What size roof vent do you need for a bathroom fan?
Short answer. Use a roof exhaust cap that matches the fan's duct outlet, which is 4 inches on most standard bathroom fans and 6 inches on many larger, higher airflow models. Never reduce the duct size along the run, and choose a cap made for exhaust with a built-in damper, not an attic box vent.
Why the size has to match
A fan is rated to move its listed airflow against a certain resistance. Necking a 6 inch outlet down to a 4 inch cap, or running a long, crushed flex duct, can cut real airflow sharply, and a bathroom that never clears its steam feeds mold and peeling paint. The cap is the last restriction in the run, so it needs the full duct size.
What the code asks for
- Exhaust must go outdoors, not into the attic, soffit, ridge vent or crawl space.
- Bathrooms generally need at least 50 CFM of intermittent exhaust or 20 CFM continuous, though many people choose larger fans for bigger bathrooms.
- The outlet should sit a few feet away from windows, doors and other openings into the house; check local rules for the exact distance.
Getting the run right
- Keep the duct as short and straight as possible from fan to cap.
- Use insulated duct through the attic so warm, wet air does not condense inside and drip back into the fan.
- Seal the duct to the fan collar and the cap collar with foil tape or clamps.
- Choose a cap with a damper and a hood opening that faces down the slope, flashed in with its upper flange under the shingles above.
When to add one
If your fan currently dumps into the attic or ends loose near the soffit, a roof replacement is the ideal time to add a cap, since the shingles are already off and the flashing can be woven in cleanly. See roof ventilation basics and roof flashing explained.