CSLB C-39 Roofing ContractorEstimating and PlanningMedium
A contractor calculates that an attic space requires a total net free ventilation area (NFA) of 432 square inches. The static roof vents specified for the job each provide 50 square inches of net free area. What is the minimum number of static vents required to meet the ventilation requirement?
- A8 vents
- B7 vents
- C9 vents
- D10 vents
Show answer & explanationAnswer & explanation
Correct answer: C. 9 vents
Divide the total required NFA by the NFA provided per vent: 432 ÷ 50 = 8.64. Since partial vents cannot be installed, the result must be rounded up to the next whole vent, giving 9 vents. Rounding down to 8 would leave the attic under-ventilated.
Why the other options are wrong
- A. Rounding down (8 × 50 = 400 sq in) fails to meet the 432 sq in requirement.
- B. Too low; provides only 350 sq in of NFA, well short of the 432 required.
- D. Overestimates the need; exceeds requirement more than necessary and adds unneeded cost.
Static Vent Quantity Takeoff
The number of individual roof vents needed is found by dividing the total required net free ventilation area (NFA) by the NFA rating of a single vent unit, always rounding up to the next whole vent.
- Total NFA ÷ per-vent NFA = number of vents needed
- Always round up — partial vents are not installed
- Vent NFA ratings are typically printed on product specification sheets
- Total NFA is often split between intake (soffit) and exhaust (ridge/roof) vents
Memory trick: Divide the total air need by each vent's share, then round UP — never leave the attic short of breath.