How is an attic ventilation fan installed?
Short answer. A roof mounted attic fan is installed by cutting a round opening between two rafters near the ridge, sliding the fan's flange under the upper shingles so it sheds water, nailing and sealing it, and having the power and thermostat wired to code. Before any of that, the attic needs enough soffit intake, or the fan will pull air out of the house instead.
Before installing
- Size the fan. A common manufacturer rule of thumb is about 0.7 cubic feet per minute (CFM) per square foot of attic floor, with extra capacity for dark roofs.
- Check intake. Many fan makers call for roughly 1 square foot of net free intake area for every 300 CFM. Most homes have less soffit venting than that.
- Close competing exhaust. A fan near a ridge vent or gable vent tends to pull air through those openings instead of the soffits, so manufacturers generally advise against combining them.
Typical roof mount steps
- From the attic, pick a spot between rafters, usually on the rear slope and within a couple of feet of the ridge, and drive a nail up through the deck as a locator.
- On the roof, mark and cut the hole to the size the instructions specify.
- Remove nails from the shingles above so the top and sides of the flange slide under them; the bottom edge sits on top of the shingles.
- Nail the flange, seal the nail heads and edges with roofing sealant, and reseal the shingles.
- Run power to the thermostat and humidistat. Wiring must meet the electrical code, and many jurisdictions require a licensed electrician or a permit.
Gable mounted fans skip the roof cut and mount behind an existing gable vent, which avoids a new roof penetration.
When to skip it
If you already have a ridge vent and good soffits, a fan usually adds little and can work against them. Solar fans avoid wiring but still need intake. Read roof ventilation basics and ridge vent vs box vents before buying.