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I currently have a drain stack that is piped like this: enter image description here

The blue pipe services a sink, and I do believe continues on to become a vent above the sink. The red pipe services a toilet, and the green services a washer and another bathroom group on the 2nd floor.

As you can see, there is quite a bit of maneuvering going on up at the top of this stack to get everything connected, and then routed down to the stack. I would like to reconfigure this drain arrangement to take up less space, to minimize the amount of boxing in I will have to do when framing this out. I'd like to run the red pipe vertically down, to connect with the top of that 45 degree offset, and run the blue pipe around to the top of the wye where the green is connecting. This is crudely illustrated below.

enter image description here

I would use a long sweep 90 to transition the red pipe downways, and a wye to tie it in. I can't find anything in my plumbing codes to prevent my doing this (IRC 2015 and IPC 2015), and the inspector I asked would only say "Yep, that's allowable." Will running the red pipe down in to the middle of that 45 degree section cause me any issues with future performance of these drains?

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    "The blue pipe services a sink, and I do believe continues on to become a vent above the sink." I just started learning plumbing codes recently, so I won't post this is as a definitive answer, but are you sure the toilet (red pipe) isn't using this as a wet vent? If so, you may have venting issues by jutting it down 90 degrees first to connect.
    – statueuphemism
    Mar 22, 2016 at 19:55
  • Sounds like you want more than code compliance, you want performance. I would seize the opportunity to think about going above-and-beyond with cleanout siting. That way you can pull a serious clog down into DIY territory, or at least, a much cheaper plumber job. As for finishing, either do like the Victorians and put a cabinet door there, or a built-in bookshelf with a lift-out backboard.
    – Harper - Reinstate Monica
    Mar 22, 2016 at 20:32

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Depending on the size of the sink drain, (if it is 1 1/4") I think you can run it through the joist. Then the toilet will have a proper vent. enter image description here

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  • A vent for the toiliet...I knew I was missing something here. I believe I'll need to bring it down and run it entirely horizontally to meet the wet vent requirements, but that still gets rid of a ton of bulk in the installation.
    – user11467
    Mar 23, 2016 at 13:29

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