bottle trap

A bottle trap is a U-shaped or S-shaped fitting installed directly beneath a sink, basin, or other drainage point. Water and waste flowing down the drain collect in the lower chamber of the trap, where a column of standing water creates a seal that prevents sewer gases and odors from rising back up through the outlet. The trap sits between the waste pipe leaving the fixture and the main drainage system, typically visible under a pedestal sink or in a cabinet.

The bottle trap earned its name from its shape: the lower section swells outward like a bottle before the outlet pipe rises away from it. Common sizes range from 32 mm to 40 mm in diameter on residential installations. They are made from plastic (PVC or polypropylene), chrome-plated brass, or stainless steel depending on whether they are exposed or hidden behind cabinetry. The depth of water seal usually sits between 50 and 100 mm, though local plumbing codes specify minimum seal depths to ensure effectiveness.

Bottle traps work well where headroom or horizontal space is limited, making them popular under wall-hung basins and compact vanities. Unlike P-traps or S-traps, which extend further into adjacent cabinetry, the bottle trap keeps most of its bulk directly below the drain outlet. They are easier to access for cleaning and removing trapped objects, since the base of the bottle can often be unscrewed to clear blockages.

Common failure points and maintenance

The standing water seal evaporates over time if a fixture sits unused for weeks, allowing sewer gas to escape into the room. Regular use keeps the seal intact. Sediment, hair, and grease accumulate in the bottle section, eventually restricting flow or causing backups. The threaded joint at the base can crack or leak under thermal stress or if tightened excessively during installation. Older chrome-plated brass traps corrode internally, leaving sharp particles or discoloring water.

Building codes in some regions restrict bottle traps in new work because their shape traps air pockets that can interfere with proper venting of the drainage system. They remain acceptable for retrofits and repairs in many jurisdictions, but local plumbing inspectors may require an S-trap or P-trap with a proper vent stack instead. Always verify the applicable code before specifying one on a new installation.