Frost depth in Illinois
The frost depth in Illinois is 36″. That is the line the ground freezes to. A footing has to sit below it, so the hole is 42″ deep — not 36″.
Illinois frost depth and the depth you dig
| What | Depth |
|---|---|
| Frost depth in Illinois | 36″ |
| Working allowance below the frost line | 6″ |
| Hole depth to dig in Illinois | 42″ |
Why the Illinois hole goes deeper than the frost line
The frost depth is where the soil freezes solid. Below that line the ground stays workable all winter, which is what lets a footing bear its load instead of being lifted by frozen soil heaving upward. The allowance on top of that is the working room the footing needs above the frozen zone — the part of the hole that sits in seasonally frozen ground rather than permanently frozen soil.
Skipping the allowance is the most common reason a footing is rejected in Illinois. It is also the most expensive kind of mistake, because a deck that has already been framed and decked has to come apart to fix it.
Where Illinois sits
Illinois is in the Lower Midwest band. Illinois publishes 36 inches statewide. Chicago and the northern counties are at the figure; the southern tip is shallower in practice but the state value is what most departments enforce.
What a Illinois frost depth number does not cover
- It does not replace your Illinois building department. The state figure is the common published value. Your county may enforce a deeper one, and in a few cases a shallower one.
- It does not cover soil conditions. Frost depth is about temperature. Fill, expansive clay, or rock at the bottom of the hole is a different problem that the depth number does not solve.
- It does not cover load. Footing diameter and thickness come from the load the deck puts on it, not from how cold it gets. See the footing calculator for that side.
- It does not cover unheated spaces. A deck attached to the house is usually over unheated space; a deck over a heated room or a crawlspace with a heat source may be exempt from part of the frost requirement. Ask.
Before you dig in Illinois
- Call the Illinois county building department and ask for the frost depth they enforce for your address. Ask for the number they put on the permit, not the state default.
- Dig to the depth above, and check what is at the bottom. Rock means a different footing. Soft fill means you are probably into a footing design question, not a depth question.
- Get the permit before the hole is in the ground. Almost every jurisdiction requires it for an elevated deck, and almost none of them check the footing afterwards.
Other Illinois footing references
- Frost depth and footings — the general version, without a state
- Frost depth by state — every state
- Deck footing calculator — concrete volume and bag count
- Deck code checklist — the rest of what a permit will ask for
Questions people actually ask
How deep do deck footings go in Illinois?
Below the frost line. In Illinois that is 36″, so the hole is 42″ — the extra 6″ is working room above the frozen zone. Your county building department enforces the number that goes on the permit.
What is the frost depth in Illinois?
36″. That is the depth the ground freezes to. Illinois sits in the Lower Midwest band.
Do I need a permit for a deck in Illinois?
Almost always, for an elevated deck. The permit is what forces the frost depth question to be answered with your county's number rather than a general one — and it is checked before the hole is dug, not after.
Can I use a shallower footing in Illinois?
Not on a cold-structure footing. A footing above the frost line gets lifted by heave, which takes the deck out of level and racks the frame. The exception is a footing on rock or on a foundation that does not move with the soil — an engineered decision, not a shortcut.