How do you do a roof and attic ventilation calculation?
Short answer. Divide the attic floor area by 300 for a balanced system, or by 150 if it is not balanced, multiply by 144 to get square inches of net free area, split it roughly half intake and half exhaust, then divide each half by the rated net free area of the vents you plan to use. For example, a 2,400 square foot attic needs about 1,152 square inches in total, roughly 576 of intake and 576 of exhaust.
Worked example: a hip roof
- Attic area: 60 by 40 feet is 2,400 square feet.
- Total net free area: 2,400 divided by 300 is 8 square feet, times 144 is 1,152 square inches.
- Exhaust share: about 576 square inches.
- Ridge vent check: a hip roof this size might have only about 20 feet of ridge. At 18 square inches per foot, that provides 360 square inches, short of 576.
- Options: add off-ridge or static vents near the peak where the manufacturer allows that layout, or use static vents alone. At about 50 square inches each, 576 square inches takes 12 vents.
- Intake: at least 576 square inches. If the continuous soffit vent you choose is rated at 9 square inches per foot, that is 64 feet, spread along the eaves.
Common errors
- Using the gross opening size instead of net free area. Screens and louvers cut the real number.
- Counting vents that are painted over or buried in insulation.
- Measuring roof surface area instead of the attic floor.
- Counting a vaulted section as part of the open attic.
When the numbers do not work, intake is usually the limiting factor. More in roof ventilation basics and how roofers measure a roof.