EXTERIOR
How to Size Roof Ventilation
Contractor method for sizing intake and exhaust ventilation from attic area and net free area.
Roof ventilation needs enough net free area (NFA) and a balance between intake and exhaust. The common rule is 1 square foot of NFA per 150 square feet of attic.
THE SHORT ANSWER
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.
Measure the attic area
Use the floor area of the attic you are ventilating. Larger attics need more net free area.
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.
- Attic area
- NFA ratio
- Intake share
- Exhaust share
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.
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.