CSLB C-39 Roofing ContractorEstimating and PlanningMedium
An attic space measures 1,800 square feet and has no vapor retarder installed, requiring a 1:150 ventilation ratio per code. What is the total net free ventilating area (NFA) required for this attic, expressed in square feet?
- A12 square feet
- B9 square feet
- C18 square feet
- D6 square feet
Show answer & explanationAnswer & explanation
Correct answer: A. 12 square feet
Under the 1:150 ratio, 1 square foot of net free ventilating area is required for every 150 square feet of attic floor area. Dividing 1,800 square feet by 150 gives 1,800 ÷ 150 = 12 square feet of total required NFA.
Why the other options are wrong
- B. Incorrect: does not match the correct division of attic area by the ratio.
- C. Incorrect: overestimates the required ventilation area.
- D. Incorrect: this would result from an incorrect ratio, not 1:150.
Attic Ventilation NFA Calculation
Code-required net free ventilating area (NFA) is calculated by dividing attic floor area by the applicable ventilation ratio (commonly 1:150 or 1:300).
- 1:150 ratio applies when no vapor retarder is present
- 1:300 ratio may apply when balanced intake/exhaust and vapor retarder conditions are met
- Total NFA is typically split between intake (soffit) and exhaust (ridge) vents
Memory trick: Divide the attic by one-fifty to breathe easy.