Deck Footing Depth in Illinois: Why DuPage Uses 42 Inches
Before you decide
- The model residential code asks only 12 inches of cover for deck footings, yet an attached deck must reach below the frost line, which DuPage villages inspect at 42 inches.
- The 42 inch rule is a margin, not a measurement: the deepest frost observed in far northern Illinois in a 1980 to 1996 survey was about 30 inches.
- Depth and width are separate questions, and on 1,500 psf soil the 2024 code asks a 25 inch round footing, 8 inches thick, under 100 square feet of deck.
- A deck can skip footings only when it is freestanding, 200 square feet or smaller, and no more than 20 inches above grade.
- Poured concrete hides its own depth, so the open hole is measured before the pour, and the village inspects it at that point.
- A shallow footing on an attached deck shows up later as a deck that rises and settles against the house, and the ledger takes the strain.
Deck footing depth in Illinois is set by frost, not by the deck. Water in the soil freezes and swells. A footing inside that layer rides up with it each winter, so the footing has to sit below it.
This guide gives the depth that applies in DuPage County and shows where the model code differs. It covers footing width, the freestanding exception and how a shallow footing shows itself. It ends with inspection and cost.
How deep do deck footings have to be in Illinois?
Below the local frost line, for any deck attached to a house. The Village of Glendale Heights in DuPage County sets the number in its village deck brochure. The footing base sits 42 inches below grade.
Carol Stream sets the same 42 inches, as its local permit rules explain. A town with a different frost line can differ. Ask the building department for its number before anyone digs.
What does the model code say about deck footing depth?
Less than most people expect. The International Code Council's 2024 residential code asks only 12 inches. Section R507.3 puts deck footings at least that far below undisturbed ground.
Frost enters through a second clause. A deck attached to a frost-protected structure needs footings that extend below the frost line. Solid rock or another approved method also works.
The American Wood Council's deck construction guide agrees. Footings go 12 inches down or below the frost line, whichever is deeper. It tells readers to ask the local authority for the frost line.
Why do DuPage villages inspect to 42 inches?
Frost depth changes with the winter, so towns pick a safe number. The Illinois State Climatologist ties frost depth in Illinois to soil, moisture and snow cover.
The same article reports the deepest observed frost from a 1980 to 1996 survey. It was 5 inches in far southern Illinois and 30 inches in far northern Illinois. The 42 inch rule leaves a foot beyond that record. A footing cannot know which winter is coming.
How wide does a deck footing need to be?
Width follows the load and the soil, not the frost. The 2024 code sizes each footing from the deck area its post carries and the soil's bearing value. Table R507.3.1 turns those into a diameter.
| Deck area on the post | Round footing, 1,500 psf soil | Round footing, 3,000 psf soil or better | Thickness at 1,500 psf |
|---|---|---|---|
| 20 sq ft | 12 inches | 8 inches | 6 inches |
| 60 sq ft | 19 inches | 14 inches | 6 inches |
| 100 sq ft | 25 inches | 17 inches | 8 inches |
| 160 sq ft | 31 inches | 22 inches | 11 inches |
These sizes assume a 40 psf live load. The code allows interpolation between rows but not extrapolation. The post must bear completely on the footing, so the top needs room for the post base.
Can a deck skip footings entirely?
Sometimes. The 2024 code exempts a freestanding deck whose joists sit on grade along their whole length. It also exempts one on precast pier blocks at grade, if the deck is 200 square feet or smaller.
The walking surface also cannot be more than 20 inches above grade within 36 inches of the edge. A deck attached to the house is not freestanding, so those exceptions do not reach it.
Blocks on the surface still move with the frost. Such a deck can rise and settle because nothing ties it to the house. The tie to the house is what makes depth matter.
What happens when the footings are too shallow?
The deck moves with the seasons. A footing with its base in frozen soil can be lifted as the ground swells, then settle on thaw. Each post moves by a different amount, so the frame twists.
On an attached deck the ledger takes the strain. The guide to a deck pulling away covers the gap that opens at the wall. A low middle can also start at one footing, as the sagging deck guide shows.
When is the footing checked?
Before the concrete goes in. Glendale Heights requires a footing inspection and a final inspection, with 24 hours of notice. It also asks for a call to the utility locating line before digging.
Concrete hides its own depth, so the open hole is the only time to measure it. The guide to a deck permit in Illinois lists the four inspections Carol Stream runs.
How do the footing options compare?
Four options appear in the code. Only the poured pier suits an attached deck in a DuPage village. The other three come with conditions, listed in the last column.
| Footing | Where it fits | Depth | The catch |
|---|---|---|---|
| Poured concrete pier | Any attached deck | Below the frost line, 42 inches locally | Dug and inspected before the pour |
| Precast pier block at grade | Freestanding, 200 sq ft or less, within 20 inches of grade | On the surface | Moves with the frost |
| Footing on solid rock | Where rock is reached | Bears on the rock | Needs rock to be there |
| Other approved system | Where the town accepts it | Set by the approval | Approval comes first |
What does footing work cost?
Footings are priced inside the deck rather than alone. These are starting prices, not quotes. The middle column is the wider Chicago-area market, for scale. The last column is what moves a job.
| The work | Starting price | Wider market | What moves it |
|---|---|---|---|
| New pressure treated deck | From $24 per sq ft | $26 to $50 per sq ft | Height, footings, stairs and railing |
| Typical 12 by 16 foot deck | From $4,495 | $4,700 to $9,300 | Footing count, depth and access |
| Joist replacement | From $175 per joist | $200 to $450 each | Span and hangers |
| Ledger board repair | From $495 | $600 to $1,500 | Flashing and band joist condition |
What cannot be determined before digging?
Which edition of the code the town enforces, and what its frost line says. Towns adopt different editions and amend them. The sizes above come from the 2024 book. The building department holds the final number.
Two good sources can differ by an inch. The council's guide gives 26 inches round for 100 square feet on 1,500 psf soil. The 2024 code gives 25. The plan reviewer decides which table applies.
What the soil will bear, and what is buried. Soil bearing is an assumption until the hole is open. Old footings and utilities only show then. Soft soil changes the plan, not the depth rule.
Deck footing depth questions
How deep do deck footings need to be in Illinois?
For an attached deck, below the local frost line. DuPage County villages such as Glendale Heights inspect to 42 inches below grade. The model code floor is only 12 inches below undisturbed ground. An attached deck follows the town's frost depth instead.
Can I put deck footings less than 42 inches deep?
Only where the town allows it. Under the 2024 code, a freestanding deck on precast blocks needs no footing. It must be 200 square feet or less and no more than 20 inches high. An attached deck needs frost depth. Ask the building department first.
How does Perfect Work decide how deep a deck footing goes?
Perfect Work Remodeling & Repairs asks the building department for its frost depth first. It sizes each footing from the deck area its post carries. It digs to the town's number. It leaves the hole open for inspection before any concrete is poured.
How wide should a deck footing be?
It depends on the area each post carries and the soil. On 1,500 psf soil the 2024 code asks 25 inches round for 100 square feet. On 3,000 psf soil it asks 17 inches. The post must bear completely on the footing.
What can frost heave do to a deck?
Frost heave lifts a shallow footing as soil water freezes and swells, then lets it settle on thaw. Posts move by different amounts, so beams twist and an attached deck pulls against its ledger. Deeper footings are the lasting fix.
What does a deck footing estimate from Perfect Work cover?
Perfect Work Remodeling & Repairs names the work, the number of footings and a starting price. A new deck starts at $24 per square foot, with footings priced inside it. If the hole finds soft soil or an old pier, the estimate is updated first.
Dig to the town's number
Footing depth comes down to one number from the building department. Below it the deck stays put. Above it the deck moves, and the ledger and the posts pay for it over the years.
A visit from Perfect Work Remodeling & Repairs starts with that number. It confirms the frost depth the village uses. It marks the post locations and explains the footing inspection before digging. Wider deck work sits with the firm's deck repair services.
Call Perfect Work at 630-297-7888 · Book a footing check