CSLB C-36 Plumbing ContractorRough-in PlumbingHard
A plumbing inspector is reviewing a rough-in for a new multi-story residential building. The inspector observes a common vent serving two fixture traps. What is the maximum developed length of the common vent, measured from the farthest trap arm to the vent stack, before it requires an additional vent or different venting method, assuming 1 1/2-inch pipe size?
- A8 feet
- B15 feet
- C12 feet
- D10 feet
Show answer & explanationAnswer & explanation
Correct answer: D. 10 feet
The California Plumbing Code (CPC) limits the developed length of a common vent based on its size. For a 1 1/2-inch common vent, the maximum developed length from the farthest trap weir to the vent stack or branch vent connection is 10 feet to ensure adequate air circulation and prevent siphonage.
Why the other options are wrong
- A. 8 feet is too short and not the maximum allowed for a 1 1/2-inch common vent.
- B. 15 feet is significantly over the limit for a 1 1/2-inch common vent.
- C. 12 feet exceeds the maximum allowed for this pipe size.
Common Vent Developed Length
The total length of a common vent pipe, measured along the centerline of the pipe and fittings from the farthest trap weir to the vent stack or branch vent connection.
- Ensures proper air circulation for multiple fixtures.
- Prevents trap seal loss due to siphonage or backpressure.
- Length limits are specified by plumbing codes based on pipe size.
Memory trick: Common vent, don't extend, 1 1/2-inch, 10 feet is the end.