How do you vent your roof, and what does each vent on a roof do?
Short answer. Venting a roof means giving the attic a way to breathe: intake vents low at the eaves and exhaust vents high near the ridge, sized to the attic. Not every vent on a roof is for the attic, though; plumbing vents, gas flues, and bath or kitchen exhaust caps are separate systems that simply pass through the roof.
What each roof vent does
- Ridge vent: attic exhaust along the peak.
- Box, hip, and turbine vents: attic exhaust high on the slopes.
- Power or solar fans: mechanical attic exhaust.
- Soffit and edge intake vents: bring outside air into the attic at the eaves.
- Plumbing vent pipes: let air into the drain system and release sewer gas, sealed with boots.
- Gas flues: carry combustion gases from furnaces and water heaters, with caps and storm collars.
- Exhaust caps: terminate bath fans, kitchen hoods, or dryers, with dampers.
How to vent a roof well
- Calculate the attic's need from its floor area: about 1 square foot of net free area per 300 square feet when intake and high exhaust are balanced.
- Provide at least half that area as intake at the eaves.
- Provide the other half as exhaust, using one type of exhaust only.
- Keep insulation out of the eaves with baffles.
- Send every exhaust fan outdoors through its own cap.
How many vents should a roof have?
It depends on attic size and vent ratings, not on a standard count. A ridge vent may provide all the exhaust a gable roof needs, while a hip roof may need a row of box vents instead. The number of plumbing and exhaust caps depends on your home's layout, and each is a potential leak point that needs good flashing.
Keep penetrations to a minimum
Every hole in a roof needs flashing that can fail. During a reroof, consolidating attic exhaust into a ridge vent and replacing all boots and collars reduces future leaks. See roof ventilation basics and roof flashing explained.