Do I need a permit for an attached deck in
Collinsville, Illinois?

Collinsville attached deck permits — the key details

Collinsville enforces the 2021 Illinois Building Code, which requires a permit for any deck — attached or not — that exceeds 30 inches above grade or 200 square feet. However, Collinsville's interpretation is stricter: attachment to the house is itself a permit trigger, regardless of height or size. This matters because a 4x8-foot, 12-inch-high ground-level deck would be exempt in many Downstate Illinois towns (under the 30-inch and 200-sq-ft threshold), but in Collinsville, the ledger attachment to the house's rim board means structural review is mandatory. The reason: ledger-board connection is a life-safety issue (IRC R507.9 specifies flashing, rim-board nailing, and lag-screw depth), and inspectors need to verify before construction starts. If your deck is fully freestanding — posts in the ground, no ledger, no rim-board connection — it may be exempt if under 30 inches and 200 square feet. But the moment you bolt or nail to the house, you need a permit.

Frost depth in Collinsville is nominally 36 inches per Downstate Illinois convention (Illinois Department of Transportation winter-hardiness maps place Collinsville, Madison County, in the 4A USDA zone and 36-inch frost line). However, the 42-inch frost depth used in Chicago and the northern Illinois metro applies in parts of western Madison County (glacial till deposits near the bluffs). The building department does not automatically impose the deeper 42-inch depth; you submit a deck plan with footing depth, and the inspector verifies. Conservative practice is 42 inches. Frost heave — the upward expansion of soil water below the frost line during winter freeze-thaw — will buckle footings and ledgers set shallower than frost depth. Decks in Collinsville have failed mid-winter when footings were set at 30 inches or less. The plan must call out footing depth in writing and on the detail drawing.

Ledger-board flashing is the single most-cited deficiency in Collinsville deck permits. IRC R507.9 requires a flashing membrane (metal or rubber) that sheds water behind the ledger and down the rim board. Many homeowners and contractors skip this or use caulk alone — both are code violations. The inspector will red-tag the framing and halt work until flashing is installed. Flashing must be continuous, extend below the rim board, and be sealed at corners. Typical flashing detail: 1/4-inch galvanized or aluminum Z-flashing or J-channel, nailed at 16-inch intervals to the rim board with galvanized nails, and sloped to shed water away from the house. If the rim board is notched for the ledger, the notch must not exceed 1.5 inches (to preserve rim-board strength per IRC R507.9.1). Lag screws or structural bolts (not nails alone) must attach the ledger; spacing is 16 inches on-center, and bolts must penetrate fully into the rim board with washers.

Guardrails and stairs are a second major review point. Any deck over 30 inches requires a guardrail 36 inches high measured from the deck surface; some jurisdictions (and some inspectors) interpret this as 42 inches to the handrail. Collinsville's standard is 36 inches. Balusters (the vertical spindles) must not pass a 4-inch sphere — this prevents child entrapment. If you have exterior stairs (deck-to-ground), risers must be uniform within 3/8 inch, treads between 10 and 11 inches deep, and the top and bottom landings must be level and not less than 36 inches wide. Stringers (the angled side supports) must be designed to carry live load (40 pounds per square foot for residential decks per IBC Table 1607.1). Many homeowners attach stairs with 2x12 stringers notched to fit treads; this is structurally weak and often rejected by inspectors. Pressure-treated stringers with bolted treads or metal-plate-connected stringer hardware (Simpson or equivalent) pass inspection faster.

Plan-review intake in Collinsville is online via the City of Collinsville municipal portal (accessible via the city website). You submit a deck plan drawing (hand-drawn or CAD) showing overall dimensions, footing locations, ledger attachment detail, flashing detail, railing detail, and stairs (if any). The drawing must call out all materials (e.g., 'pressure-treated Southern Pine, grade 2, rim joist 2x10, posts 6x6, bolts 1/2-inch lag screws 16 in. o.c.'), footing depth (36 or 42 inches), and frost-line note. The city charges $150–$400 depending on the deck's estimated cost (permit fee = 1.5–2% of material cost). Plan review typically completes in 1–2 weeks; once approved, you receive a permit card and can schedule footing inspection. Footing inspection happens before concrete is poured — the inspector checks the post holes, verifies depth, and ensures they are spaced per your plan. Framing inspection follows once ledger is attached and rim board is installed; this is where flashing and fastener spacing are verified. Final inspection is after rails, stairs, and any decking are installed. Total timeline from permit to final: 3–5 weeks if inspections happen back-to-back and no deficiencies are found.

Three Collinsville deck (attached to house) scenarios

Frost depth and footing failure in Collinsville: why 36 vs. 42 inches matters

Collinsville straddles two frost zones. Southern Madison County (Collinsville proper, along the Mississippi River lowlands) is in the 4A USDA hardiness zone with a nominal 36-inch frost line per Illinois Department of Transportation standards. However, glacial-till soil west of Collinsville (toward the Illinois bluffs) and deposits in the northern part of the county can reach 42 inches in worst-case years. The City of Collinsville Building Department does not impose a single frost depth; instead, you specify it on your permit application, and the inspector verifies it is defensible. Conservative practice in Collinsville is 42 inches to avoid mid-winter heave that buckles footings and ledgers. Two to three times per decade, a Collinsville deck fails when footings frost-heave upward during a hard freeze-thaw cycle; if the footing was set at 30 inches, it will rise 2–4 inches and crack the deck frame or separate the ledger from the house.

The mechanism is straightforward: soil moisture below the frost line freezes and expands (ice lenses form). If a post footing is set above the frost line, the soil around it freezes, expands, and lifts the post. A properly designed footing sits below the frost line, so the expansion happens beneath it and does not move the post. Collinsville's building code (adopted from the 2021 IBC) defers to the frost-line map in the code appendix; for Collinsville, that map shows 36–42 inches depending on exact location and historical weather. Your permit plan must call out footing depth and reference the frost-line standard; the inspector will check the note and your site's soil/elevation. If you are in a valley or near a spring, the inspector may ask for a deeper footing (42 inches) even if the nominal line is 36 inches.

Cost implication: digging six post holes to 42 inches instead of 30 inches adds 12 inches of digging (roughly 1 cubic yard of soil moved per hole, or ~$300–$600 in labor if hand-dug, ~$150–$300 if machine-dug). The concrete (one bag per hole, roughly $20 per hole) is negligible. The insurance and resale peace of mind is substantial. Never set a Collinsville deck footing shallower than 36 inches; if your inspector approves 36 and a freeze-thaw fails it five years later, you have no recourse under code.

Ledger flashing: the number-one inspection failure in Collinsville residential decks

Every month, at least one Collinsville homeowner gets a red-tag (stop-work order) on a deck framing inspection because the ledger flashing is missing, non-continuous, or installed incorrectly. The reason: water infiltration behind the ledger board rots the rim board and band joist, which support the house's foundation and floor. A rotted rim board is a structural failure that can cost $5,000–$15,000 to repair. IRC R507.9 requires a flashing membrane — metal (galvanized steel, aluminum) or synthetic rubber — that sits between the ledger and the rim board and extends below the rim board to shed water. Caulk alone does not work; caulk fails in 3–5 years and allows water to pool behind the ledger.

Correct flashing detail: 1/4-inch galvanized Z-flashing (or J-channel) is nailed to the rim board with galvanized nails at 16-inch intervals, sloped slightly downward to shed water away from the house. The flashing must extend at least 4 inches below the rim board on the house exterior side. At corners, the flashing must be continuous and sealed (a corner piece or a gutter splash guard is often added). The ledger board itself is bolted to the rim board with 1/2-inch lag screws or structural bolts, spaced 16 inches on-center, with washers. The bolts must penetrate fully into the rim board (typically a 2x10 or 2x12) and seat firmly. Many contractors skip flashing or use roofing felt or tar paper, which are not approved; Collinsville inspectors will reject these. Plan detail must show flashing explicitly; a note like 'Z-flashing per IRC R507.9' is not enough. A drawing or specification sheet must accompany your permit.

Common rejection: contractor installs the ledger board, nails it, caulks the seams, and then calls for inspection. Inspector arrives, sees no flashing, and red-tags the job. Contractor must then remove the ledger, install flashing, reinstall the ledger, and call again. This adds 1–2 weeks and $500–$1,000 in labor. Avoid this by submitting a detailed flashing spec with your permit plan. If your plan drawing shows flashing, the inspector will verify it is correct during the ledger inspection (after the ledger is bolted but before decking is laid).

Common questions