CSLB C-39 Roofing ContractorEstimating and PlanningMedium
An attic space measures 2,400 square feet and has an approved vapor retarder installed, qualifying it for the 1/300 ventilation ratio with a balanced 50/50 split between intake and exhaust vents. How many square inches of net free area (NFA) must be provided by the exhaust (ridge) vents alone?
- A384 sq in
- B1,152 sq in
- C768 sq in
- D576 sq in
Show answer & explanationAnswer & explanation
Correct answer: D. 576 sq in
Total required NFA = (2,400 sq ft × 144 sq in/sq ft) ÷ 300 = 1,152 sq in. Splitting this 50/50 between intake and exhaust gives 576 sq in for the exhaust vents.
Why the other options are wrong
- A. This is less than half the required total NFA.
- B. This is the full total NFA, not the exhaust-only share.
- C. This exceeds the 50/50 balanced share.
Balanced Attic Ventilation NFA
When a vapor retarder is present, the required attic ventilation ratio is 1/300 of the attic floor area, with the total net free area typically split evenly between intake and exhaust vents.
- 1/300 ratio applies when a vapor retarder is present
- 1 sq ft = 144 sq in for NFA conversion
- Balanced systems split total NFA 50/50 intake/exhaust
Memory trick: Half goes low, half goes high, air must flow.