Missouri sets no statewide frost line depth and has no statewide residential building code for site built houses: the one place the state adopts the International Residential Code is the Public Service Commission's rule for factory built modular units, and construction rules for everything else are written by the county or the city, so the depth a footing must reach is a local answer.
Missouri leaves the frost line depth to the local building department, which is where the number for an address comes from.
For the purpose of promoting the public safety, health and general welfare, to protect life and property and to prevent the construction of fire hazardous buildings, the county commission in all counties of the first and second classification, as provided by law, is for this purpose empowered, subject to the provisions of subsections 2 and 3 of this section, to adopt by order or ordinance regulations to control the construction, reconstruction, alteration or repair of any building or structure and any electrical wiring or electrical installation, plumbing or drain laying therein, and provide for the issuance of building permitsSection 64.170 of the Revised Statutes of Missouri, County commissions control construction, Missouri Revisor of Statutes, 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 Missouri 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. Kansas City is the coldest of the 3 cities on the record in Missouri.
363degree days
average freezing index a winter
Kansas City Intl AP
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Kansas City | Missouri | 363 | 1,087 | 107 |
| St. Louis | Missouri | 235 | 771 | 92 |
| West Plains | Missouri | 152 | 502 | 104 |
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 Missouri marked at 250 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, North Carolina at 12 inches and Connecticut at 42 inches. That is a reading of other states’ codes rather than a depth for an address in Missouri: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Missouri code read September 23, 2026 from Section 64.170 of the Revised Statutes of Missouri, County commissions control construction, Missouri Revisor of Statutes. 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.