Total novice here. The sink in our bathroom often has a bad smell coming out of it.

We've done bleach as well as baking soda & vinegar, but it seems to keep coming back.

I noticed that the sink doesn't drain straight down. Could this P trap configuration be the problem, or part of it? Based on browsing here it seems like the offset before the vertical drop might be a little long.

picture of plumbing under bathroom sink

  • I assume those stubs have knockouts?
    – Huesmann
    Oct 20, 2023 at 12:28

1 Answer 1


The fact that it's an S-trap with no vent is far more likely to be the root problem than the horizontal offset (which is certainly odd in itself) is.

Assuming you're not up for running a proper vent, replace the black elbow with a sanitary Tee, and run pipe as far as possible upwards from the top of the Tee to a threaded adapter for an AAV (air admittance valve, mechanical vent, "Studor" a brand name of a common AAV) - those need to be accessible for replacement when needed, which is one reason that they thread in. But the higher you can locate them, the longer they'll last, generally.

  • Thanks for your answer! Would you suggest I swap out that top black corner for a piece with a vent?
    – orchest
    Oct 20, 2023 at 0:44
  • I was already editing when you commented.
    – Ecnerwal
    Oct 20, 2023 at 0:47
  • I really appreciate it! Is the reason this could cause a smell because without a vent the S trap will siphon itself dry?
    – orchest
    Oct 20, 2023 at 0:50
  • That is indeed the usual reason for them to stink. And why S-traps are forbidden by code in most civilized locations, (there's a tricky variant where the riser pipe is larger that won't siphon, but yours does not look to be built that way, and it's only allowed by code in some locations. If replacing the whole pipe with 2" would be easier you could try it, but it has to be 2" from there until it connects to a larger than 2" pipe if you do that. And the venting working that way depends on length limits as well)
    – Ecnerwal
    Oct 20, 2023 at 0:55
  • 2
    this is the ikea sink trap design and allows drawers to work as if there was no trap. the pipe diameter is a little smaller but hair clogs are pretty easy to resolve without disassembly. the two horiz pieces will actually pull apart and are sealed with an o-ring. Oct 20, 2023 at 5:21

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