I have been going through new houses under construction in my area to learn about Drain, Waste, and Venting (DWV). I found this configuration (reference image below) and it does not look to be correct. A short section looks like a wet vent for the Clothes Washer.

  1. Is this permissible in the Plumbing Code?

  2. Will there be problems with this type of configuration?

  3. Would the water in the Clothes Washer Trap be syphoned out when the Utility Sink is being used?

  4. Should a Sanitary Tee, like this one, be used instead (reference image below)?

  5. What other configuration would have been acceptable?

Drain, Waste, Venting - Clothes Washer and Utility Sink

2-in x 2-in x 2-in x 2-in dia Schedule 40 Tee

Revised Feb 2, 2021 @ 3:05 PM: Found this information at bfdrona.com:

Install The Sink Drain and Vent lines

DWV Sink Drain and Vent Lines Installation

1 Answer 1


I would suggest spending some time reading the code applicable to your area - for me, that's the IPC (with a few local changes) and it's easily available on-line.

Pretty much the entire point of vents (that pipe in the middle that goes up) is to prevent siphoning of traps. So, no, that should not be subject to siphoning the trap when using the other fixture, since there appears to be an adequate vent. That's a perfectly acceptable arrangement - well, it might be missing a required clean-out, at least under UPC, as I understand UPC (but I don't spend a lot of effort on it since it does not apply to me.) What code section makes you think it's not?

That is a "Double-Tee" or "Cross" - it's not particularly superior to using a pair of Tees. At toilet scale it can even be a problem (and you'll find that in the codes if you read them.)

  • I have read the 2018 North Carolina State Building Code: Plumbing Code: codes.iccsafe.org/content/NCPC2018/chapter-7-sanitary-drainage (Free ICCSafe.org registration required) Maybe someone could assist me with determining where in the code this installation is compliant. Feb 2, 2021 at 20:24
  • ICC 911.3 and and table 709.1 - the reason that your bfdrona picture needs the additional dry vent is how far the washer is from the drain pipe. There's nothing that says you can't dry vent the heck out of everything (you can always exceed code, so long as you don't violate it) but it's not needed in your original picture.
    – Ecnerwal
    Feb 2, 2021 at 20:34
  • Okay, I was looking at Section 911 and Table 911.3 Common Vent Sizes. Feb 2, 2021 at 20:38
  • "911.3: The upper fixture shall not be water closet or clothes washer." So since the Clothes washer is the lower fixture and appears to be 2", all is good. I missed that. Nice. Feb 2, 2021 at 20:40

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