Everything I see mentions P-traps. I seem to have instead an S-trap. What are the downsides of this, and is it something that should be fixed?


  • 3
    I have an S-trap under my kitchen sink and really don't experience problems from siphoning because of it. I generally wouldn't bother with it unless you are having immediate plumbing problems or you are remodeling. Nov 13, 2012 at 11:58
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    Unrelated, you might want to pin that dishwasher hose up as high as you can within the cabinet space, trim any excess of the hose so that there are no dips or loops. This is recommend by almost all dishwasher manufactures.
    – pdd
    Nov 16, 2012 at 18:00

2 Answers 2


You have an "S" trap because the drain exits through the floor and is probably unvented.

A "P" trap is for a drain in the wall that is teed for a vent stack.

The major disadvantage comes because "S" traps are usually used in locations where they are either not vented or poorly vented on the outflow side and so can siphon off the water in the trap, leading to sewer gas backing up into the building.

For a short term fix, after using any quantity of water, pulling the plug in the sink will empty it vigorously and pull enough water out of the trap to break the water seal, run a pint out of the tap afterwards to make sure there's a water seal.

For long term repair, you would have to install a tee on the drain, run the output from the P trap into one leg and a vent to who knows where from the other side. Plumber consultation recommended before you end up with one of those plumber's nightmare constructions.

It is illegal in most localities to have S traps in new construction because of the siphoning.

  • Great answer... but judging from the signs of water damage from a long time ago, and the drain pipe connector I would say that this building is far from new construction and perfectly acceptable for the time period. Nov 13, 2012 at 11:56
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    Hence the "new "construction" sentence at the end. I'll bold it. For established construction that didn't come under code violation at the time of construction, it's a case of fix it if you easily can or be aware and make sure you fill the trap after emptying large quantities of water. Nov 13, 2012 at 16:58
  • So bottom line, if it's a problem - and it can be a problem - you can workaround it by running some slow water to ensure the trap is filled, and you can fix it by replacing it with a properly vented P-trap or a mechanical vent. But if it's not a problem, don't worry about it. Nov 14, 2012 at 7:14
  • Yep, pretty much. Nov 14, 2012 at 7:53
  • I've sometimes seen piping arrangements below sinks that look like they're designed to be siphon-resistant, but I don't recall seeing such things for sale even for purposes of retrofitting existing S-traps. Is there some problem with such things?
    – supercat
    Oct 17, 2014 at 2:01

This is incorrect. S-traps are now against plumbing code. A P-trap can be used whether the drain line comes from the floor or the wall. The issue with S-Traps is on rare occasions, when a bunch of water goes down the drain, it can pull all the water down the drain emptying the trap which would allow sewer gases (which are toxic and flammable) to come out from the pipe. Simply use a tee wye with a automatic air vent and a P-trap. Done deal.

  • Are you saying that they're a "must replace" item when found in existing construction, then? Because that's not the norm when Codes are changed...also, this answer would be much stronger if you could point to a Code citation (IRC, most likely) regarding this... Aug 31, 2019 at 4:34
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    Vinny did not state that it must be replaced, Only that it is in the Uniform Plumbing Code (2018) as prohibited. *Chapter 10,1004.0 Traps ..."S" traps, bell traps, and crown-vented traps shall be prohibited. Apr 30, 2020 at 15:45

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