Short definition
An S-trap is a banned trap geometry that exits vertically downward instead of horizontally. The vertical drop creates a continuous water column during full-flow drainage that self-siphons the trap dry every time the fixture is used. Modern code (UPC 1004.1, IPC 1002.3) prohibits S-traps in all new work, but they’re common under sinks in pre-1970 Washington homes.
What it is
Compared side by side with a P-trap, an S-trap looks like the same U-bend but tilted: instead of the outlet running horizontally to a vented branch, the outlet drops straight down through the floor. That vertical discharge is the problem. As water leaves the trap and falls, it creates a continuous column with the standing trap water. Surface tension, plus the falling water’s pull, draws the trap seal down with the discharge — and by the time the fixture finishes draining, the trap is empty.
The result is a trap that holds water between uses but loses the seal during every use. Sewer gas then enters the home until the next fixture run refills the trap.
Why it matters to a homeowner
If you have a faint sewer smell from a single sink and the trap looks like an “S” laid on its back rather than a “P,” you’ve found the cause. S-traps are still functional plumbing in the sense that water drains, but they don’t do their main job — blocking sewer gas — most of the time.
Existing legal-non-conforming S-traps are typically grandfathered in until renovation work cuts into the line, at which point UPC requires replacement with a properly-vented P-trap. A pre-purchase home inspection that flags “S-trap under kitchen sink” is signaling future remodel cost, not an immediate hazard, but the sewer-gas exposure is real.
When you’ll encounter this term
- A pre-1970 Seattle, Tacoma, or Spokane home with a faint sewer smell coming from one sink.
- Pre-purchase inspection report flags “S-trap under kitchen sink.”
- Renovation opens a kitchen wall — UPC requires replacement.
- DIY DWV review identifies a trap that drops straight through the floor instead of into the wall.
Common variants / not the same as
- S-trap vs. P-trap. Vertical exit (banned) vs. horizontal exit (current standard).
- S-trap vs. drum trap. Both are pre-1950 banned-or-restricted styles. S is a U-then-U bend. Drum is a canister.
- S-trap vs. Q-trap. Q-trap exits at 45° (between P and S geometry). Also restricted.
Common failure modes
- Self-siphons every drain event — trap empty when next user comes; sewer gas migrates back.
- Often paired with no vent at all in pre-1970 installs — fundamental DWV non-compliance.
- Chronic faint sewer smell at the fixture, especially first thing in the morning.
Washington note
S-traps are ubiquitous in pre-1970 Washington single-family stock. Most that are still functioning have either had a vent retrofit added — turning the S into a vented P — or have lived with chronic minor sewer-gas presence for decades. Replacement during any kitchen or bath remodel is standard practice in Seattle, Tacoma, Bellevue, Olympia, and Spokane jurisdictions. If a remodel quote includes “replace S-trap with P-trap and add vent,” that’s the right answer at code-compliant cost.