S-trap
An obsolete drain trap configuration that can siphon its own water seal, allowing sewer gas into the building.
What is S-trap?
What is S-trap?
A trap holds a small volume of water that blocks sewer gas. An S-trap — where the outlet turns down and continues vertically — can be siphoned empty by the flow of the drain itself, breaking that seal. The modern replacement is a P-trap with a properly vented arm, so that air enters behind the flow rather than being pulled from the trap. S-traps are common under older sinks and in unpermitted work, sometimes alongside flexible accordion tubing, which is likewise not accepted because its corrugations trap debris. Both are frequent and easily described findings.