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Home / Code Guides / Dryer Vent

Dryer Vent Code — The 35-Foot Limit, Elbow Deductions & Termination

Dryer exhaust is one of the few residential systems where the code gives you a hard length budget and then charges you for every turn. Getting it right is mostly arithmetic, plus one 2021 change that catches remodelers.

RequirementModel Code StandardSection
Duct materialExhaust duct is 4 in. nominal diameter, metal, with a smooth interior and a wall thickness of at least 0.0157 in. (No. 28 gage)IRC M1502.4.1
Length budgetMaximum developed length is 35 ft from the dryer connection to the termination, before deductionsIRC M1502.4.6.1
Fitting deductionsEach fitting is deducted from the 35 ft according to Table M1502.4.6.1 — a 4 in. mitered 90-degree elbow costs 5 ft and a mitered 45 costs 2 ft 6 in., while smooth-radius fittings in larger diameters cost far lessIRC Table M1502.4.6.1
Transition ductThe flexible transition between the appliance and the duct may be up to 8 ft long, must be listed to UL 2158A, and may not be concealed in constructionIRC M1502.4.3
TerminationThe outlet terminates outside the building with a backdraft damper and no screen, kept at least 3 ft from any other opening, including ventilated soffit openingsIRC M1502.3
Length labelWhere the developed length exceeds 35 ft, a permanent label or tag identifying the equivalent length must be posted near the dryer connectionIRC M1502.4.7

Worth knowing

  • The 2021 edition prohibits ordinary domestic booster fans in dryer exhaust (M1502.4.5). Where a run needs help, the listed device is a dryer exhaust duct power ventilator listed to UL 705 under M1502.4.4, and installing one changes how the length limit is applied.
  • Sheet-metal screws are banned in dryer duct, but not by M1502. The 1/8 in. protrusion limit is the mechanical-chapter rule; the outright prohibition on screws sits in the fuel-gas chapter at G2439.3.
  • Makeup air and the louvered closet door are fuel-gas provisions too. G2439.5 requires makeup air where the dryer moves more than 200 cfm, and the 100 sq in. closet opening is G2439.5.1. Neither appears in M1502.
  • Two IRC chapters give different numbers for the same duct. M1502 sets the metal thickness at 0.0157 in. and support spacing at 12 ft; G2439 says 0.016 in. and 4 ft. Where a gas dryer is involved, expect the inspector to enforce the tighter pair.
  • California is not on 35 ft. The mechanical code in use there allows 14 ft of developed length including two 90-degree bends, with a deduction for each additional elbow.
Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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