What size opening do you need for a ridge vent?
Short answer. The opening has two dimensions: the width of the slot, typically about 1 inch on each side of the peak, and the length, which must be long enough that the vent's rated net free area meets the attic's exhaust requirement. For a balanced system, required exhaust in square inches is roughly the attic floor area times 0.24; divide that by the vent's rating per foot to get the minimum slotted length.
Worked examples
- 1,600 sq ft attic: 1,600 x 0.24 is 384 square inches of exhaust. At 18 square inches per foot, that is about 21 feet of slotted ridge.
- 2,200 sq ft attic: 528 square inches. At 18 per foot, about 29 feet; at 12.5 per foot, about 42 feet.
Counting usable length
Only the slotted length counts. Subtract the uncut ends, often about 6 inches at each gable end and about 12 inches at walls and chimneys, plus any ridge over spaces that do not connect to the attic, such as a vaulted porch.
When the ridge is too short
- Choose a vent with a higher net free area rating per foot.
- Add static or off-ridge exhaust only where the attic is divided or the manufacturer permits the combination.
- Make sure intake is at least equal; the ridge opening does little without it.
Do not widen the slot to compensate
Cutting wider than the instructions specify does not raise the vent's rated airflow and can let water in. If the math still comes up short, the honest answer may be a different exhaust plan rather than a bigger hole. Read roof ventilation basics and how roofers measure a roof.