How long is ridge vent, and how long should a ridge vent be?
Short answer. Ridge vent is commonly sold in rigid sections about 4 feet long or in rolls of 20 feet or more. How long the vent on your roof should be depends on your attic size and the vent's rating, but most roofers run it nearly the full length of the ridge, stopping short of the ends as the instructions require.
Common product lengths
- Rigid sections: usually around 4 feet each, joined end to end. Easy to handle and resistant to crushing.
- Rolls: flexible vent material in rolls commonly 20 feet or longer, unrolled along the ridge and cut to length.
- Metal roof vent material: rolls or strips sized for vented ridge closures, cut to the ridge length.
How long the run should be
- Calculate the exhaust needed: attic floor area divided by 300, times 144, divided by 2. A 1,800 square foot attic needs about 432 square inches of exhaust.
- Divide by the vent's rating per foot. At 18 square inches per foot, that is 24 feet of open slot. At 12.5, it is about 35 feet.
- Compare that to the ridge length available after the end stops, typically at least 6 inches from each end.
Full length or partial?
Many roofers install the vent along the whole ridge for a uniform look and cut the slot as long as the instructions allow, which adds exhaust capacity. That is generally fine as long as intake keeps up. If the ridge is too short for the required amount, add hip vents or consider box vents placed high on the slopes.
Ridges with hips and intersections
On complex roofs with several short ridges, each section is vented separately, with end stops at each intersection and cap shingles carried over the ends. See ridge vent vs box vents and how roofers measure a roof.