Section 504.1/Clothes Dryer Exhaust
IMC 504 covers dryer exhaust duct length limits, rigid metal construction, fitting deductions, and screenless termination.
Clothes dryer exhaust ducts must be constructed of rigid metal (not flex duct for concealed runs) with a smooth interior surface. The maximum exhaust duct length is 35 feet with deductions for each fitting: 5 feet for each 90-degree elbow, 2.5 feet for each 45-degree elbow. The exhaust must terminate to the outdoors with a backdraft damper. The exhaust outlet must not be screened (screens trap lint, creating a fire hazard). The duct must be supported at intervals not exceeding 12 feet and joints must be sealed with mechanical fasteners (not screws that protrude into the duct and catch lint).
Why this section exists
Dryer exhaust contains hot air, moisture, and highly flammable lint. Lint accumulation in long, improperly installed exhaust ducts is a leading cause of residential and commercial fires. The 35-foot maximum length (with fitting deductions) ensures the dryer can push exhaust through the duct without excessive back pressure, which would cause lint to accumulate. The prohibition on screws and screens prevents lint from catching on protrusions and blocking the duct.
What plan reviewers look for
Plan reviewers measure the dryer exhaust duct length on the mechanical plan from the dryer connection to the exterior termination. They count fittings and apply the deductions to verify the equivalent length is within 35 feet. They check the duct material (rigid metal required for concealed portions). They verify the termination has a backdraft damper and no screen. They check that the duct routing avoids unnecessary elbows.