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When is a P-Trap not an S-Trap?


caryseidner
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This has bugged me for a while, but I finally was able to snap a couple of good photos that (I think) illustrate my question.

Here is a P-Trap beneath a pedestal sink, or is it an S-Trap? Clearly this pipe moves in an S shape. How long does the horizontal portion, directly downstream of the 'P', have to be to make it a 'P' and not an 'S'. Or is that not the criteria?

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I would consider that an S-trap. The trap arm, the pipe crossing in front of the stop-valve, should be near horizontal. It could be reconfigured using a longer tailpiece (or a tailpiece extension) to achieve that.

"How long does the horizontal portion, directly downstream of the 'P', have to be to make it a 'P' and not an 'S'. Or is that not the criteria?"

If you are talking about the short section before the 45 bend, it doesn't matter in this case. The whole trap arm (including all bends) has to be near horizontal until it reaches the vented drain in the wall.

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Originally posted by kurt

I'm not sure what I'd call that, but I don't think it's an S trap.

An S trap is where the drain extends vertically down. This is someones bizarre idea of an offset with an incorrect pitch, not an S trap.

Kurt, I agree it's not a classic S-trap, but anything that slopes down that much is not vented correctly to prevent siphonage (albeit, it seems unlikey that full siphonage could actually occur). I guess I should have used "effectively acting as a S-trap"?

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Count me as one who doesn't consider that an S trap. Normally, I believe that the highest likelihood for a problem for that design would be due to the fact that there is too much slope to the "horizontal" section -- leading to the possibility that the more solid components of the waste stream (soap, hair, etc. ) could separate out because the water will exit more rapidly than the more dense materials will. If that happens, you can get a build up of gunk in the trap.

However, that is a pedestal sink. I assume it is in a seldom used half bath. If so, the real likelihood of a problem in anything close to a reasonable time span is pretty small.

But it's still wrong.

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In addition to everything already said, the section of pipe that is passing through the drywall would need to be one size larger, and then within five feet from the trap, attach to a verticle vent two sizes larger than the pipe in the photo. IRC Section P3105 Fixture Vents. It would still be ugly.

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This is a great question and it is not spelled out in the building codes at all.

I was, however able to dig something up from New Jersey some time back (I don't know if they still have this language) but essentially they said that the outlet from the trap weir had to be at least 3" in length to be condidered a P trap.

I have been using 3" as a decision maker ever since then but it is not in the IRC or IPC.

Maybe Darren can dig this back up since he is a Jersey boy

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  • 1 year later...

Hello - I was having my vanities redone and the plumber did something that looked odd. How do I post a photo here? It's an s-shape but he said wasn't an s-trap. Basically he couldn't raise the main pipe to get to where it as needed for the vanity, so he added a small curve (kinda s-shaped) behind the drywall before the straight pipe coming out to the p-trap.

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Hello - I was having my vanities redone and the plumber did something that looked odd. How do I post a photo here? It's an s-shape but he said wasn't an s-trap. Basically he couldn't raise the main pipe to get to where it as needed for the vanity, so he added a small curve (kinda s-shaped) behind the drywall before the straight pipe coming out to the p-trap.

Does the pipe exit through the wall or floor? Like others have said above, typically a "S" trap will exit through the floor, with no vent to allow fresh air in the drain. You can add pictures by clicking the blue "upload" links when you post a reply. Pics always help, but make sure they are not too close.

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The guideline I use is that the trap arm going from the trap to the drain, needs to allow air back to the trap. If not, air will be sucked down the sink drain, wrong. A nearly horizontal trap arm lets air flow back. The arm enters the wall where it hits a Tee with air coming down from above to displace the waste flowing down. That's the basic setup.

It sounds like the plumber is trying to get the correct configuration under the sink. So you probably have a good P-trap there.

If there is an elbow pointing down on the end of the trap arm where it enters the wall, there will not be good venting. It should be a Tee, with a pipe going up (a vent) as well as a drain going down.

Make sure air can freely come down from the vent through the roof to your sink without going through another trap or restriction in the wall.

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