What is the best roof vent for a bathroom fan?
Short answer. The best roof vent for a bathroom fan is a dedicated exhaust roof cap with a built-in backdraft damper, a screen, a collar sized to the fan's duct (usually 4 inch) and a flashing flange that weaves into the shingles. Low profile metal or high impact plastic caps both work; a sealed duct connection and correct flashing matter more than the brand.
Features to look for
- Damper: keeps outside air, rain and insects out when the fan is off.
- Correct collar size: matched to the duct so the connection seals without choking airflow.
- Flashing flange: wide enough to sit under the upper shingles and over the lower ones.
- Screen: keeps birds out without being so fine that lint clogs it.
- Durability: metal caps resist UV but dent in hail; plastic caps do not dent the same way but can crack with age.
What not to use
- A ridge vent or soffit vent as the outlet; the code bars discharge into either.
- A static attic box vent with the duct merely pointed at it.
- A roof outlet for a clothes dryer; dryer vents belong on a wall where lint can be cleaned out.
Installation details
- Place the cap above the bathroom for the shortest duct path, away from valleys.
- Cut the hole to the collar size between rafters.
- Weave the flange into the shingle courses.
- Connect the duct with clamps or foil tape and insulate it in the attic.
- Avoid sags in the duct where condensation can pool.
Learn more in roof flashing explained and roof ventilation basics.