S-Trap Under a Sink: What It Means on Your Home Inspection Report

Under every sink, the drain pipe dips into a curved bend called a trap. The bend holds a little water, and that plug of water is what keeps sewer gas from coming up the drain into the room. Public Health – Seattle & King County puts it simply: each fixture must be trapped to keep sewer gas from coming up into your house.

The usual trap is a P-trap: after the bend, the pipe runs sideways into the wall, so the whole thing looks like a P. Your inspector found an S-trap instead, where the pipe leaving the bend turns back down, usually into the floor, and the assembly looks like a sideways S. It drains, and at rest it holds water like any trap. The problem is what happens while the sink empties. (Our glossary explains P-traps and vent stacks.)

Why It Matters

A trap only works if its water stays put, and the codes we read say that takes a vent. The 2016 California Plumbing Code, for example, requires every fixture trap to be protected against siphonage by a vent pipe, and King County describes a trap as a liquid seal that works "when properly vented." The University of Houston's Engines of Our Ingenuity explains why: water rushing down the drain can drop the pressure on the far side of the trap below the pressure on the sink side, and that can siphon the trap's water down the pipe, "leaving it empty." A vent lets air reach both sides, so the siphon can't form.

All three codes we read single out the S shape: California's 2016 plumbing code, Illinois's plumbing code, and HUD's construction standard for manufactured homes all list S-traps as prohibited. Your local code may differ. Illinois makes one exception, for toilets and similar fixtures that "depend on self-siphonage for their proper operation": a toilet is built to siphon. Under a sink, the same siphon pulls out the water that's supposed to stay behind.

The City of Kerrville, Texas, describes the result: without proper venting, low pressure in the drains can siphon the traps whenever you drain a fixture, and when a trap is dry, sewer gas comes straight into the house. It lists gurgling and a rotten-egg smell among the signs of venting trouble.

We rate this monitor: it isn't a fire, shock, or carbon monoxide hazard, it doesn't damage the house, and between siphons it holds its seal like any trap. But a siphoned trap lets sewer gas straight into the house, which is why the codes we read prohibit it. Move it up to repair soon if you smell sewer gas at that fixture or hear gurgling when it drains; Kerrville says that when traps or vents are missing, you may need a plumber to install them immediately.

What to Do Before Closing

• Ask the inspector for specifics: which fixtures have S-traps, and whether they noticed an odor or gurgling. Under the InterNACHI standards of practice, inspectors check sinks for functional drainage and inspect the drain, waste, and vent system.
• Have a licensed plumber look at it. The plumber can tell you whether there's a vented drain line nearby to connect a P-trap to, or whether the fix needs new venting, and what your local code requires.
• Talk to your real estate agent about asking the seller to correct it or offer a credit within your inspection contingency.

What to Do After You Move In

The fix belongs to a licensed plumber: replace the S-trap with a P-trap that connects to a vented drain. HUD's standard calls for P-traps that are "effectively vented," and California's 2016 code says no trap shall be installed without a vent unless the code provides otherwise. HUD's standard for manufactured homes also defines a mechanical trap vent device, which opens to let air in and closes to keep sewer gas out, and allows it only as a secondary vent, with the drain system still vented outdoors. Whether your local code accepts one under a sink is a question for your plumber.

Until it's fixed, if you notice a sewer smell at that fixture, run water into the drain to refill the trap. Kerrville suggests the same for any dry trap. If the smell keeps coming back, call the plumber sooner. Kerrville notes a plumber can smoke-test the drain system to find where gas is getting in.

Can You Fix It Yourself?

Not this one. Our DIY vs. hire-a-pro guide puts clearing a P-trap in the DIY column, but replacing an S-trap is a different job. An S-trap's drain usually drops into the floor, and a P-trap bolted onto a drain with no vent hasn't fixed anything: the codes we read require the trap to be vented. Vent work runs inside the walls and may need a permit, which makes it a plumber's job. What you can do is refill a trap that smells and keep an eye out for gurgling.

Frequently Asked Questions

Are S-traps illegal?

All three codes we read prohibit them: California's, Illinois's, and HUD's standard for manufactured homes. Whether an existing S-trap must be replaced is up to your local code official; a licensed plumber can tell you what your jurisdiction requires.

Is an S-trap a deal breaker?

It depends on how many fixtures have one and whether a vent is close by, because that decides how big the repair is. A plumber's look before your contingency ends answers both. If the seller doesn't fix it, put it in your maintenance budget.

Can an S-trap cause a sewer smell?

Yes. When it siphons, the water seal is gone and sewer gas can come up through the drain. A smell that shows up after the sink drains, or gurgling, can point to venting trouble, which Kerrville says can siphon traps.

The Bottom Line

An S-trap holds water until the sink drains hard enough to siphon it out, and then nothing blocks sewer gas. It doesn't damage the house, so it's a monitor item, but it's worth fixing: a licensed plumber replaces it with a P-trap tied into a vent. Treat a sewer smell or gurgling as the signal to do it now. See more findings in our inspection report decoder.

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