Ridge vent: what it is, how much an attic needs, and how it's installed
A ridge vent is a long exhaust vent along the peak of the roof: a slot is cut through the sheathing on both sides of the ridge and the vent goes over it, under the ridge cap shingles. Divide the attic's high vent area by the vent's net free area per foot to get the feet you need. A 1,500 sq ft attic at the code's 1/300 rule needs about 360 square inches high, or 22 feet of a vent rated 16.9 square inches per foot.
Anthony Hicks30+ years in construction. Updated November 5, 2026.
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What a ridge vent is
A ridge vent is an exhaust vent that runs along the ridge, the peak where two roof slopes meet. The roofer cuts a slot through the sheathing along the ridge, lays the vent over it and covers the vent with ridge cap shingles, so from the ground it looks like an ordinary ridge.
Warm, damp attic air rises and leaves through the slot; cooler air comes in low through the soffit vents under the eaves. The guide to roof vents compares a ridge vent with box, turbine, power and gable vents.
How much ridge vent an attic needs
Start from the code’s minimum vent area (IRC R806.2): 1/150 of the attic floor, or 1/300 when 40 to 50% of the vent area is high, within 3 feet of the ridge, and the rest low. A ridge vent is the high part. Split the total about half high and half low, then divide the high half by the vent’s net free area per foot.
Net free area is the open area a vent leaves after its baffles and mesh. The maker prints it per foot. GAF’s Cobra Exhaust Vent, for example, lists 16.9 square inches per foot for the hand-nailed version and 14.1 for the nail-gun version.
| Attic floor | High half at 1/300 | Feet at 16.9 sq in/ft | Feet at 14.1 sq in/ft |
|---|---|---|---|
| 1,000 sq ft | 240 sq in | 15 ft | 18 ft |
| 1,500 sq ft | 360 sq in | 22 ft | 26 ft |
| 2,000 sq ft | 480 sq in | 29 ft | 35 ft |
| 2,500 sq ft | 600 sq in | 36 ft | 43 ft |
Feet are rounded up. At 1/150 the figures double. When the ridge is shorter than the feet you need, add box vents near the ridge, or work out the attic at 1/150 against the soffit intake you have.
Balance it with the intake
The intake at the soffits has to carry at least as much as the ridge lets out. GAF puts it plainly: the exhaust at the ridge should never exceed the soffit ventilation. When the soffits are short of the ridge’s figure, add intake before adding ridge vent.
How a ridge vent goes on
- Cut the slot. The slot runs along the ridge on both sides, stopped short of each end of the ridge, at the width the vent’s instructions give.
- Lay the vent. It runs over the slot from end to end, centered on the ridge.
- Cap it. Ridge cap shingles go over the vent, nailed through it with the nails the maker calls for.
The code’s rule is that vents are installed to the maker’s instructions (R806.4), so the slot width, the distance kept from the ends and the nails come from that sheet. GAF’s Cobra vent is rated for slopes from 2:12 to 20:12; the roof pitch guide shows how to read yours.
Common questions
Do I need soffit vents with a ridge vent?
Yes. The ridge vent lets air out; soffit vents let air in. Without intake, a ridge vent can’t move air through the attic the way it’s meant to.
Can a ridge vent leak?
A ridge vent sits under the cap shingles with baffles or mesh against rain and snow; the code calls for vents protected against them (R806.1), and for the vent to go on to its maker’s instructions (R806.4).
Is a ridge vent part of a roof replacement?
It’s a line of its own on the estimate, priced by the foot. The roof replacement cost guide lists vents among the extras on top of the shingles, and the roofing calculator gives the ridge length with the rest of the roof.
How long is my ridge?
On a gable roof, the ridge runs the full length of the roof, overhangs included. On a hip roof with the same pitch all around, it’s the roof’s length less its width. A roof report or a measured sketch gives the ridge in feet with the eaves, rakes and valleys.