Frost line depth in Iowa

Iowa has no statewide frost-depth table, which surprises people who expect a state with this much winter to legislate one. What it has instead is a set of municipal design criteria that land in the same place. Des Moines publishes 42 inches in its construction-codes design criteria, and Johnson County publishes the same figure in its residential deck guidance.

The practical effect is that 42 inches is the working assumption across most of the state, and the honest caveat is that it is a working assumption rather than a statewide rule. A contractor quoting a footing or a service-line trench in Sioux City is going to the Sioux City building department for the number, not to a state table, and so should you.

Iowa also generates a particular kind of plumbing call that follows from its housing stock rather than its code. Basements are the norm here, which keeps most supply piping inside conditioned space, and it moves the winter risk to the sump and the outside spigot rather than to the pipes in the walls.

Across all 150 Iowa stations with published normals, the median first 32°F night falls around October 10 and the median station records 77 nights a year below 20°F, which is the coldest threshold NOAA counts and a decent proxy for a hard freeze rather than a light frost. The state’s spread runs from SHELDON, where the median first freeze is September 28, to KEOKUK LOCK DAM 19 at October 27.

The typical Iowa station crosses freezing for the first time around 10 October and records about 77 nights a year below 20°F. The northwest corner runs roughly two weeks ahead of the Mississippi river towns: Sheldon in O’Brien County reaches its median first freeze on 28 September, while Keokuk at the southeast tip holds out until 27 October. That is a month of difference inside one state, and it is why the winterize-by date is worth checking by ZIP rather than by state.

Where these numbers come from

The Iowa frost figure was researched against the body that publishes it, and it’s recorded with a confidence note of “local only, varies” in our source ledger. Freeze dates are NOAA normals for stations in the state, picked by a fixed rule rather than by hand: first-order stations, spread west to east. The full method is on our methodology page.

• Freeze dates and cold-night counts: NOAA NCEI U.S. Climate Normals, 1991 to 2020, annual and seasonal by station. Retrieved 2026-09-09. Field definitions are in the by-variable readme.
• ZIP code locations: U.S. Census Bureau 2023 ZIP Code Tabulation Area gazetteer, matched to the nearest station by great-circle distance.
• City of Des Moines, Construction Codes (Design Criteria / Local Condition Information) The Iowa frost depth figure and basis.
• Johnson County, Iowa, Residential Guidelines: Decks The Iowa frost depth figure and basis.
• 2021 International Residential Code, Section P2603.5 (Freezing), ICC free code viewer The water service line burial rule.

The full method, including how we pick a station and what we do when there is not one close enough, is on our methodology page.