Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsMedium
A newly constructed multi-story commercial building requires a drainage system for its roof. What is the minimum required slope for a horizontal 6-inch roof drain leader that collects stormwater and discharges into the building storm sewer?
- A1/16 inch per foot
- B1/8 inch per foot
- C1/2 inch per foot
- D1/4 inch per foot
Show answer & explanationAnswer & explanation
Correct answer: B. 1/8 inch per foot
For horizontal drainage piping carrying stormwater, the minimum slope for pipes 3 inches to 6 inches in diameter is 1/8 inch per foot. This ensures adequate flow velocity to prevent solids from settling.
Why the other options are wrong
- A. This slope is generally too shallow for storm drainage and is typically for larger pipes (8 inches and above) or specific applications.
- C. This slope is typically used for smaller (1 1/4 to 2-inch) drainage pipes or where faster flow is desired.
- D. This is the common minimum slope for smaller (2-1/2 to 3-inch) sanitary drainage pipes, but not strictly required for 6-inch storm drainage.
Storm Drainage Pipe Slope
The minimum slope for horizontal storm drainage piping ensures proper flow of rainwater and prevents accumulation of debris. This slope is often less steep than sanitary drainage due to the nature of the waste.
- Prevents standing water and clogs.
- Slope varies by pipe diameter.
- Typically 1/8" per foot for 3-6 inch pipes.
- Measured as fall per linear foot of pipe.
Memory trick: Storm Drains Need Just a Slight Decline to Flow Fine.