Plain English:
504.9 domestic dryer ducts: This is the "hard" duct from the wall box to outside. It must comply with 504.9.1-504.9.6.
504.9.1 material and size
Smooth interior metal, at least 0.016 in thick
Nominal 4 in diameter only
This is the duct in walls/ceilings, not the short flex at the dryer
504.9.2 duct installation
Support at max 4 ft intervals
Insert end of each section points in direction of airflow
No screws/fasteners sticking more than 1/8 in into the duct
Stud/joist cavities must allow the duct to run without being crushed
504.9.3 transition ducts-the short connector between the dryer outlet and the wall box is the "transition duct." Code treats this differently than the in-wall duct:
Must be a single listed and labeled UL 2158A product
Max length 8 ft
Cannot be concealed in construction (kept exposed and accessible behind the unit)
504.9.4 duct length (equivalent length)
Default: max 35 ft from the transition-duct connection to the outside terminal
Each elbow reduces that 35 ft using equivalent lengths in Table 504.9.4.1
Alternatively, you can use the specific dryer manufacturer's limit, or the DEDPV manufacturer's limit when a listed dryer exhaust duct power ventilator is used
504.9.5 length identification: If equivalent length is more than 35 ft, permanently label the equivalent length within 6 ft of the duct connection.
504.9.6 exhaust duct required: If a dryer space is provided, a dryer exhaust duct system must be installed and capped if the dryer is not yet in place.
Exception: a listed condensing (ductless) dryer installed before occupancy.
On Plans: Why it matters
Dryer issues usually come back to the same mistakes: wrong duct material, crushed duct in framing, flex buried in walls, no length calc, or an overlength run with no design considerations taken.
Code Path: Where to show it
M-001: Add an IMC 504 note that domestic dryer exhaust uses a min. 4" smooth metal per 504.9, a single exposed UL 2158A transition duct (max 8 ft), and meet maximum allowed equivalent lengths.
M-101: Keep routes short and straight and note "DRYER DUCT EQ. LENGTH = XX FT (PER 504.9.4)" for any long/complex run.
Check: Do
Coordinate dryer location early so the run stays short.
Check the actual dryer cut sheet for allowed duct length and fitting equivalents.
Review Risk: Don't
Don't use foil flex or plastic duct in building cavities.
Don't ignore equivalent length; elbows add up fast.
Field Tip:
On laundry room views, add a callout: "DRYER DUCT EQ. LENGTH = ___ FT (MAX = ___ FT PER DRYER/IMC 504.9.4)." Fill it during design. It shows reviewers you ran the 504.9 check and gives the field a clear limit not to exceed.
Masterbuild QA Lens
Dryer exhaust termination must be a listed termination cap designed for dryer exhaust - not a standard HVAC supply or exhaust cap. Confirm the cap prevents re-entry of exhaust into the building, is accessible for cleaning, and maintains the minimum opening dimension. Backdraft damper and lint screen requirements must be noted on the termination detail.
Drawing / Submittal Check
Domestic dryer duct (504.9-504.9.6) must show the complete duct path from the dryer collar to the termination, with elbows counted and the equivalent length calculation on the drawings or a schedule. Confirm the duct diameter is 4 inches throughout. Show the maximum allowable length table reference on the drawings.
Common Review Risk
The most common domestic dryer duct
When To Escalate
Escalate when the domestic dryer duct layout cannot meet the maximum equivalent length with any reasonable routing - the building geometry or the unit layout makes a compliant duct path impossible. These situations require either a powered exhaust booster (with the interlock requirement), a manufacturer's alternative system, or an AHJ-approved exception.
Duct System Coordination
Domestic dryer duct length limits (504.6-504.8) are calculated using a deduction table for each elbow. Show the equivalent length calculation on the duct drawing so the installer and inspector can verify compliance without guessing.