Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsHard
A plumber is installing a main building drain in a commercial facility. The drain will receive discharge from several fixtures, totaling 180 DFU. What is the minimum required diameter for this building drain if it has a slope of 1/8 inch per foot?
- A3 inches
- B6 inches
- C5 inches
- D4 inches
Show answer & explanationAnswer & explanation
Correct answer: B. 6 inches
According to plumbing codes, a building drain with a slope of 1/8 inch per foot can carry a maximum of 180 DFU for a 6-inch pipe. Smaller pipes at this slope cannot accommodate 180 DFU.
Why the other options are wrong
- A. A 3-inch pipe at 1/8 inch per foot slope can only carry a maximum of 31 DFU.
- C. A 5-inch pipe at 1/8 inch per foot slope can only carry a maximum of 144 DFU.
- D. A 4-inch pipe at 1/8 inch per foot slope can only carry a maximum of 84 DFU.
Building Drain Sizing (DFU & Slope)
Determining the minimum diameter of a building drain based on the total drainage fixture units (DFU) and the pipe's installed slope.
- DFU represent the probable discharge load.
- Steeper slopes allow smaller pipe diameters for the same DFU.
- Lower slopes require larger pipe diameters.
- Sizing tables are crucial for accurate determination.
Memory trick: DFU Downslope Diameter Decides.