Possible Ways to Vent a Bathroom Sink Drain Pipe

Branch Vent

• A standard vent attaches directly to the drain pipe with a sanitary tee fitting. The drain pipe travels straight down while the vent pipe travels straight up. The sanitary tee connects directly to the P-trap for the sink's drain. The vent pipe attaches to the main plumbing vent in the home's attic. The main vent exits through the roof and provides venting for all the waste and drain pipes in the plumbing system. Before it attaches to the main vent, it may join other vent pipe in the attic.

Main Vent

• The fixtures in many bathrooms connect directly to the main vent, or stack. If the sink's drain pipe can connect directly to the stack without traveling more than a few feet horizontally, the drain does not need a dedicated vent. This configuration is common in many homes with one bathroom. If the stack is beyond the horizontal run limit -- check local codes -- a vent that rises from the drain pipe and connects to the stack before reaching the attic works like a branch vent.

Wet Vent

• A wet vent is a both a drain and a vent. A drain pipe from a fixture on a floor above the sink travels down to join with the main drain and waste pipes. This drain pipe may also serve as both the vent and the drain pipe for the lower level sink drain. Avoid using a toilet drain as the vent for a lower level sink. The large volume of water that passes through the pipe may pull the water out of the sink's trap and allow sewer gases to escape into the home. Wet vents are not approved in all areas for this reason.

AAV Vents

• Air Admittance Valves allow a drain to vent without having a vent pipe connect to the main vent pipe. Most codes only permit these valves in installations where connection to the home's main vent is impossible or impractical. The AAV valve allows air to enter the pipe when the pipe is draining. It closes to prevent sewer gas from entering the home. AAV vents require prior approval by a building inspector before installation.