CSLB C-39 Roofing ContractorLow-Slope RoofingHard
A designer must size two overflow scuppers to evenly split roof drainage duty for a 3,000 sq ft roof area during a 4 in/hr design rainfall event. The manufacturer's scupper sizing chart indicates that at a 4 in/hr rainfall rate, a scupper with a 4-inch opening height requires 1 inch of width for every 150 square feet of roof area drained. What width should each of the two scuppers be?
- A15 inches
- B10 inches
- C30 inches
- D20 inches
Show answer & explanationAnswer & explanation
Correct answer: B. 10 inches
Total required width = 3,000 sq ft ÷ 150 sq ft/inch = 20 inches. Since two scuppers share the load evenly, each scupper needs 20 ÷ 2 = 10 inches of width.
Why the other options are wrong
- A. This would result from dividing incorrectly or using the wrong ratio.
- C. This overestimates the total width beyond what the chart requires.
- D. This is the total required width for a single scupper, not each of two.
Scupper Width Sizing
Overflow scupper width is sized based on roof area and design rainfall intensity using manufacturer or code sizing charts, then divided among multiple scuppers sharing the drainage load.
- Sizing charts relate roof area to required opening width at a given rainfall rate and height
- Multiple scuppers split the total required width evenly
- Scupper height is typically fixed (e.g., 4 inches) per code minimums
Memory trick: Split the total width by the number of exits.