The five rules of M1502
- M1502.4.1 — Rigid metal duct, smooth interior, ≥ 0.016 in. thick.
- M1502.4.2 — Joints sealed without screws protruding into the airstream.
- M1502.4.5.1 — 35 ft maximum equivalent length (5 ft / 90°, 2.5 ft / 45°).
- M1502.4.3 — UL 2158A transition duct, ≤ 8 ft, fully exposed.
- M1502.3 — Independent exterior termination, backdraft damper, no screen.
Common M1502 violations
Sheet-metal screws in joints
Every screw catches lint. M1502.4.2 prohibits fasteners that protrude more than ⅛ inch.
Concealed transition duct
Foil flex inside a wall or floor cavity is the most cited violation in the field.
Termination near windows
M1502.3 requires ≥ 3 ft clearance from operable openings to prevent moist exhaust re-entry.
Unlisted booster fans
Generic in-line fans are not approved. M1502.4.6 requires a listed dryer-exhaust booster, interlocked with the dryer.
Inspection checklist (HowTo)
- Confirm duct listing. M1502.4.1 — duct must be 0.016-in. thick (28-gauge minimum) rigid metal with smooth interior. No flexible duct in concealed spaces.
- Verify joint method. M1502.4.2 — joints sealed without protruding fasteners. Foil tape or UL 181B-FX mastic only.
- Measure equivalent length. M1502.4.5.1 — total ≤ 35 ft including elbow deductions, unless dryer instructions specify otherwise.
- Check transition duct. M1502.4.3 — UL 2158A-listed, ≤ 8 ft, exposed and not concealed within construction.
- Inspect termination. M1502.3 — independent exterior termination, ≥ 3 ft from any opening, equipped with a backdraft damper, no screen.
- Verify booster (if used). M1502.4.6 — listed for dryer exhaust, interlocked with dryer operation, accessible for service.
- Document & report. Produce a written compliance report citing each M1502 subsection with measurements, photos, and Pass/Caution/Fail verdict.
FAQ
What is IRC M1502?
Section M1502 of the International Residential Code governs clothes-dryer exhaust systems in one- and two-family dwellings. It defines duct material, length, termination, transition duct, and listing requirements. Most U.S. jurisdictions — including Chicago via 18-28-504 — adopt M1502 with local amendments.
What is the maximum length under M1502?
M1502.4.5.1 sets a 35-foot maximum equivalent length from the dryer connection to the outside termination. Elbows reduce the allowable length: deduct 5 feet per 90° elbow and 2.5 feet per 45° elbow. Dryer manufacturers may publish longer allowable lengths in their installation instructions.
Can the duct be screwed together?
M1502.4.2 prohibits the use of screws or fasteners that extend more than ⅛ inch into the duct interior, because protrusions catch lint. Joints must be sealed with foil tape or UL 181B-FX listed mastic — not duct tape.
What about booster fans?
M1502.4.6 permits a listed dryer-exhaust booster fan when the equivalent length exceeds the manufacturer's maximum. The fan must be operationally interlocked with the dryer.
Need a documented M1502 inspection?
Forensic report with code citations, photos, and measurements.
Related: Chicago 18-28-504 Guide · Our Services