Traps and water seals

A trap is a bend in a drain pipe that always holds a little water. That water plug, the seal, blocks foul air from the drain while letting waste flow through. Every fixture, from a washbasin to a floor drain, needs one.

Common shapes are the P-trap, which leaves horizontally, the S-trap, which leaves vertically and is more prone to losing its seal, the bottle trap under basins, the gully trap outside kitchens and bathrooms, and the intercepting trap where a building drain joins the sewer.

Seals are lost in several ways: self-siphonage when a fixture's own outflow pulls the water out behind it, induced siphonage when another fixture's discharge creates suction, back-pressure that blows the seal out, evaporation in fixtures that are rarely used, and capillary action through a thread or hair hanging over the weir. Venting, correct pipe sizing and gradients, and occasionally topping up rarely used floor drains, prevent these.

The required seal depth for each fixture is set in the plumbing code; we cite rather than reproduce those figures.

Traps must also be easy to clean, since hair, grease and debris collect at the bend. Access through a cleaning eye or a removable bottle makes routine maintenance simple. In buildings with long periods of disuse, such as schools in vacation or hotels with empty floors, a regular flush keeps seals from drying out.