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This is in our bathroom vanity, sitting behind a U shaped drawer that the P-trap tucks into. Kind of a loopy routing that looked almost like an S trap when I looked at it, but now I'm wondering if, with the long (about 7 inch) horizontal run after the p trap and before it turns down and makes the 180° turn, it's okay and just looks funny? The long lower section the enters the wall is about exactly 1/4" drop per foot, the whole thing looks more angled than it is in the photo because of the camera angle.

  • 1
    I'm doubting my earlier assertion that this is an S-trap. The horizontal run may mean that it's not. I'll leave it to one of our plumbers to answer.
    – isherwood
    Oct 7 '19 at 20:45
  • 2
    Just making the tail piece longer in order to make the P trap lower would have saved a lot of trouble (and a lot of fittings). I'm not sure if it qualifies as an S trap, but it's certainly not an optimal configuration.
    – JPhi1618
    Oct 7 '19 at 20:51
  • I agree with jphil1618, I would extend the tail from the sink and eliminate 3 ea. 90’s that plumbing mess is a clog waiting to happen, is it a slow draining sink? Getting rid of the mess may speed it up.
    – Ed Beal
    Oct 7 '19 at 21:08
  • Thanks for all the comments! Hard to tell in the photo, but the line from the wall has an angled fitting, I think a 60°, so they built the loop to get the drain where it needed to be without hitting the U-shaped drawer, as a straight shot at that angle would have done. There was also no way to just turn 90 off the wall and come up the middle as the drawer was designed, again because of the 60° angle, if that makes sense.. The alternative was to glue the face of the drawer in place as a false front and lose the drawer, I was just worried that what they came up with would be considered an S?
    – Eric
    Oct 7 '19 at 21:34

That trap is prone to siphoning. Against code.

The critical trap arm length prevents the air path from the vent to the trap from being blocked. If the air path is blocked then the trap can siphon. The air path from the stack to your trap can be blocked given the weird 90s you have in that configuration.

See image B:


  • Suspected that was the case, but I wasn't sure why. Thanks for the great explanation 👍
    – Eric
    Oct 7 '19 at 22:03

I believe the first answer does answer your question. However to modify the situation so that it meets code, you could turn the feed off the sink towards the back lengthen it to get within 4-6" of the wall. Install a 90 down to the input of a P running towards the drain where the 60 is at the same level or slightly higher for the output of the P to angle out from the wall into a 90 that is inline with the 60. Cut the lengths into the 90 so that the 60 is aligned and the output from the P makes up the odd angle. This should all be behind the u-drawer but away from the rear wall by about 4" while allowing the P trap to be positioned so that it is within slopes above the drain sani-T and only questionable thing is the final 90 into the 60. BTW you could use a 45 from the sink toward the back wall if it doesn't lower the pipe below the T's level I guess this amounts to putting the P trap in the lower pipe of your picture with the joint where the halves come together just out from the rear wall. Then go up as far as you can on the input side to make the connection to the sink.

If you can make it straight back, you could use a 45 to angle down to a 90 into the P trap's input. The front half of the P is parallel to the rear wall and the exit half of the P goes on the angle needed to reach the 90 aligned with pipe at 60 degrees from the T. The 90 is flat, parallel to the floor.

I think it would work.

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