CSLB C-39 Roofing ContractorFlashing, Ventilation and InsulationMedium
A single-story home has an attic floor area of 2,400 square feet. The attic will be vented using the 1/300 ratio with a balanced system (50% intake at the eaves, 50% exhaust at the ridge). How many square inches of net free ventilation area are required at the ridge (exhaust side) alone?
- A576 square inches
- B2,304 square inches
- C288 square inches
- D1,152 square inches
Show answer & explanationAnswer & explanation
Correct answer: A. 576 square inches
Total required vent area = 2,400 ÷ 300 = 8 sq ft = 1,152 sq in. With a balanced 50/50 split, the ridge (exhaust) portion is 1,152 ÷ 2 = 576 sq in.
Why the other options are wrong
- B. This doubles the actual total requirement in error.
- C. This is only a quarter of the total required area.
- D. This is the full total, not just the exhaust half.
1/300 Balanced Ventilation Ratio
When a vapor retarder is present, attics may use 1 sq ft of net free vent area per 300 sq ft of attic floor area, split evenly between intake and exhaust.
- Formula: Attic area ÷ 300 = total sq ft of vent area.
- Convert to sq in by multiplying by 144.
- Balanced systems split total evenly: 50% low, 50% high.
Memory trick: 'Divide by 300, then split it in two.'