The Dryer Vent Code, Translated: What IRC M1502 Requires of Your Laundry Room
Rigid metal in the walls, 35 feet of developed length, one listed transition duct, and no screen on the outlet. Four rules that most houses break.
Nearly every dryer-vent problem we are called to is a code violation that has been running quietly for years. Not through recklessness — the requirements are specific, unglamorous, and nobody hands them to a homeowner at closing. Here they are in plain terms.
01Rigid metal, smooth wall, inside the building
Concealed duct must be smooth-wall rigid metal at four inches, joined so the interior stays smooth in the direction of airflow, with no screws penetrating into the airway.
The reason is mechanical rather than bureaucratic. Every corrugation is a shelf and every projecting screw tip is a snag — and concealed duct cannot be inspected or effectively cleaned, so whatever collects in there stays.
02Thirty-five feet, and elbows cost more than you think
The limit is on developed length, not tape-measure distance. Fittings add equivalent length: roughly five feet for a ninety-degree elbow, about two and a half for a forty-five.
A run measuring fifteen feet with four ninety-degree turns has a developed length near the maximum before any straight duct counts. This is why an interior laundry room is a harder problem than one on an outside wall, and why we measure rather than eyeball.
03One flexible piece is allowed, and it has rules
The visible connector between machine and wall may be flexible, but it must be a listed transition duct, a single unbroken length no longer than eight feet, and it must stay visible — it cannot be concealed or pass through a wall.
Generic foil hose sold beside it is not the same product. And it must not be crushed, which is where the real-world failure lives: push a dryer flat against the wall over a rigid elbow and you have folded the one part permitted to be flexible.
04Outdoors, with a damper, and without a screen
The duct terminates outside the building, with a backdraft damper, away from openings back in. Venting into an attic, crawlspace or garage is never permitted — a single load discharges several litres of water plus lint into a space with no way to handle either.
And the counterintuitive one: no screen on the termination. Mesh clogs with lint into a solid mat, usually within a season, at which point the screen is the blockage. Animal exclusion is a damper or a wide-opening guard, never fine mesh.
05Gas dryers add a second layer
A gas dryer is a fuel-burning appliance. Moving one to clean behind it means disconnecting a gas line, and reconnecting means a leak test — licensed work in Texas.
It also raises the stakes on a blocked exhaust: combustion products share the same restricted path out as the moisture does.
If your laundry duct is flexible foil inside a wall, has more elbows than you can count, or terminates behind a screen, you have found the reason the dryer is slow — and the fix is a compliant rebuild rather than another cleaning.
Every recommendation in this guide starts from one place: a graded, photographed inspection. Book a vent inspection and pair it with a full vent rebuild & replacement — one standard, one paper trail, every job.