Can you roof vent a dryer, and why is my dryer vent on the roof?
Short answer. A dryer can be vented through the roof when the dryer manufacturer's instructions allow it and the duct run stays within the length limits, using a roof cap made for dryers with a backdraft damper and no screen. Dryer vents end up on the roof when the laundry room sits in the middle of the house with no short path to an outside wall, but roof terminations clog more easily and need regular cleaning.
The rules that apply
- Outdoors only. Dryer exhaust must terminate outside the building, never in the attic.
- Manufacturer first. The code defers to the dryer manufacturer's installation instructions on termination and duct length, and some manufacturers discourage roof terminations.
- Damper, no screen. The termination needs a backdraft damper, and screens are not allowed because they catch lint.
- Length limits. Under the residential code's default rule, the duct is limited to about 35 feet, reduced for each elbow, unless the dryer maker allows a longer run. A vertical run up through an attic uses that budget quickly.
- Rigid metal duct. Smooth wall metal duct, not foil or plastic flex, should be used for concealed runs.
Why roof terminations cause trouble
Lint has to be pushed up and out against gravity. When airflow drops, lint settles in the vertical run and around the cap flap, which slows drying and creates a fire risk. Roof caps meant for bath fans often have screens or small openings that clog fast. And in winter, warm, moist exhaust condenses in a long, cold duct.
What to do if yours is on the roof
- Have the duct and roof cap cleaned at least once a year.
- Make sure the cap is dryer rated and the duct is insulated where it passes through the attic.
- If drying times get long, check the cap flap first.
- At your next reroof, confirm the cap is properly flashed and ask whether a sidewall route is possible.