CSLB C-39 Roofing ContractorLow-Slope RoofingHard
A 2-inch diameter primary roof drain is rated by the plumbing code drainage table to handle 6,750 square feet of roof area at a rainfall intensity of 4 inches per hour. If the project's design rainfall intensity is only 2 inches per hour, what is the maximum roof area that single drain can serve?
- A3,375 sq ft
- B10,125 sq ft
- C6,750 sq ft
- D13,500 sq ft
Show answer & explanationAnswer & explanation
Correct answer: D. 13,500 sq ft
Drainage capacity is rated for a specific rainfall intensity, and the maximum serviceable roof area scales inversely with rainfall intensity. Since 2 in/hr is half of 4 in/hr, the drain can handle twice the roof area: 6,750 sq ft x (4 ÷ 2) = 13,500 sq ft.
Why the other options are wrong
- A. 3,375 sq ft incorrectly halves the area instead of doubling it.
- B. 10,125 sq ft does not match the correct inverse proportion calculation.
- C. 6,750 sq ft ignores the change in rainfall intensity entirely.
Drain Capacity vs. Rainfall Intensity
Plumbing code drainage tables rate roof drain capacity for a reference rainfall intensity (commonly 4 in/hr); the serviceable roof area scales inversely with actual design rainfall intensity.
- Lower rainfall intensity allows a larger service area per drain
- Higher rainfall intensity requires a smaller service area or additional drains
- Formula: New Area = Rated Area x (Rated Intensity ÷ Design Intensity)
Memory trick: Less rain per hour means each drain can cover more ground.