It allows sewer gases to escape to the outside through a vent pipe, which extends above the roof. ... Without a vent, an S-trap can allow the water in the fixture to drain so quickly that it often siphons the trap dry and exposes the room to sewer gases.

Are s traps vented?

Venting Issues

You can't vent an S-trap in this way because it drops immediately to the drain. If you run water through an unvented S-trap, the vacuum created in the pipe can pull water out of the trap, rendering it useless.

Does every trap need a vent?

Every fixture is required to have an internal or external trap; double trapping is prohibited by plumbing codes due to its susceptibility to clogging. Every plumbing fixture must also have an attached vent. ... All plumbing waste fixtures use traps to prevent sewer gases from leaking into the house.