I have a couple plumbing questions.

I’m adding in a washing machine and sink. My space is tight and in the right bay where the Sink Drain is above is an electric panel. The part out of the wall I was going to leave out of the wall. As it will be under the free standing sink. The exit drain line will be enclosed in a chase. Of the the vertical pipes Left is the washer (2” Pipe, 6” of FF and 18”-30” in length). The right is the vent to roof.

I understand about the P-Trap Arm length and the distance to vent needs to be twice the size of the pipe. But that’s typically for the Vent on the other side of the drain line. The way this is plumbed (mocked up) fits nicely in the wall, but I didn’t know if I needed the 4” separation as the vent is on the other side of the ‘S’ trap.

I wanted to be sure that It would not cause a siphon.


mockup of plumbing

EDIT: This is an as-built photo of what I went with. Though I like pdd's solution better

How I ended Plumbing

  • IPC 909.3 "Crown Vent" appears to prohibit this, no matter which direction you wave your hands, as I read it. I'll let someone else manage a detailed answer with the whys...
    – Ecnerwal
    May 17, 2021 at 12:54
  • Yes that is pretty clear, though references to images I see about it all have the Vent on the other side of the Drain thus needing to have the 2x pipe diameter distance to get to the Air. Looks like I need to get even more creative? Thank you.. )-:
    – scooter133
    May 17, 2021 at 14:50

2 Answers 2


Regardless of the S-trap that this arrangement makes, I don't see how the sink is being vented as the vertical potion to the 90º would exceed the allowable fall on the fixture drain.

I would eliminate the S-trap and resolve the venting by using stacked tees. The lower being for the washing machine and the upper being for the sink. Then offset the vent back into the wall. See below sketch.

enter image description here

Note that you and also arrange this so that you don't have to drill the studs.


As nobody else has come to the plate yet: reasons crown vents have been banned (for a long time) include (I'll community wiki, feel free to add, or write a whole new answer):

  • Easily plugged/blocked. You get a clog, you plunge it, the mass of hairy gook is sent directly from the trap into the vent line, the vent is now plugged. Your proposed setup is perfect for this problem.
  • High flow into the trap (draining a full sink, say) can cause the vent opening to be blocked by water, encouraging siphoning. With the vent further down the pipe, the water level in the pipe falls, leaving airspace at the top for the vent. Can also lead to gradual clogging if fats floating on the water congeal in the vent.
  • Several sources claim faster evaporation, but I don't know that I buy that argument.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.