Dryer exhaust duct: max 35 ft straight (subtract 5 ft per 4-in 90° mitered elbow); 4-in metal smooth-wall duct
2025 RCNYS §M1502.4.6 Dryer exhaust duct max length (35 ft)Description
Dryer exhaust ducts are limited to 35 ft from the transition duct connection at the dryer to the outlet terminal. Each fitting reduces the allowed length per Table M1502.4.6.1 (a 4-in mitered 90° elbow = 5 ft equivalent length). The duct must be 4-in nominal smooth-wall metal (No. 28 gage minimum), supported every 12 ft, with the insert end facing airflow direction. Transition ducts (UL 2158A listed) limited to 8 ft. Booster fans prohibited. Where length exceeds 35 ft, the equivalent length must be permanently labeled within 6 ft of the connection.
Why this exists
Long or sharp-elbowed dryer ducts trap lint — a leading cause of dryer fires. Architects locating laundry rooms on interior walls / upper floors must plan the exhaust path before construction; routing through 35+ ft of duct with multiple elbows often forces relocating the laundry or specifying a UL 705 power ventilator.
Measurements
| Property | Operator | Value | Unit | Note |
|---|---|---|---|---|
maxLength | max | 35 | ft | Straight-length maximum |
ductDiameter | exact | 4 | in | Nominal duct diameter |
supportInterval | max | 12 | ft | Duct support spacing |
mitered90Equiv | exact | 5 | ft per elbow | 4-in mitered 90° elbow equivalent |
Categories
Applies to
- Jurisdiction: New York State
Source
Solver enforcement
Browsable only — the solver does not currently enforce this directive (no spec-level data to check against). This entry exists so the architect personas can cite it in conversation and the user can read what the rule says.
Related directives
- Garage / dwelling fire separation · IRC R302.6
- Natural light in habitable rooms · IRC R303.1
- Habitable space minimum ceiling height · IRC R305.1
- Egress window net opening dimensions · IRC R310.2
- Stair riser, tread, and headroom · IRC R311.7
Last reviewed 2026-05-15.