CSLB C-39 Roofing ContractorFlashing, Ventilation and InsulationMedium
A ventilation product is rated with a net free area (NFA) of 50 square inches per linear foot. If a contractor installs 24 linear feet of this vent along a ridge, how many square feet of net free area does the installation provide?
- A6.25 sq ft
- B10.0 sq ft
- C8.33 sq ft
- D12.5 sq ft
Show answer & explanationAnswer & explanation
Correct answer: C. 8.33 sq ft
Total NFA = 50 sq in/ft × 24 ft = 1,200 sq in. Converting to square feet: 1,200 ÷ 144 = 8.33 sq ft. This conversion is essential when comparing vent product ratings (given in sq in) against ventilation code requirements (given in sq ft ratios).
Why the other options are wrong
- A. Incorrect — this would result from dividing by the wrong linear footage.
- B. Incorrect — overstates the result by rounding up incorrectly.
- D. Incorrect — would require significantly more linear footage.
NFA Unit Conversion
Net free area (NFA) is often listed in square inches per linear foot on vent products but must be converted to square feet to compare with code ventilation ratios.
- 1 square foot = 144 square inches
- NFA per foot × total linear feet = total sq in NFA
- Divide total sq in by 144 to get sq ft
Memory trick: 144 square inches make one square foot — always divide!