Does every plumbing fixture need its own vent?

Q: What size drain pipe do I need for a bathroom?

A full bathroom group (water closet, lavatory, tub/shower) is about 5–6 DFU. A 3-inch branch handles the water closet and the group, while the lavatory and tub branches are typically 1-1/2 to 2 inches. The building drain serving a bathroom is 3 inches minimum, because any drain carrying a water closet must be at least 3 inches.

Q: How many fixtures can a 2-inch drain handle?

Under the IPC, a 2-inch horizontal branch at 1/4 inch-per-foot slope carries up to about 6 DFU, and a 2-inch stack handles more. That covers a shower, lavatory, and kitchen sink, but never a water closet — toilets require a minimum 3-inch drain regardless of fixture-unit count.

Question

Does every plumbing fixture need its own vent?

Accepted Answer

Every trap must be protected by a vent to prevent siphoning, but it does not always need its own individual vent. The IPC allows wet venting, common venting, and circuit venting so a single vent can serve multiple fixtures when arranged within the code’s distance and size limits. Size the vent from its DFU load and developed length.

Does every plumbing fixture need its own vent?

Answers