Most dryer vent problems we find are code violations the homeowner never knew about. Here are the rules that matter, in plain English. This is a homeowner guide, not legal advice; check with your city inspector for permits.
Material: rigid or semi-rigid metal
The duct must be metal, at least 0.016-inch thick, with a smooth interior. White vinyl flex and thin foil accordion duct do not meet code for the concealed run. A short flexible metal transition hose behind the dryer is allowed if it's listed for the purpose and not run through walls.
Joints: tape, not screws
Joints must be fastened without screws or fasteners that protrude into the airflow, because they snag lint and start a plug. Foil tape and crimped joints are standard.
No screens at the termination
The exterior cap must have a backdraft damper and no screen. Screens trap lint and are the single most common violation we remove.
Maximum developed length: 35 feet
The default limit is 35 feet of total run, with each 90-degree elbow counting as 5 feet and each 45 as 2.5 feet. A manufacturer can allow longer if their installation manual says so. Runs over the limit need a listed booster fan.
Terminate outdoors
Dryer exhaust must go outside, at least 3 feet from openings into the building. Never into an attic, crawl space, garage or soffit cavity.
Why it matters
Each of these rules exists because the alternative traps lint or hides a fire. If we find a violation during cleaning, we photograph it and explain the fix. Many are cheap: a new cap, a semi-rigid hose, a few feet of rigid duct. Learn about our inspection add-on.