CSLB C-39 Roofing ContractorFlashing, Ventilation and InsulationHard
An attic has a floor area of 2,400 square feet and will be vented using the 1/300 ratio, split evenly between soffit intake and ridge exhaust. If the ridge vent product provides 18 square inches of net free area per linear foot, how many linear feet of ridge vent are required to meet the exhaust-side requirement?
- A64 linear feet
- B16 linear feet
- C32 linear feet
- D48 linear feet
Show answer & explanationAnswer & explanation
Correct answer: C. 32 linear feet
Total required NFA = 2,400 ÷ 300 = 8 sq ft = 1,152 sq in. Split evenly, the exhaust (ridge) side needs 576 sq in. At 18 sq in per linear foot, required length = 576 ÷ 18 = 32 linear feet.
Why the other options are wrong
- A. Incorrect — would double-count the total required NFA.
- B. Incorrect — underestimates by using only a quarter of the total NFA.
- D. Incorrect — overstates the requirement; not consistent with the 50/50 split.
Ridge Vent Sizing Calculation
Ridge vent linear footage is determined by dividing the required net free area (in sq in) for the exhaust side by the vent's rated NFA per linear foot.
- 1/300 ratio: total NFA (sq ft) = floor area ÷ 300
- Convert sq ft to sq in by multiplying by 144
- Split total NFA 50/50 between intake and exhaust for balanced systems
Memory trick: Divide by 300, split in half, then divide by the vent's rating per foot.