footings and lets each city or county set the design criteria in Section R301, but it does put one hard statewide number on the other thing people dig for: a buried water supply line has to have at least 42 inches of cover, whatever the local frost line turns out to be.
Idaho leaves the frost line depth to the local building department, which is where the number for an address comes from.
(c) Local jurisdictions may amend by ordinance the following provisions of the Idaho residential code to reflect local concerns: (i) Part I, Administrative; (ii) Part II, Definitions; (iii) Part III, Building Planning and Construction, Section R 301, Design Criteria; and (iv) Part IX, Appendices.Idaho Code 39-4116 Local government adoption and enforcement of building codes, Idaho Legislature, 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 Idaho does publish is below, and it is worth having in hand first.
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. Idaho writes both of them out.
Idaho throws out the model footing width tables and writes its own Table R403.1 sized by soil bearing value and stories, with spread footings at least six inches thick, but it sets no frost depth of its own, so how deep the footing has to reach is the frost line the local jurisdiction puts into Section R301 design criteria.
Section R403.1.1. Delete and replace with the following: R403.1.1 Minimum size. Minimum sizes for concrete and masonry footings shall be as set forth in Table R403.1 and Figure R403.1(1). The footing width (W) shall be based on the load bearing value of the soil in accordance with Table R401.4.1. Spread footings shall be at least six (6) inches in thickness (T).IDAPA 24.39.30 Rules of Building Safety (Building Code Rules), Idaho Building Code Board, read September 22, 2026. Read it at the source
Idaho deletes the Uniform Plumbing Code's frost based rule for buried water supply piping and replaces it with one flat statewide number: the cover over building supply yard piping has to be at least 42 inches below grade.
Section 609.1 Installation. Delete the following sentence: Building supply yard piping must be not less than twelve (12) inches (305 mm) below the average local frost depth; and replace it with the following: The cover must be not less than forty-two (42) inches (1068mm) below grade.IDAPA 24.39.20 Rules Governing Plumbing, Idaho Plumbing Board, 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. Idaho Falls is the coldest of the 3 cities on the record in Idaho.
825degree days
average freezing index a winter
Idaho Falls
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Idaho Falls | Idaho | 825 | 2,025 | 169 |
| Boise | Idaho | 241 | 931 | 116 |
| Coeur d'Alene | Idaho | 221 | 732 | 128 |
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 Idaho marked at 429 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 Massachusetts at 4 feet, Connecticut at 42 inches and North Carolina at 12 inches. That is a reading of other states’ codes rather than a depth for an address in Idaho: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Idaho code read September 22, 2026 from Idaho Code 39-4116 Local government adoption and enforcement of building codes, Idaho Legislature. 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.