Traps and Cleanouts: Seals, Vents, and Access
Trap purpose and design
A trap holds a water seal that prevents sewer gas from entering the building. The seal depth is defined by code, and the trap must be self-cleaning. Traps must be installed level, with a smooth interior and an accessible cleanout. Each fixture must have its own trap, except where fixtures are permitted to share one through a continuous waste arrangement.
Prohibited arrangements
Bell traps, drum traps (except where specifically allowed), and traps with moving parts are generally prohibited. Double trapping and trap-seal in series are prohibited because they can cause air lock or loss of seal. The trap arm must be vented within the allowed distance to prevent siphonage.
Cleanouts
Cleanouts provide access for clearing blockages. They are required at the base of stacks, at the building sewer, at changes of direction greater than a specified angle, and at intervals along horizontal runs. Cleanouts must be accessible, with clearance in front of the opening, and must be gas- and watertight when concealed.
Traps in questions
Watch for questions about trap seal loss, improper trap types, and concealed cleanouts without access. Also watch for the distance from the trap to the vent.
Worked example
A sink trap is installed in a cabinet. The cleanout on the trap must be accessible by removing the cabinet doors or a removable panel, not by cutting the wall. If the trap arm exceeds the allowed distance to the vent, the trap may siphon and lose its seal.
Rule of thumb
Every trap needs a vent within the allowed distance and a cleanout that can be reached without destructive work.