What is a typical ridge vent NFA per foot, and how many feet does your attic need?
Short answer. Most shingle-over ridge vents are rated somewhere between about 12.5 and 18 square inches of net free area per linear foot, with many rigid vents at 18. On the balanced 1/300 path, a 2,000 square foot attic needs about 27 feet of an 18 rated vent or about 39 feet of a 12.5 rated vent.
The quick formula
Exhaust needed in square inches equals attic floor area times 0.24 on the balanced 1/300 path. Divide by the vent's rating per foot to get the feet of open slot.
Feet of open slot needed
- 1,200 square foot attic: about 288 square inches of exhaust. About 16 feet at 18 per foot, or 23 feet at 12.5.
- 1,800 square foot attic: about 432 square inches. About 24 feet at 18, or 35 feet at 12.5.
- 2,400 square foot attic: about 576 square inches. About 32 feet at 18, or 46 feet at 12.5.
- 3,000 square foot attic: about 720 square inches. About 40 feet at 18, or 58 feet at 12.5.
Double these if the attic does not have balanced high and low vents.
Reading the rating
- The rating is printed on the packaging and data sheet, usually in square inches per linear foot.
- Narrower versions of a vent made for 10 inch caps may carry a lower rating than the wide version.
- The rating assumes the slot is cut to the specified width.
When the ridge is not long enough
Many hip roofs fall short. Options are a higher rated vent, a vent approved for hips cut near the top, or box vents as the single exhaust type. Intake at the soffits must at least match whichever you choose.
The ratios behind the math are in roof ventilation basics; measuring the ridge is covered in how roofers measure a roof.