Frost line depth in Illinois

Illinois regulates residential construction at the municipal level, so there is no state table and no state requirement to adopt one. Chicago states the number plainly in its porch construction guidelines: the bottom of the footing must be a minimum of 42 inches below grade. The Village of Flossmoor writes the same figure into its building code as three feet six inches.

Forty-two inches is the working assumption across the Chicago metro, and it does not travel downstate. Peoria, 130 miles southwest, publishes a 36-inch minimum footing depth in its own design criteria, and a Carbondale winter has more in common with Kentucky than with Cook County. Two published figures six inches apart inside one state is the reason to ask your own building department rather than reuse Chicago's number.

For plumbing the number matters mostly as a floor for the service line: the model-code rule puts it at least six inches below the frost line and never less than twelve inches down, so a 42-inch jurisdiction implies four feet.

Across all 170 Illinois stations with published normals, the median first 32°F night falls around October 21 and the median station records 49 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 MT CARROLL, where the median first freeze is October 7, to ALTON - MELVIN PRICE L&D at November 5.

Illinois runs nearly 400 miles north to south and its freeze record shows it. The median station reaches freezing on 21 October with about 49 nights a year below 20°F, but Mt Carroll in the northwest hills gets there on 7 October while the lock and dam station at Alton, near St. Louis, waits until 5 November. Chicago O’Hare, the station most Illinois readers will recognise, has a median first freeze of 28 October and averages 50 nights a year below 20°F.

Where these numbers come from

The Illinois frost figure was researched against the body that publishes it, and it’s recorded with a confidence note of “local only, verified” 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 Chicago Dept. of Buildings, Porch Design & Construction Guidelines ('bottom of the footing must be a minimum of 42 inches below grade') The Illinois frost depth figure and basis.
• Village of Flossmoor Building Code, Section 19.1.2, Bottom of Footings ('not less than 3'6" below adjoining finished grade') The Illinois 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.