How many roof vents should a house have?
Short answer. There is no fixed number; a house should have enough roof vents to provide about half of the attic's required ventilation as exhaust, with the other half as soffit intake. For a typical box vent rated around 50 square inches, that works out to roughly one vent for every 200 square feet of attic floor on the balanced 1/300 path, so a 1,500 square foot attic needs about eight.
How the count is calculated
- Attic floor area divided by 300 gives the total net free area in square feet when the vents are balanced and the exhaust is high on the roof.
- Multiply by 144 for square inches, then take half for exhaust.
- Divide by the vent's rated net free area, printed on the label.
Typical counts with 50 square inch box vents
- 1,000 square foot attic: about 240 square inches of exhaust, so 5 box vents.
- 1,500 square feet: about 360 square inches, so 8 box vents.
- 2,000 square feet: about 480 square inches, so 10 box vents.
- 2,500 square feet: about 600 square inches, so 12 box vents.
If the exhaust is not placed within 3 feet of the ridge, the code's 1/150 ratio applies instead, which doubles these numbers.
Why rules of thumb mislead
Advice like "four vents per house" ignores attic size and vent ratings, which vary widely. A ridge vent changes the math entirely, since each foot of it may provide 12 to 18 square inches. Some roofs also have separate attic spaces over garages or additions that need their own vents.
Do not forget intake
The intake side should be at least equal to the exhaust. Eight box vents on an attic with blocked soffits will still underperform. And keep to one type of exhaust per attic. See ridge vent vs box vents and roof ventilation basics.