The S-Trap

The purpose of a plumbing trap under a sink is to prevent odorous sewer gases from entering the house back through the sink and tub drains.

There are two basic types of traps: P and S configurations – both have a trap section, which is a U-shape pipe where water sits in the U and creates a water seal, so that sewer gases at the street sewer system can’t move past the water.


The modern P-trap discharges water through a horizontal pipe through a wall… Whereas, the S-trap shown in the photo discharges water vertically down towards the floor.

The problem with “S” traps is that fast draining water from a sink or bathtub may siphon out the water seal and allow sewer gas to enter the house.

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