CSLB C-39 Roofing ContractorFlashing, Ventilation and InsulationHard
A vaulted cathedral ceiling assembly (no attic space) covers 3,600 square feet and will use the 1/150 ratio with vents split equally between the eave and ridge. How many square inches of net free area must be provided at the EAVE intake side?
- A1,728 square inches
- B864 square inches
- C1,152 square inches
- D3,456 square inches
Show answer & explanationAnswer & explanation
Correct answer: A. 1,728 square inches
Total vent area = 3,600 ÷ 150 = 24 sq ft = 3,456 sq in. Split 50/50 between eave and ridge, the eave (intake) portion is 3,456 ÷ 2 = 1,728 sq in.
Why the other options are wrong
- B. This is only a quarter of the total requirement.
- C. This equals the exhaust-only value under a 1/300 ratio, not this scenario.
- D. This is the full total, not just the intake half.
Cathedral Ceiling Ventilation Math
Vaulted ceilings without attic space follow the same net free area math as attics, using 1/150 or 1/300 ratios and typically requiring balanced airflow channels.
- Total sq ft = ceiling area ÷ ratio denominator.
- Multiply by 144 to convert to sq in.
- Balanced systems split the total evenly between intake and exhaust.
Memory trick: '150 rule, then split the tool.'