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The core difference: where the trap exits
Both traps do the same essential thing: a U-shaped bend holds a small plug of water, the trap seal, that seals the pipe and stops sewer gas from rising into the room. The difference is what happens after the bend. A P-trap curves down and then turns horizontal, exiting sideways into a drain that is vented, so it looks like a P lying on its side. An S-trap curves down, back up, and then straight down again into the floor, so it looks like an S.
That downward exit on the S-trap is the whole problem. When water drains, gravity pulling a full column straight down past the trap creates suction, and that suction pulls the water right out of the trap behind it. A P-trap exiting sideways into a vented line does not develop that pull, so the seal stays put. Same job, two geometries, and only one of them holds water reliably.
Why S-traps siphon dry and are banned
Self-siphoning is the failure mode that got S-traps banned. As a sink full of water rushes down and out the bottom of an S-trap, the moving column acts like a siphon and sucks the trap empty in its wake. The trap reseals with the last trickle of water, but if the fixture drains hard and is not refilled, the seal can be left low or broken. A broken seal is an open pipe straight to the sewer, and sewer gas, which carries methane and hydrogen sulfide, flows into the living space.
That is a health and safety issue, not a nuisance, which is why the S-trap is prohibited in modern plumbing code across the U.S. and many other countries. If you smell sewer gas at a sink that has an S-trap, the dry trap is a prime suspect, and the same symptom shows up in our guide to tracing a sewer smell in the house. A P-trap with proper venting simply does not fail this way.
How a P-trap stays sealed: the role of the vent
A P-trap holds its seal because of venting. When water drains, air enters the system through the vent behind the moving water, relieving the suction that would otherwise pull the trap dry. With that air supply, the trap fills, drains and reseals every time without being siphoned. The vent is what makes the P-trap reliable, which is why the two are designed as a pair.
Where running a full vent to the roof is impractical, code often allows an air admittance valve (AAV), a one-way valve mounted near the fixture that opens to admit air when the drain pulls a vacuum and snaps shut otherwise. An AAV gives a P-trap the air it needs without a long vent run. Either way, the principle is the same: the plumbing vent does its job of feeding air in so the trap never siphons dry.
Converting an S-trap to compliant P-trap
Fixing an S-trap means giving it a vent and a sideways exit. The clean version reworks the drain so the trap arm runs horizontal into a vented wall drain, turning the S geometry into a proper P-trap. Where the drain comes up through the floor and there is no wall stack to tie into, the common solution is to install a P-trap with an air admittance valve under the sink, which provides the venting locally and stops the siphoning.
This is a modest job for a plumber and a common one during a sink or vanity replacement, since it brings an old fixture up to code at the same time. Replacing or reworking the trap assembly is inexpensive on its own; understanding what a P-trap replacement runs helps you judge a quote, and it is often bundled with the larger fixture work. The result is a trap that holds its seal every drain cycle and keeps sewer gas where it belongs.
- ·Rework the drain to exit sideways into a vented line, or
- ·Install a P-trap with an air admittance valve under the fixture
- ·Often done alongside a sink or vanity replacement
- ·Brings an old, non-compliant trap up to current code
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