Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsMedium
A plumber is installing a main vent stack for a 4-story building. The building's main soil stack is 4 inches in diameter. What is the minimum required diameter for this main vent stack?
- A5 inches
- B2 inches
- C4 inches
- D3 inches
Show answer & explanationAnswer & explanation
Correct answer: C. 4 inches
The main vent stack, also known as a vent stack, must be at least the same diameter as the soil or waste stack it serves, or larger. Therefore, for a 4-inch soil stack, the minimum vent stack diameter is 4 inches.
Why the other options are wrong
- A. 5 inches would be acceptable but is larger than the minimum required.
- B. 2 inches is too small for a 4-inch soil stack and would lead to inadequate venting.
- D. 3 inches is too small for a 4-inch soil stack.
Main Vent Stack Sizing
The minimum diameter requirement for the vertical vent pipe that extends through the roof and serves as the primary air inlet/outlet for the DWV system.
- Must be at least the same size as the largest soil or waste stack.
- Ensures adequate air flow for the entire system.
- Prevents siphoning and backpressure.
Memory trick: Stack's Same Size, Skyward Stream.