Connection of a Sink to an Upstairs Vent Pipe

Connecting the Vent

• Most sink P-traps extend horizontally from the lowest point to the wall, and from there the drain can slope gradually or vertically to the sewer. Plumbers usually use 2-inch pipe to construct the drain. The plumbing code requires that the diameter of the vent pipe be half the diameter of the drain, but since 1 1/4 inches is the minimum allowable vent diameter, it's common to simply use a 1 1/2-inch vent pipe. It must connect to the drain within 5 feet of the trap. To connect it, you usually glue in a 2-by-2-by-1 1/2-inch sanitary tee, with its perpendicular port facing up and sweeping toward the sewer.

Running the Pipe

• The sink vent pipe can't run horizontally until it is 6 inches above the overflow line of the sink, but from that point, the only restriction is that it must maintain a 1/4-inch-per-foot slope toward the drain. Plumbers often run vents horizontally to clear a window or other obstruction over the sink, and then extend them vertically, or almost vertically, through the wall to the attic. They extend them that far so they can make connections above the overflow line of the highest plumbing fixture in the house. That way, the vent will never inadvertently fill with water.

Wet Vents

• If the sink you are venting is between the toilet and the main soil stack, and the toilet is too far from the stack for adequate venting, you can use the sink drain to vent the toilet. This is called wet venting, and the requirements are that the diameter of the sink drain be more than the 1 1/2-inch minimum drain size -- 2-inch pipe is usually sufficient -- and that the vent run continuously up from the sink drain. In this case, maintaining a 2-inch diameter on the vent pipe ensures that the toilet will be properly vented.

Tying In

• The horizontal section of pipe that ties into the stack vent, which is the vertical vent connected to the soil stack, is called a branch vent. You connect it to the stack vent with a vent tee, which is a tee without a sweep, similar to a tee for water connections. The procedure is usually simple. You simply cut the stack vent pipe with a hacksaw, glue in the tee with plastic pipe cement and glue the branch vent to the perpendicular port. A house with more than one bathroom might have more than one such connection to the stack vent.