Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsMedium
When installing a horizontal drainage pipe, what is the minimum required slope for a 3-inch (75 mm) pipe according to most plumbing codes?
- A1/8 inch per foot (10 mm/m)
- B1/16 inch per foot (5 mm/m)
- C1/4 inch per foot (20 mm/m)
- D1/2 inch per foot (40 mm/m)
Show answer & explanationAnswer & explanation
Correct answer: A. 1/8 inch per foot (10 mm/m)
For horizontal drainage pipes 3 inches (75 mm) in diameter or larger, most plumbing codes permit a minimum slope of 1/8 inch per foot (10 mm/m). Pipes smaller than 3 inches typically require 1/4 inch per foot.
Why the other options are wrong
- B. This slope is generally too shallow for any drainage pipe and would lead to insufficient scouring.
- C. 1/4 inch per foot is the standard minimum slope for horizontal drainage pipes smaller than 3 inches, and also acceptable for larger pipes, but not the minimum for 3-inch pipe.
- D. While an acceptable slope, 1/2 inch per foot is more than the minimum required and could lead to solids being left behind if the flow velocity is too high and liquid volume too low.
Drainage Pipe Slope
The downward inclination of horizontal drainage piping, measured in inches per foot, to ensure proper flow and self-scouring.
- Ensures waste moves effectively.
- Prevents solids from settling.
- Varies by pipe diameter.
Memory trick: Slope it right, keep it clear, no clogs, no fear!