How to Plumb an In-Wall Drain Vent

Measure the diameter of the drain and waste pipe that serves the plumbing fixture you are venting. The pipe diameter is printed on the exterior wall of most pipes. The diameter of your vent pipe must be at least half the size of the drainpipe and under no circumstances less than 1-1/4 inches in diameter. Note that many codes call for main vent stacks to equal the diameter of the main building drain; consult your local building authority for main vent stack requirements.

Determine the maximum distance of the vent pipe from the fixture's trap. This distance varies by your municipality's plumbing code. Regardless of code, the determining factor is the trap's diameter. In general, 5 feet is the maximum distance for 1 1/4-inch traps, 6 feet for 1 1/2-inch, 8 feet for 2-inch and 12 feet for 3-inch. Check with your building authority to confirm the proper vent location.

Drop a plumb bob from the trap's wall drain stub to the wall's bottom plate. Mark the location of the bob's weight on the plate. Stretch a tape measure from this mark to the dimension equal to the maximum distance between the trap and vent. Mark this dimension on the wall's bottom plate. Your vent pipe must fall between these two marks.

Choose a location for your vent pipe between the two marks; the vent must branch from the fixture's drainpipe. Consider the vent's destination, either roof or main vent stack, when you choose its branch location. Building codes prescribe the maximum vent pipe length, according to the vent pipe's diameter and the diameter of the drainpipe that the vent services. Maximum lengths range from 25 to 150 feet or more. Consult a vent pipe sizing and length table, available from your building authority, to determine the maximum length. Keep in mind the maximum length when you choose your pipe's location.

Mark the location where the vent will connect to the drainpipe on the top side of the drainpipe's wall. If you are working on a new system and the drainpipe isn't installed, mark the location on the wall's bottom plate. Stretch a plumb bob between the mark and the wall's top plate. Use the bob to transfer the mark's location to the underside of the top plate.

Select a hole saw equal to or slightly larger in diameter than the required vent pipe. Mount the hole saw onto a power drill and drill through the mark at the underside of the top plate. Wipe debris from the inside of an ABS tee's openings with a clean rag. If the vent pipe and main drain are vertically aligned and meet the fixture's drain at a right angle, you must use a sanitary tee to accept the fixture's drain.

Coat the tee's bottom opening and main drainpipe's end with ABS cement. Insert the tee over the pipe and twist it roughly 1/4-turn while keeping the side of the tee that meets the fixture's drainpipe aligned with the drainpipe's path. Hold the tee in position for approximately 30 seconds.

Measure the distance from the tee's top opening to the vent's destination, either the main stack or roof. Mark and cut a length of ABS pipe to this size with a chop saw. Clean the pipe's ends with an emery cloth and wipe them with a rag. Use ABS cement to join the pipe to the vent's upper opening by running the pipe through the hole in the wall's top plate. Connect the remaining open end of the tee to the fixture's drainpipe with a length of pipe and ABS cement.