Illinois

Statewide summary — confirm with your building department before designing

AUDIT 2026-08-12 (physical design data pass). Illinois publishes no ground snow load at all, anywhere, for any part of the state, and the min and max columns are left null as a confirmed absence. The reason is structural rather than an oversight: Illinois has never adopted a statewide building code that assigns climatic design criteria, and even the 2025 change described below does not create one. The statutory text was read in full at the General Assembly site. Where the codes actually come from. Under the Illinois Residential Building Code Act, 815 ILCS 670/15, a contract to build new residential construction in a non-building code jurisdiction must adopt a residential building code as part of the construction contract, agreed between the home builder and the home purchaser; they may pick the International Residential Code or any municipal or county residential code in effect within 100 miles, and if they agree on nothing, the International Residential Code plus the Energy Efficient Building Act code and the state plumbing code are adopted into the contract by operation of law. Section 10 of the same Act defines a non-building code jurisdiction as a municipality or county that has not adopted a residential building code, or that is required to but has not identified its adopted code to the Capital Development Board. So outside the jurisdictions with their own codes, the residential code in Illinois is a term of a private construction contract, and Table R301.2 arrives unamended with its cells to be completed locally. For commercial work in the same jurisdictions, 20 ILCS 3105/10.09-1 requires a certification of inspection against the IBC and IEBC before occupancy, again unamended. What changed on 1 January 2025, and what it does not do. Public Act 103-0510 added 20 ILCS 3105/10.18(e), which requires that from that date any municipal or county building code must regulate the structural design of non-residential new buildings at least as stringently as a baseline building code, rehabilitation work at least as stringently as a baseline existing building code, and residential buildings at least as stringently as a baseline residential code. Each baseline is defined as the edition of the relevant ICC code first published in the current year or the preceding nine calendar years with the least restrictive structural provisions, and the section expressly defines structural design to include snow loads. The subsection is declared a limitation on home rule. This is a real statewide floor and the earlier framing that Illinois has no statewide code needs qualifying to that extent; but it is a stringency floor referencing a model code, not a table of Illinois values, and it assigns no psf figure to any Illinois county. Section 10.18(a) to (d) separately requires municipalities and counties to report the model code and edition they have adopted to the Capital Development Board, which must publish that on its website, so the practical route to the number governing a given lot is that registry followed by the local ordinance. Other statewide Illinois codes were checked and none carries a snow load. 71 Ill. Adm. Code 600 was retrieved in full and is the Illinois Energy Conservation Code together with the commercial and residential stretch codes; its only occurrences of the word snow are an energy-code requirement that construction documents for a solar-ready zone show the ground snow load, and a section on snow melt systems. The Illinois Accessibility Code and the Illinois Plumbing Code are statewide but structural loads are outside their scope. No value on this row was changed by this pass. Ground snow load 25 psf, wind speed 105 mph, seismic design category A and frost depth 42 inches all remain figures that no Illinois authority publishes; each is filled in by the local building official under whatever code that jurisdiction has adopted. The seismic warning in the note below is the most important thing on this row and is unaffected by anything above. PRIOR AUDIT NOTE (retained in full and still accurate): No value on this row was changed by this audit, but the seismic category on it is the single most misleading figure in this state record and you should read this before relying on any of them. IRC Table R301.2 is filled in by the local jurisdiction, not by the state. Seismic design category A is defensible only for northern Illinois. Under ASCE 7-16 at Site Class D and Risk Category II, Chicago, Rockford, Peoria and Quincy band to A on short-period spectral response and to B on the full determination; Champaign and Springfield are B to C; the Metro East St Louis suburbs around Belleville and the Effingham area are C to D; Mount Vernon and Carbondale are D0; Metropolis is D1; and Cairo, at the New Madrid zone, is E, with mapped short-period spectral response above 1.5 g. The value was left at A because it is correct for the Chicago metropolitan area and roughly three quarters of the state population, but it is dangerous south of about Effingham and anyone building there must run a site-specific determination. Ground snow load 25 psf is the Chicago and Rockford value under ASCE 7-16; central Illinois is 20 psf and the far south 15 psf. Ultimate design wind speed 105 mph is the ASCE 7-16 interior base contour at Risk Category II; Illinois site values interpolate to about 106 to 108 mph, so 110 mph is the conservative reading, and some Illinois jurisdictions still publish 115 mph carried over from ASCE 7-10. Frost depth 42 inches matches long-standing Chicago practice but Illinois sets frost depth locally, downstate is shallower, and this figure was not verified against any government source. The termite designation moderate encodes the IRC moderate-to-heavy band, which is the correct band for Illinois, but this was not confirmed against an Illinois authority. The winter design temperature is ASHRAE 99 percent data, is not published by any Illinois authority, and is unverified.