Short definition
Self-siphonage is the trap-seal-loss mechanism in which a fixture’s own discharge creates a full-flow water column in its drain that pulls the trap seal out as the water leaves. The classic configuration that guarantees it is the banned S-trap; even a properly-shaped P-trap can self-siphon if the trap arm is too long, sloped wrong, or unvented within the prescribed distance.
What it is
When a fixture drains, water moving through the trap and into the trap arm fills the pipe momentarily. If air can’t enter from a vent within range, the moving column siphons everything behind it — including the standing water that forms the trap seal. The fixture finishes draining empty.
Self-siphonage is the same physics as siphoning a fish tank: a continuous water column from a higher elevation to a lower one will pull water through any open inlet. Vents prevent it by introducing air at the right point in the pipe so the column can’t stay continuous.
The two design rules that prevent self-siphonage in P-traps are the trap arm length limit (UPC Table 1002.2) and the trap arm slope limit (the trap arm can’t drop more than one pipe diameter total, or the vent connection ends up below the trap weir). Violating either rule produces self-siphonage even with a code-compliant P-trap.
Why it matters to a homeowner
You’ll usually hear this term from a plumber explaining why a fixture keeps smelling like sewer despite a fresh P-trap. The fix is almost never the trap itself — it’s the venting or the trap arm geometry feeding into it. A new P-trap that self-siphons after a remodel is a common sign that the trap arm is too long or sloped wrong.
Common scenarios that cause self-siphonage
- S-trap installation — vertical drop exit. Virtually guaranteed to self-siphon every flow event.
- P-trap with trap arm sloped too steeply — the pipe drops more than its diameter total; the vent connection ends up below the trap weir.
- P-trap with trap arm exceeding UPC max length.
- Disposer flush followed by a full sink drain — the high-flow pulse can self-siphon a borderline-compliant P-trap.
Common variants / not the same as
- Self-siphonage vs. induced (aspiration) siphonage. Self = same fixture’s flow. Induced = a neighbor’s flow.
- Self-siphonage vs. compression / back-pressure. Self pulls. Compression pushes water up through the trap.
- Self-siphonage vs. evaporation. Self is dynamic during use. Evaporation is static during disuse.
- Self-siphonage vs. capillary action. Capillary is wicking through hair or string. Self is hydraulic.