Do I need a ridge vent, and how much ridge vent do I need?
Short answer. You need a ridge vent if your attic is vented, has soffit intake, and does not already have enough exhaust high on the roof; it is the most common way to provide that exhaust on gable roofs. How much you need depends on your attic's floor area and the vent's rating, typically enough linear feet to supply about half of the required net free area.
When you need one
- You are replacing the roof and the attic has soffits but little or no high exhaust.
- Existing box vents are cracked, leaking, or too few.
- The attic shows heat or moisture symptoms: mildew on the decking, rusty nails, damp insulation, or very hot upstairs rooms.
- Your shingle manufacturer's ventilation requirements are not being met.
When you do not
- The attic is sealed with spray foam under the deck.
- Existing exhaust is adequate and the attic is dry, unless you are reroofing and want to replace box vents.
- There is no intake and no way to add it.
How much ridge vent you need
- Find the attic floor area. Example: 2,000 square feet.
- Divide by 300 for the balanced path: about 6.67 square feet, or 960 square inches in total.
- Take half for exhaust: about 480 square inches.
- Divide by the vent's rating per foot. At 18 square inches per foot, that is about 27 feet of open ridge. At 12.5 square inches per foot, it is about 39 feet.
If the ridge is shorter than the number you get, you need a higher rated vent, supplemental hip vents, or a different exhaust strategy. Many roofers run the vent the full length of the ridge for a uniform look even when less is required.
Make sure intake at least matches the exhaust. Learn more in ridge vent vs box vents and roof ventilation basics.