How do you calculate how much ridge vent you need?
Short answer. Divide the attic floor area by 300, multiply by 144 to get total square inches of net free area, and take half of that as the exhaust the ridge vent must supply. Divide that by the vent's rating per linear foot to get the feet of open slot you need, then check that your ridge, minus the end setbacks, is long enough.
Worked example
- Attic floor area: 2,400 square feet.
- Total net free area: 2,400 divided by 300 is 8 square feet. Times 144, that is 1,152 square inches.
- Exhaust share: half, or 576 square inches. The other 576 must come from intake.
- Feet of vent: with a vent rated 18 square inches per foot, 576 divided by 18 is 32 feet. With a vent rated 12.5, it is about 46 feet.
- Check the ridge: a 40 foot ridge with 6 inch setbacks at each end gives 39 feet of usable slot. The 18 rated vent works; the 12.5 rated vent falls short.
If the ridge is too short
- Pick a vent with a higher rating per foot.
- On hip roofs, consider a product approved for hips, cut only near the top.
- Otherwise, use box vents placed high as the single exhaust type instead.
- Do not mix ridge vent with box vents or a fan on the same attic.
Check the intake side
Intake should be at least equal to exhaust. If the soffits can supply only 400 square inches, more ridge vent will not help; adding intake will.
Common mistakes
- Using roof area instead of attic floor area.
- Using the vent's outside dimensions instead of its net free area rating.
- Counting ridge length over porches or vaulted rooms with no attic below.