Frost Line Depth in Illinois
42 inches in Chicago and the north, 36 in central Illinois, 24 to 30 along the Ohio River
Written by M., a general contractor in Richmond, VA · Last updated · Reviewed against the current Chicago Building Code and municipal IRC adoptions
How Deep Is the Frost Line in Illinois?
The frost line depth for footings in Illinois runs from 42 inches in Chicago, the collar counties and Rockford to 24 inches in the southern tip around Carbondale and Cairo. Peoria, Bloomington and Champaign use 36 inches; Springfield uses 30 to 36. Illinois has no statewide residential building code, so each municipality or county that adopts the IRC fills in Table R301.2(1) itself, and unincorporated areas in many downstate counties have no code at all.
| State | Avg. Depth (in.) | Range (in.) | Major City Examples | Code basis |
|---|---|---|---|---|
| Illinois | 36 | 20 - 50 | Chicago, Aurora, Naperville | No statewide residential code; Chicago Building Code 42 in, municipal Table R301.2(1) elsewhere |
Returns the code depth for the nearest listed city and the regional note for that part of the state. A ZIP outside Illinois still resolves to its own state.
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All 50 states and the national map are on the frost line depth by ZIP code page. Footing width comes from the footing size chart and soil bearing capacity; roof loads from snow load by ZIP code.
What the Illinois Code Says
Illinois is one of a handful of states with no statewide residential building code. The state mandates only the Illinois Plumbing Code, the Energy Conservation Code and accessibility rules; structural requirements come from whatever code each municipality or county adopts, typically a recent edition of the International Residential Code with local amendments. Chicago is the exception, with its own Chicago Building Code (Title 14B of the Municipal Code, now aligned with the 2018 IBC/IRC), and the city writes a 42 inch frost depth into it. The collar counties and their suburbs, from Naperville and Aurora to Joliet, Elgin and Waukegan, use 42 in as well.
North of Interstate 80 the answer is 42 in almost everywhere, including Rockford, DeKalb and the Quad Cities on the Illinois side. Central Illinois steps down: Peoria, Bloomington-Normal, Champaign-Urbana and Decatur use 36 inches, and Springfield's adopted table lists 30 inches (some Sangamon County townships use 36). South of Effingham the values drop again to 24 to 30 in, and Carbondale, Marion, Cairo and the Metro East communities across from St. Louis (Belleville, Edwardsville, Alton) mostly use 24 to 30 in, matching Missouri's St. Louis at 30.
Where a jurisdiction has adopted the IRC, it keeps the R403.1.4 minimum of 12 in below undisturbed ground, the R403.1.4.1 exemption for freestanding accessory structures of 600 sq ft or less, and R403.3 frost-protected shallow foundations, which are common for heated slab buildings in northern Illinois. Where no code has been adopted, there is no legal frost depth, but lenders and insurers expect the regional norm and Illinois's heavy glacial clay heaves badly, so use the values here as a floor. For comparison, Wisconsin requires 48 in statewide and Indiana runs 24 to 42 in by county; see the national frost line table.
Frost Depth Zones in Illinois
With no statewide code the values are municipal, but they follow a clear north-to-south gradient.
| Zone or region | Footing depth | Where it applies |
|---|---|---|
| Chicago, collar counties and the north (Chicago, Naperville, Aurora, Joliet, Elgin, Rockford, DeKalb) | 42 in | Cook, DuPage, Lake, Kane, Will, McHenry, Kendall, Winnebago, DeKalb counties |
| Quad Cities and northwest (Moline, Rock Island, Galesburg) | 42 in | Rock Island, Henry, Knox counties |
| Central (Peoria, Bloomington-Normal, Champaign-Urbana, Decatur) | 36 in | Peoria, McLean, Champaign, Macon counties |
| Springfield and west-central (Springfield, Quincy, Jacksonville) | 30 to 36 in | Sangamon, Adams, Morgan counties |
| Metro East and south (Belleville, Edwardsville, Carbondale, Marion, Cairo) | 24 to 30 in | St. Clair, Madison, Jackson, Williamson, Alexander counties |
Illinois Frost Line Depth by City and County
Values below are the frost line depths each municipality applies. Illinois suburbs adopt codes independently of their county, so neighboring towns can differ; the county column is a guide to jurisdiction in unincorporated areas.
| City | County | Required Footing Depth (in.) | Source |
|---|---|---|---|
| Chicago | Cook | 42 | Chicago Building Code (Title 14B), frost depth 42 in |
| Aurora | Kane / DuPage | 42 | City of Aurora Building and Permits, IRC Table R301.2(1) |
| Naperville | DuPage / Will | 42 | City of Naperville TED Business Group, IRC Table R301.2(1) |
| Joliet | Will | 42 | City of Joliet Building Division, IRC Table R301.2(1) |
| Rockford | Winnebago | 42 | City of Rockford Construction and Development Services, IRC Table R301.2(1) |
| Elgin | Kane | 42 | City of Elgin Building Division, IRC Table R301.2(1) |
| Waukegan | Lake | 42 | City of Waukegan Building Department, IRC Table R301.2(1) |
| Peoria | Peoria | 36 | City of Peoria Community Development, IRC Table R301.2(1) |
| Champaign | Champaign | 36 | City of Champaign Building Safety, IRC Table R301.2(1) |
| Bloomington | McLean | 36 | City of Bloomington Building Safety, IRC Table R301.2(1) |
| Springfield | Sangamon | 30 | City of Springfield Building and Zoning, IRC Table R301.2(1); 36 in in some townships |
| Decatur | Macon | 36 | City of Decatur Neighborhood Inspections, IRC Table R301.2(1) |
| Belleville | St. Clair | 30 | City of Belleville Building Department, IRC Table R301.2(1) |
| Carbondale | Jackson | 24 | City of Carbondale Building and Neighborhood Services, IRC Table R301.2(1) |
| Moline | Rock Island | 42 | City of Moline Inspections, IRC Table R301.2(1) |
Illinois has no statewide residential code; each municipality sets its own frost depth or has none. Where your township has no building department, use the nearest listed city's value as a floor and expect your lender to ask for it.
Decks, Fences and Water Lines in Illinois
- - Deck footings: Any deck attached to the house must bear below the frost line, so piers go to 42 in the north, 24 to 36 downstate in or deeper. Freestanding decks under the IRC R403.1.4.1 size limit may sit on pier blocks if the local inspector allows it. Size the pier with the deck footing size chart.
- - Fence posts: Code rarely requires frost depth for a fence that carries no structure, but posts set shallower than the frost line heave and lean within a few winters in Illinois. Use one third of the post height with a minimum of 24 in, and go to 42 in the north, 24 to 36 downstate in for gate and corner posts. Chicago-area fence contractors set 6 ft privacy fence posts 42 in as a matter of course. Bag counts are in the post hole bags guide.
- - Water lines: IRC P2603.5 puts exterior water service at least 6 in below the frost line and never shallower than 12 in, so the code floor here is your municipality's frost depth plus 6 in (48 in in Chicago). Many Illinois utilities require more; check the water department standard before trenching and call 811 first.
Neighboring states: Wisconsin (48 in), Indiana (30 to 42 in by county), Iowa (42 in), Missouri (30 in St. Louis), Kentucky (24 in). When a project sits near a state line, the permit follows the jurisdiction where the footing is, not where the contractor is based.
Frequently Asked Questions
What is the frost line depth in Illinois?
42 inches in Chicago, the collar counties and northern Illinois; 36 inches in Peoria, Bloomington and Champaign; 30 to 36 in Springfield; and 24 to 30 inches in southern Illinois and the Metro East. Illinois has no statewide code, so each municipality sets the value.
What is the frost line in Chicago?
42 inches to the bottom of the footing, per the Chicago Building Code. Every suburb in Cook, DuPage, Lake, Kane, Will and McHenry counties uses the same 42 in.
How deep do deck footings need to be in Illinois?
To the local frost depth: 42 in in the Chicago area, 36 in central Illinois, 24 to 30 in the south. Attached decks always need frost-depth piers; a freestanding deck of 600 sq ft or less can use the IRC R403.1.4.1 exemption where the municipality allows it.
How deep should fence posts be in Illinois?
One third of the post height and at least 24 in for line posts; the local frost depth for gate and corner posts, which means 42 in in the Chicago area. Illinois glacial clay heaves posts hard; use gravel under the post and crown the concrete.
How deep are water lines buried in Illinois?
Frost depth plus 6 in under the Illinois Plumbing Code (IRC P2603.5 equivalent), so 48 in in Chicago. The City of Chicago Department of Water Management requires 5 ft (60 in) of cover to the top of the service; most suburbs require 5 to 5.5 ft.
My township has no building code. What frost depth should I use?
The nearest city's value, and treat it as a minimum. Unincorporated Illinois often has no residential code, but frost does not care, and a lender or buyer's inspector will expect the regional norm: 42 in north of I-80, 36 in central Illinois, 30 in the south.
Sources
- No statewide residential code; Chicago Building Code 42 in, municipal Table R301.2(1) elsewhere. Retrieved September 27, 2026.
- City of Chicago Department of Buildings. Chicago Building Code (Municipal Code Title 14B). Retrieved September 27, 2026.
- Illinois Capital Development Board. Building codes in Illinois (no statewide residential structural code). Retrieved September 27, 2026.
- International Code Council. 2024 International Residential Code, Table R301.2(1), R403.1.4 and R403.1.4.1. codes.iccsafe.org.
- National Weather Service, North Central River Forecast Center. Frost Depth Map. weather.gov.
City values are the frost line or minimum footing depths published or applied by each jurisdiction as of the retrieval date. Codes change on a three-year cycle and local amendments change between cycles; confirm with the building department before you submit a permit.