Illinois Deck Building Code, in Plain English: What a Deck Has to Meet

What this comes down to

• Illinois has no deck code of its own. Since 2025, every local code must be at least as strict as the IRC.
• Some DuPage villages are stricter than the model code. Lombard requires deck guards at 24 inches, not 30.
• The rules a deck must meet are the ones its village, city or county adopted, amendments included.
• Deck footings in DuPage County go 42 inches deep, below the local frost line.
• Nails may not hold a deck to a house. The ledger needs lag screws or bolts, with flashing above it.
• The state code law covers new homes, not repairs. Deck repairs answer to the local code official.

Every number on a deck inspection is a length. The one that moves most between towns is the guard trigger.

Anyone searching for the Illinois deck building code meets a surprise. There is no single state deck code to download. What a deck has to meet is set by the village, city or county that issues its permit.

This guide explains how that works since the state baseline arrived in 2025. It then walks through what an inspector actually checks: footings, the ledger, guards and stairs. The numbers are the ones DuPage homeowners meet.

Does Illinois have a statewide deck building code?

Not a deck code as such, but since January 1, 2025 there is a floor. Public Act 103-0510 set it. Every local code in Illinois must now be at least as strict as the model codes on structure.

The Capital Development Board administers that rule. It sets out its answers on codes in plain terms. Local codes must regulate house structure at least as strictly as the IRC.

The IRC is the International Residential Code. It is the model code most Illinois towns adopt for houses and decks. The International Code Council publishes it and issues a new edition every three years.

The state itself does not review plans, issue permits or inspect decks. The Board says the local unit of government handles all of that. It also says the state cannot overrule a local code official.

Which code actually applies to my deck?

The one adopted by whoever issues the permit for the address. Inside a village or city, that is the municipality. In unincorporated areas, it is the county. Each adopts an IRC edition, then amends it.

Editions differ from town to town. One DuPage village may build to the 2018 IRC while its neighbor uses the 2021 or 2024 edition. The Board keeps a directory of what each town has reported adopting.

Finding the right permit office is therefore the first step, before any drawing. The walkthrough of the permit application covers who issues it and what the office asks for.

Why can a local code be stricter than the IRC?

Because the state sets a minimum, not a maximum. A village can amend the model code upward, and several in DuPage County do. The deck rules can change at a street that marks a village line.

The Village of Lombard shows how far that goes. Its deck inspection checklist requires guards on any surface 24 inches above grade. The IRC sets that trigger at 30.

Lombard also asks for solid stair risers once a stair is 16 inches high. It wants joist hangers on joists over six feet long, and post holes at least 8 inches across. Those are local rules, and they are what gets inspected.

How deep do deck footings have to be in Illinois?

Below the frost line, which DuPage County sets at 42 inches. Lombard's checklist asks for the same depth, measured from grade. The hole must also be dry and clear of debris before the inspector signs it off.

Frost depth matters because wet soil expands as it freezes. A footing that stops above the frost line gets lifted each winter and settles each spring. Over a few seasons the deck racks, and the ledger takes the strain.

The footing has to sit in soil that never freezes. In DuPage that means the bottom of the hole is 42 inches down.

Frost depth also varies across the state, deeper in the north than the south. A number read on a national website can be wrong for a DuPage deck. The guide to footing depth and permits covers one town's version.

How does a deck have to attach to the house?

With lag screws or through bolts into the house band joist, never with nails. The IRC has long barred nails loaded in withdrawal from holding a deck to a wall. It is the first connection examined when a deck fails.

Engineers at Washington State University showed why, in research published in STRUCTURE magazine. Their tests on deck connections found a hard fact about nails. Smooth nails lose about 75 percent of their grip through wet and dry cycles.

In the same work, a joist hanger fastened with screws was rated at 750 pounds in withdrawal. The nailed version was rated at 150 pounds. That is a fivefold difference for the same hanger on the same ledger.

The bolts carry the load and the flashing keeps the band joist dry. A ledger needs both to last.

Lombard's checklist spells out the local detail. Lag bolts must reach 1 1/2 inches into the rim joist or studs. Flashing must run from behind the siding and house wrap, down over the top of the ledger.

A rotted ledger is a different problem from a missing bolt. It cannot be fixed by adding fasteners. The signs, and what a repair involves, are in the guide to a rotted ledger board.

When does a deck need a guard, and how tall?

Under the IRC, whenever the walking surface is more than 30 inches above the floor or grade below. The code sets out where guards are required, measured anywhere within 36 inches of the open edge.

That last clause catches many homeowners out. A deck 24 inches high at the house can still need guards if the yard falls away. What counts is the drop within 3 feet of the edge, not the height at the door.

Baluster gaps, guard height and handrail height are all measured with a tape. None of them can be judged by eye.

Once required, a guard stands at least 36 inches tall on the deck and 34 inches along a stair. No opening may pass a 4 inch sphere, about the size of a small child's head. Lombard's checklist uses the same limits.

Guards also have to resist a push. A post bolted only through a thin rim board can meet the height and still fail the load. The guide to a wobbly deck railing explains why that happens.

What does code say about deck stairs?

Risers may be no taller than 7 3/4 inches and treads no shallower than 10 inches. Within one flight, riser heights and tread depths may not vary by more than 3/8 inch. Lombard's checklist repeats those limits.

A stair with four or more risers needs a graspable handrail on at least one side. It sits 34 to 38 inches above the tread nosing. The stair must be at least 36 inches wide, and Lombard wants a metal connector at the rim.

Stairs are where older decks most often fall short. A stair built decades ago may have uneven risers or no handrail at all. Rebuilding it brings it under the current code, even when the deck itself stays.

Does the code apply when repairing an old deck?

For structural repairs, usually yes, but the local code decides rather than the state. The Capital Development Board says the state residential code law covers new construction only. Repairs are left to the local authority.

Many DuPage towns treat new footings, beams, joists, ledgers, stairs and guards as permit work. Board replacement over a sound frame often is not. The permit office for the address gives the definitive answer.

An existing deck is not usually forced up to current code just because the code changed. What gets rebuilt, though, is rebuilt to the code in force now. A new stair on an old deck meets today's riser and handrail rules.

What are the key deck code numbers at a glance?

The table sets the IRC baseline beside one DuPage village's local rule. It shows how far a local amendment can move a number. It also shows why the permit office for the address has the last word.

Requirement	IRC baseline	Lombard, as an example	What the inspector checks
Guard required when the drop exceeds	30 inches	24 inches	Height to grade within 36 inches of the open edge
Guard height on the deck	36 inches	36 inches	Measured from the deck surface
Opening in a guard	Under a 4 inch sphere	Under a 4 inch sphere	Balusters, the bottom gap and the gap at the house
Footing depth	Below the frost line	42 inches from grade	Hole depth, dry and clear, before concrete
Ledger fastening	Lag screws or bolts, no nails	Lags into the rim, flashed above	Fastener size, spacing and flashing
Stair riser, maximum	7 3/4 inches	7 3/4 inches	No more than 3/8 inch variation in a flight
Stair tread, minimum	10 inches	10 inches	Measured nose to nose
Handrail	Four or more risers	Four or more risers	34 to 38 inches high, graspable, returned

The deck is low at the door and high at the rail. The code measures the drop at the edge.

What cannot be determined from the code book?

Which edition and amendments apply to a given address. That takes the adopting ordinance or a call to the permit office. A general summary, including this one, cannot promise a local number has not been changed.

Whether an existing deck meets code. Ledger fasteners sit behind the rim and flashing behind the siding. Footing depth is underground. None of it can be judged from the yard without opening something up.

And whether the house can carry a ledger at all. A rim may be rotted or hidden behind brick veneer, and a ledger may not hang on veneer. Those decks need posts and a beam of their own at the house side instead.

Illinois deck code questions

Is there a statewide deck code in Illinois?

There is no separate state deck code. Since January 1, 2025, Illinois law has set a floor instead. Every local code must be at least as strict as the International Residential Code on structure. The rules in force are still the ones the village, city or county adopted, amendments included.

How does Perfect Work check a deck against the local code?

Perfect Work Remodeling & Repairs starts with the permit office for the address. That office sets the code edition and local amendments. The deck is then checked where inspectors look. That means footing depth, ledger fasteners and flashing, guard height and openings, and stairs. Any shortfall is listed with its fix.

How high can a deck be without a railing in Illinois?

Under the IRC, a guard is required once the deck surface is more than 30 inches above grade. That height counts anywhere within 36 inches of the edge. Some DuPage villages are stricter, and Lombard requires guards at 24 inches. The local code for the address is the one that counts.

How deep do deck footings need to be in DuPage County?

Footings go below the frost line, which DuPage County sets at 42 inches. Lombard's deck checklist uses the same depth, measured from grade, with an 8 inch minimum hole. Shallower footings get lifted by frost each winter, and the deck slowly racks out of level.

Does Perfect Work pull the permit for deck work?

Yes. On a deck job that needs one, Perfect Work Remodeling & Repairs applies for the permit. It prepares the drawings the office asks for and books the inspections. The permit fee sits on the estimate as its own line. The homeowner sees exactly what the town charges.

Do I have to bring my old deck up to code?

Not simply because the code changed. An existing deck is generally judged by the rules it was built under, unless it is unsafe. Anything rebuilt, such as a stair, a guard or a ledger, has to meet the code in force now. The local code official makes the final call.

Getting a deck that passes the first time

The Illinois deck building code is really a local code sitting on a state floor. The numbers that matter are the ones the permit office enforces. A few DuPage villages set them stricter than the IRC.

Perfect Work Remodeling & Repairs checks footings, ledger, guards and stairs on every visit. Each is measured against the code of the town the deck sits in. New decks and code repairs both fall under deck repair and replacement.

Call Perfect Work at 630-297-7888 · Book a deck check