IL · set by local jurisdictions
How deep do deck footings and fence posts have to go in Illinois? Below the frost line. Here's the number, and the winter behind it.
The requirement
42″
Footing bottoms in Illinois must reach 42 inches below grade — the frost line. Across Illinois it runs roughly 30″–42″ by county.
Reference: Capital Development Board Act, 20 ILCS 3105 (P.A. 103-0510, statewide IRC baseline effective 2025); frost depth set by municipal and county codes, with 42 inches standard in Chicago-area codes (2025 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the Illinois frost line. The hole bottom has to reach it — a few inches deeper seats the base cleanly below.
| Bag size | Exact | Buy (with spare) |
|---|
The buy column adds a 10% spare margin and rounds up. Running out mid-pour is the one mistake you can't fix.
Northern Illinois gets long stretches of below-freezing weather that push frost several feet into the ground in a hard winter, which is why the Chicago area requires 42 inches. Winters ease toward the Ohio River, so frost penetrates noticeably less in the far south.
The depth tracks how much freezing a winter actually delivers. NOAA's Air Freezing Index keeps a running score of how far and how long air temperatures sit below freezing; deeper scores mean deeper frost, and building codes translate that climate record into the footing depth above. Water expands about 9% when it freezes — when that happens under a footing, the ground lifts it. That's frost heave, and it cracks foundations, tilts fence lines, and pops deck piers. Getting below the frost line puts your concrete where the ground never freezes.
Chicago, Aurora, and Rockford all require 42 inches, while southern Illinois jurisdictions allow roughly 30 inches or less. Illinois' 2025 baseline law requires adopted codes to meet IRC structural minimums, but code adoption and frost depth remain local decisions.
Treat this page as the starting number, not the permit. County and city amendments are where the real precision lives, and inspectors enforce the local figure — one phone call to the building department settles it.
42 inches is the working number (set by local jurisdictions). Across Illinois it runs roughly 30″–42″ by county. Confirm the exact figure with your local building department — county amendments are common.
Bury one third of the above-grade height, never less than 24″, and never shallower than the frost line — in Illinois that means at least 42″. The post depth calculator checks both rules for you.
Below the frost line — 42″ minimum, bearing on undisturbed soil beneath it. Size the diameter with the deck footing calculator, then figure bags with the depth pre-filled above.
Yes — 811 covers every state, and the locate is free. Call a few business days before any post hole or footing dig and crews mark the buried utilities.