P-trap

A P-trap is a curved section of drainpipe, usually 1.5 to 2 inches in diameter, that dips below the level of the outlet before rising again. The water that sits in the lowest point of the curve—the seal—blocks sewer gas, insects, and odors from backing up into the fixture or building. The name comes from its profile: the vertical inlet, the curved bottom, and the outlet leg roughly resemble the letter P.

In the field, P-traps are the standard trap for sinks, lavatories, showers, and floor drains. They come in chrome or PVC, screw together with slip nuts and washers, or are solvent-welded. The trap arm—the horizontal run from the fixture to the vent—must slope at least 1/4 inch per foot toward the main drain. If the slope is wrong or the trap is undersized, water won't drain cleanly and odor problems follow. Cleanout plugs at the bottom make clearing blockages straightforward.

Code requires a trap on every fixture, and the seal must be at least 2 inches deep but not more than 4 inches in most jurisdictions. That depth matters: too shallow and gases slip through; too deep and siphoning pulls the seal dry. Watch for traps drying out in unused fixtures—a dried trap is a dead trap, and the gases come back.