A plumber is installing a main vent stack for a 5-story commercial building. The building's largest soil or waste stack is 4 inches in diameter. According to most plumbing codes, what is the minimum required diameter for the main vent stack?
- A2 1/2 inches
- B2 inches
- C3 inches
- D4 inches
Show answer & explanationAnswer & explanation
Correct answer: C. 3 inches
Plumbing codes typically require the main vent stack to be at least one-half the diameter of the largest soil or waste stack it serves, but never less than 2 inches, and not less than the size of the building drain or sewer. For a 4-inch soil stack, half would be 2 inches. However, for a 5-story building, vent sizing tables (e.g., IPC Table 906.1 or UPC Table 904.1) for stack vents or main vents serving multiple stories often require a 3-inch vent for a 4-inch soil stack to ensure adequate ventilation for the entire system's height and DFU load.
Why the other options are wrong
- A. 2 1/2 inches is not a standard vent pipe size for this application.
- B. 2 inches might be half the soil stack diameter, but often insufficient for a main vent stack in a 5-story building.
- D. 4 inches would be the same size as the soil stack, which is generally oversized for a main vent stack unless dealing with extremely high DFU or very tall buildings.
Main Vent Stack Sizing
Determining the minimum diameter of the main vent stack for a multi-story building based on the diameter of the largest soil or waste stack and the building's height/DFU.
- Ensures adequate air supply and pressure equalization for the entire DWV system.
- Minimum size is typically half the largest soil/waste stack, but not less than 2 inches.
- Height of building and total DFU significantly impact final size.
- Requires consulting plumbing code sizing tables.
Memory trick: Main Vent: Tall Building, Half the Stack, Check the Chart, No Turn Back.