Plain English:
504.1 installation: Dryers follow manufacturer instructions and exhaust outdoors. Only listed condensing (ductless) dryers can skip the duct.
504.2-504.4 penetrations and terminations
Seal annular spaces with noncombustible materials and maintain fire ratings. No fire/smoke dampers in dryer ducts.
Terminate outside with a backdraft damper, no screens, and no fasteners protruding into the airstream. If the manufacturer is silent, keep at least 3 ft from openings and provide >=12.5 in^2 open area.
504.5-504.8 power assist, makeup air, protection
Use only listed DEDPVs to extend runs; generic booster fans are not allowed.
If EA >200 cfm, provide makeup air. Dryer closets need at least 100 in^2 free area or another approved method.
Where the duct is within 1 1/4 in of the finished face of framing, protect it with 16-gauge steel shield plates.
504.9 domestic ducts
Use 4" smooth metal duct (min 0.016 in thick), supported every 4 ft and not crushed in cavities.
Transition duct = connector from dryer to wall: single UL 2158A piece, max 8 ft, not concealed.
Limit duct length per 35 ft rule with fitting equivalents or per dryer/DEDPV instructions. If equivalent length exceeds 35 ft, label it within 6 ft of the connection.
If a dryer space is provided, install an exhaust duct and cap it unless a listed condensing dryer is installed.
504.10-504.11 commercial and multistory
Commercial systems keep fan motors outside the airstream, run continuously or interlocked, maintain 6 in clearance to combustibles, and use UL 2158A transition ducts (single, <=8 ft, exposed).
Common multistory shafts: fire-rated shaft, straight 26-gauge rigid duct, no dampers or screens, fan outside the airstream on standby power with monitoring, 12 in x 12 in cleanout at base, and the shaft serves only clothes dryers.
On Plans: Why it matters
Dryer fires and failed inspections usually trace back to the same misses: bad duct material, buried flex, overlength runs with no calcs, illegal booster fans, or shafts treated like "just another exhaust."
Code Path: Where to show it
M-001: "IMC 504 CLOTHES DRYER EXHAUST" note covering discharge outdoors, no screens/dampers, DEDPV-only power assist, makeup air >200 cfm, 4" smooth metal ducts, length basis, and common-shaft rules.
M-101: Clear routing, penetrations, and terminations with distances to openings and grade.
Check: Do
Coordinate dryer locations early to keep runs short, straight, and terminations as req'd.
Review Risk: Don't
Don't use screens, fire/smoke dampers, or protruding screws in dryer ducts.
Don't combine common dryer shafts with other exhaust systems.
Field Tip:
Add a "IMC 504 DRYER EXHAUST CHECK" note next to each dryer
Masterbuild QA Lens
For the dryer exhaust summary: confirm the permit set shows the duct length from appliance to termination, duct material and gauge, number of elbows and equivalent lengths used in the calculation, backdraft damper and screen type at termination, and booster fan if required. If these items are not on M-501, expect a comment.
Drawing / Submittal Check
For any multifamily or hotel project with dryer exhaust: M-001 must include a dryer exhaust system summary showing the number of dryers, the system type (individual or common), the total equivalent duct length per unit for individual systems, and the common duct testing protocol for common systems.
Common Review Risk
Dryer exhaust on multifamily renovation projects - where new washer/dryer units are being added to existing buildings - generates the most dryer exhaust comments. The existing duct routing often cannot accommodate the required path to the exterior, or the required duct diameter exceeds the available space in the wall. Identify the routing constraint before the permit is filed; a routing compromise that was not coordinated with the AHJ before submittal will cycle.
When To Escalate
Escalate when the dryer exhaust system for a large multifamily project has not been tested or when the test results show flow rates below the design values at any unit. Underperforming dryer exhaust systems in occupied multifamily buildings are a persistent maintenance problem - resolving them after occupancy is expensive and disruptive.
Rated Assembly Coordination
Dryer exhaust duct compliance at rated assemblies requires a listed penetration method - firestop, damper, or listed sleeve - at each crossing. Confirm the penetration detail and access to the backdraft damper are shown on the construction documents.
Duct System Coordination
Clothes dryer exhaust duct compliance depends on smooth metal construction, verified equivalent length, listed termination fitting, and backdraft damper at the outlet. Every one of those items needs to appear on the drawing or schedule.