Fixtures, Fixture Fittings, and Fixture Traps
Fixture installation
Fixtures must be installed level, securely supported, and spaced for use and cleaning. Water closets generally need at least 15 inches from the centerline to a side wall and 30 inches of clear width; lavatories and sinks need usable clearance and a secure backing. Fixture outlets must be connected with approved fittings, and the fixture must be accessible for repair. Floor-mounted fixtures need a durable, watertight joint at the floor.
Traps and trap arms
Each fixture, except those specifically exempted, must have a trap with a water seal of 2 to 4 inches. The trap arm runs from the trap weir to the vent fitting and must not exceed the length allowed for its size: approximately 2 feet 6 inches for 1 1/4-inch, 3 feet 6 inches for 1 1/2-inch, 5 feet for 2-inch, 6 feet for 3-inch, and 10 feet for 4-inch. The vent must connect at least two pipe diameters from the trap weir. S-traps, crown vents, and double traps on a single fixture are prohibited.
Worked example
A lavatory has a 1 1/4-inch trap and the vent is 8 feet from the trap weir. The maximum trap-arm length for 1 1/4-inch is about 2 feet 6 inches, so the installation fails. Increasing the trap arm to 2-inch does not help because the 8-foot arm still exceeds the 5-foot limit; the vent must be moved closer or the layout redesigned.
Traps
Do not confuse the trap arm with the fixture tailpiece. Do not rely on an AAV where the code requires a through-roof vent. Check the trap seal after installation, and do not set a fixture so that its outlet is below the trap weir.