Water Trap

A water trap (P-trap or S-trap) is a curved section of drain pipe that retains a small volume of water, forming a seal that prevents sewer gases from entering living spaces. Every plumbing fixture — sinks, showers, bathtubs, floor drains, and toilets — incorporates a water trap. The trap's water seal is continuously renewed each time water flows through the fixture.

The water trap is a simple but ingenious solution to a fundamental plumbing challenge. Sewer gases contain hydrogen sulfide, methane, and other compounds that are not only foul-smelling but potentially hazardous to health. The thin layer of water held in the trap acts as an effective barrier, allowing wastewater to pass through while blocking gas from traveling back up the pipe.

A water trap can dry out if the fixture is not used for an extended period, such as in a vacation home or a rarely used guest bathroom. When the water evaporates, sewer odors enter the room. The fix is simple: run water through the fixture for 30 seconds to restore the seal. In buildings left unoccupied for long periods, adding a small amount of cooking oil to the drain can slow evaporation.

If sewer odors persist despite the trap being full of water, the issue may be a cracked trap, a blocked vent pipe, or negative pressure in the drainage system causing the water seal to be siphoned out. These problems require a plumber to diagnose and repair.