HOW TO USE IT
How to use the Roof Ventilation Calculator
- 1
Measure the attic area: Use the floor area of the attic you are ventilating. Larger attics need more net free area.
- 2
Apply the NFA ratio: The 1:150 rule means 1 sq ft of NFA per 150 sq ft of attic. Some installations use 1:300 with balanced ridge and soffit.
- 3
Balance intake and exhaust: A typical split is half intake (soffit) and half exhaust (ridge). The calculator takes the intake percentage and derives the exhaust.
- 4
Size the vents: 1,200 sq ft ÷ 150 = 8 sq ft of total NFA, split into 4 sq ft intake and 4 sq ft exhaust. Use the Roof Ventilation Calculator, then match vents to their rated NFA.
THE MATH
Roof ventilation calculation method
Divide attic area by the NFA ratio for total NFA, then split it between intake and exhaust. A 1,200 sq ft attic at 1:150 needs 8 sq ft of NFA, with 4 sq ft each intake and exhaust.The 1:150 rule means 1 sq ft of NFA per 150 sq ft of attic. Some installations use 1:300 with balanced ridge and soffit.
PRACTICAL EXAMPLE
Worked example for Roof ventilation
Divide attic area by the NFA ratio for total NFA, then split it between intake and exhaust. A 1,200 sq ft attic at 1:150 needs 8 sq ft of NFA, with 4 sq ft each intake and exhaust.
FAQ
Frequently asked questions
How do you size roof ventilation?
Divide attic area by the NFA ratio for total NFA, then split it between intake and exhaust. A 1,200 sq ft attic at 1:150 needs 8 sq ft of NFA, with 4 sq ft each intake and exhaust.
What should I verify before using the result?
Balanced intake and exhaust matters as much as total area. Unbalanced systems short-circuit.
Is this a final quote or design?
No. Use it as a planning estimate, then confirm measurements, local rates, product requirements, permits and jobsite conditions before bidding or building.