CSLB C-39 Roofing ContractorLow-Slope RoofingHard
To ensure positive drainage at a roof drain, a contractor installs a sump (depression) in the tapered insulation around the drain bowl. The surrounding field insulation is 2 inches thick, and the drain bowl must sit flush with insulation that is recessed 1/2 inch below the surrounding field surface. What insulation thickness should be installed directly at the drain?
- A2 inches
- B2.5 inches
- C1.5 inches
- D1 inch
Show answer & explanationAnswer & explanation
Correct answer: C. 1.5 inches
The drain sump must be recessed 1/2 inch lower than the surrounding 2-inch field insulation, so the insulation thickness at the drain equals 2 in − 0.5 in = 1.5 inches, allowing the drain bowl to sit properly flush and prevent ponding around it.
Why the other options are wrong
- A. This would make the drain level with the field, not recessed as required.
- B. This raises the drain above field level, which is incorrect.
- D. This recesses the drain 1 inch below field level, deeper than specified.
Drain Sump Insulation Thickness
A sump is a shallow depression in tapered insulation around a roof drain, recessed slightly below the surrounding field to ensure the drain sits flush and water flows properly into it.
- Sump thickness = field thickness − recess depth
- Prevents ponding immediately around the drain bowl
- Typically achieved with factory-tapered sump insulation panels
Memory trick: Subtract the dip from the field to find the drain depth.