What 18-28-504 actually says
Chapter 18-28-504 of the Chicago Construction Codes governs clothes-dryer exhaust ducts. It mirrors IRC M1502 with Chicago-specific amendments. The five rules that catch most homes:
- Rigid metal duct — smooth interior, 26-gauge galvanized steel typical.
- Maximum 35 ft equivalent length (less 5 ft per 90° elbow, 2.5 ft per 45°).
- UL 2158A-listed transition duct, ≤ 8 ft, never concealed in walls.
- Independent termination to the exterior with a backdraft damper, no screen.
- 4-inch minimum diameter, no reductions along the run.
Where Chicago homes typically fail
Plastic or foil transition duct
The single most common 18-28-504.9.3 violation we document. Foil collapses and traps lint.
Concealed transition behind drywall
Many remodels bury the flex duct inside the wall — explicitly prohibited.
Termination into the soffit
Vents discharging into a soffit recirculate moist, lint-laden air back into the attic.
Screen on the exterior cap
Screens trap lint and create back-pressure. Not permitted on dryer terminations.
Run length over 35 ft equivalent
Common in row houses and basement-laundry retrofits — measured, not estimated.
Inspection checklist (HowTo)
- Verify duct material. Confirm the entire run is rigid metal (typically 26-gauge galvanized steel) with smooth interior. Replace any plastic, vinyl, or foil sections.
- Measure equivalent length. Measure straight run plus elbow deductions (5 ft per 90°, 2.5 ft per 45°). Total must be ≤ 35 ft unless dryer manufacturer lists otherwise.
- Inspect transition duct. Transition between dryer and wall must be UL 2158A-listed, ≤ 8 ft, and fully visible (not concealed in wall or floor).
- Check termination. Exterior termination must have a backdraft damper, no screen, and discharge directly outside — never into an attic, crawlspace, or soffit.
- Measure airflow. Use an anemometer at the exterior hood. Healthy 4-inch vent reads roughly 1,200–1,500 FPM with a clean lint trap and dryer running on high heat.
- Document & cite. Produce a written report citing 18-28-504 with photos, measurements, and a Pass / Caution / Fail verdict per item.
FAQ
What is Chicago Building Code 18-28-504?
Chapter 18-28-504 of the Chicago Construction Codes governs clothes-dryer exhaust ducts: rigid metal construction, smooth interior, equivalent length not exceeding 35 feet (less deductions for fittings), independent termination to the outside, backdraft damper, and 4-inch minimum diameter. It mirrors and amends IRC M1502 for installations within the City of Chicago.
Does Chicago require a licensed inspector for dryer vent work?
Permitted construction must be performed by a licensed contractor. Routine cleaning does not require a permit, but any duct re-route, wall penetration, or dryer relocation does. A documented compliance inspection is commonly required for property transfers, condo board approval, and insurance claims following a dryer fire.
What is the maximum allowed dryer vent length under 18-28-504?
35 feet equivalent length, measured from the dryer connection to the outside termination. Each 90° elbow deducts 5 feet; each 45° elbow deducts 2.5 feet. Manufacturer specifications may allow longer runs only if explicitly listed for the dryer model.
Are flexible foil or plastic transition ducts allowed?
No. 18-28-504.9.3 prohibits flexible foil and plastic transition ducts. Only UL 2158A-listed semi-rigid aluminum or rigid metal transition duct is permitted, and it must not be concealed within construction.
Need a documented 18-28-504 inspection?
Pass / Caution / Fail report with photos, citations and measurements.
Related: IRC M1502 Guide · Our Services