Tennessee adopts the 2018 International Residential Code and lists every change it makes to it, none of which touches frost, so the frost line depth stays a number the local building department fills in and that is who to ask.
Tennessee leaves the frost line depth to the local building department, which is where the number for an address comes from.
International Residential Code (IRC), 2018 edition, published by the International Code Council, Inc., ... and Appendix Q except that: 1. Section R313 Automatic Fire Sprinkler Systems is not mandatory ... 3. Figure R301.2(2) Seismic Design Categories is deleted and replaced with Figure R301.2(2) Seismic Design Categories Site Class D from 2015 IRC.Chapter 0780-02-23, One and Two Family Dwellings and Townhouses, Official Compilation of Rules and Regulations of the State of Tennessee, 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 Tennessee 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. Gatlinburg is the coldest of the 3 cities on the record in Tennessee.
81degree days
average freezing index a winter
Gatlinburg 2 SW
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Gatlinburg | Tennessee | 81 | 276 | 106 |
| Nashville | Tennessee | 78 | 271 | 71 |
| Memphis | Tennessee | 46 | 167 | 51 |
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 Tennessee marked at 68 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 North Carolina at 12 inches, Massachusetts at 4 feet and Connecticut at 42 inches. That is a reading of other states’ codes rather than a depth for an address in Tennessee: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Tennessee code read September 23, 2026 from Chapter 0780-02-23, One and Two Family Dwellings and Townhouses, Official Compilation of Rules and Regulations of the State of Tennessee. 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.