S-Trap
An older S-shaped trap configuration, now banned by code, that can siphon its own water seal dry and let sewer gas into the room.
An S-trap looks like a P-trap with an extra downward bend, forming an S shape, and it drops straight down into the floor instead of turning into a vented wall connection. It was standard in older homes but is prohibited by modern plumbing code for a specific reason: an S-trap can siphon itself dry. As a full fixture drains, the rushing water acts like a siphon and pulls the trap seal down the drain with it, leaving the bend empty and the sewer-gas barrier gone.
The result is a fixture that intermittently smells of sewage, often worst right after the sink or tub drains, then improves once enough water refills the trap. Because the smell comes and goes, homeowners chase it for a long time before realizing the trap design itself is the cause. A gurgle as the fixture finishes draining is a classic tell that the trap is siphoning.
The proper fix is to reconfigure the drain into a P-trap that ties into a vent, so air can break the siphon and the seal stays put. Sometimes that is a straightforward swap, and sometimes it means opening a wall to add proper venting, which is why an inherited S-trap can turn a quick fix into a small project. Any time the drain is being reworked, code requires the S-trap to go.
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- Cleanout : A capped access point on a drain or sewer line that lets a plumber insert a snake or camera directly into the pipe to clear clogs or inspect it.
- Branch Drain : A horizontal drain line that carries waste from one or more fixtures over to the main vertical stack, sitting between the fixture trap and the soil stack.
- Soil Stack : The main vertical drain pipe that collects waste from every fixture branch and carries it down to the sewer, extending up through the roof as the vent.
- Drum Trap : An older cylindrical trap, common under bathtubs in homes built before the 1970s, now outdated because it clogs easily and is hard to clean.