How is a roof ventilation fan installed?
Short answer. A roof-mounted attic fan goes in by cutting a round hole between two rafters near the ridge, sliding the fan's flashing under the shingles above and over the shingles below, fastening it to the deck, and wiring it to a thermostat. It only helps when the attic has enough soffit intake and no ridge vent or other exhaust competing with it.
The steps
- Check the attic first. Confirm there is real soffit intake; a common manufacturer rule of thumb is about 1 square foot of net free intake for every 300 CFM of fan capacity. Make sure there is no ridge vent on the same attic.
- Pick the spot. Usually the rear slope, centered between rafters, roughly 2 feet below the ridge, away from valleys and plumbing stacks.
- Cut the opening to the template size without cutting a rafter.
- Flash it in. Unseal the shingles above, slide the top of the flange under them, let the bottom flange lie on top of the shingles below, nail per the instructions, and seal the nail heads.
- Wire it. Hard-wired fans need a circuit, a junction box and an adjustable thermostat, sometimes a humidistat. Many jurisdictions require an electrical permit. Solar fans skip the wiring but need a panel in full sun.
- Keep the fire cutoff. Many units include a high temperature switch so the fan shuts off rather than feeding an attic fire.
Common mistakes
Installing a fan on a roof that already has a ridge vent is the big one. The fan pulls air in through the ridge vent a few feet away, sometimes along with rain, while the eaves get no airflow at all. The second is a fan with too little intake, which pulls cooled air out of the house through light fixtures and the attic hatch and raises summer power bills. Leaving old box vents open next to a fan causes the same kind of shortcut.
Compare the approaches in roof ventilation basics and see why heat matters in attic heat and shingles.