Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsMedium
A plumber is installing a main vent stack for a 6-story commercial building. The building's main soil stack is 6 inches in diameter. According to plumbing codes, what is the minimum required diameter for the main vent stack?
- A3 inches
- B5 inches
- C4 inches
- D2 inches
Show answer & explanationAnswer & explanation
Correct answer: C. 4 inches
For a main soil stack 6 inches in diameter, the main vent stack must be at least 1/2 the diameter of the soil stack, but never less than 2 inches, and generally not less than 4 inches for a building of this size. 6 inches / 2 = 3 inches, but for a 6-story commercial building, 4 inches is the commonly accepted minimum to ensure adequate ventilation.
Why the other options are wrong
- A. 3 inches is half the diameter of the soil stack, but for larger commercial buildings, 4 inches is often the practical and code-required minimum.
- B. 5 inches is larger than the minimum typically required for a 6-inch soil stack, though it would provide ample ventilation.
- D. 2 inches is too small for a main vent stack serving a 6-inch soil stack in a multi-story commercial building.
Main Vent Stack Sizing
The process of determining the minimum diameter of the main vent stack, which extends through the roof and protects the entire drainage system from pressure fluctuations.
- Minimum size is often tied to the diameter of the largest soil or waste stack it serves.
- Generally, the main vent stack must be at least one-half the diameter of the largest stack connected to it.
- Never smaller than 2 inches, and typically not less than 3 or 4 inches for multi-story or large commercial buildings.
- Total DFUs and developed length also factor into final sizing.
Memory trick: The VENT is HALF the STACK, but never too slack!