CSLB C-39 Roofing ContractorFlashing, Ventilation and InsulationMedium
A single-story home has an attic floor area of 3,000 square feet. The contractor will use the 1/300 ventilation ratio with a balanced system split evenly between soffit intake and ridge exhaust vents. How many square inches of net free area must the soffit vents alone provide?
- A720 square inches
- B2,880 square inches
- C360 square inches
- D1,440 square inches
Show answer & explanationAnswer & explanation
Correct answer: A. 720 square inches
Total required NFA = 3,000 sq ft ÷ 300 = 10 sq ft = 1,440 sq in (10 × 144). Splitting this evenly between intake and exhaust gives 1,440 ÷ 2 = 720 sq in for the soffit vents.
Why the other options are wrong
- B. This double-counts the total required area.
- C. This is only a quarter of the total required area.
- D. This is the full required NFA, not the soffit-only portion.
1/300 Ratio Split Calculation
Under the 1/300 ratio, total attic floor area is divided by 300 to get total NFA in square feet, converted to square inches, then split between intake and exhaust.
- 1 sq ft = 144 sq in
- Total NFA = attic area ÷ 300
- Balanced systems split NFA 50/50
Memory trick: Divide by 300, convert to inches, then cut it in half.