I’m refurbishing my master bathroom, and during demolition I noticed some potentially incorrect plumbing as part of the DWV system. I’ve attached a rough drawing of the DWV layout.

This bathroom was original to the house, and at some point the previous owner finished the attic and added a bathroom directly above.

They tied the new bathroom in to the main DWV pipe used by the master bathroom, meaning waste flows down past the master bathroom. You can see in the diagram that there’s an upwards facing sanitary tee in the vent pipe work for the master fixtures, which of course means waste from upstairs is partially flowing in to those vents, and then back in to the main waste pipe.

Is there any situation in which this is correct/usable? It’s obviously been working fine for many years — the attic was finished ~15 years ago — but I don’t want to tile back over shoddy work. For code purposes, I’m based in Washington state. Tying the master bathroom in to a new roof vent stack would be difficult, so I’d like to avoid that if I can (i.e. utilize accessible AAVs).

The plan for the refurbishment is to keep the fixture count the same (I was hoping to not have to touch the vent piping layout too much), but shift them over (i.e. swap the vanity and toilet, and move them West a bit).

Hopefully the drawing is clear — the black piping is 3” ABS, and the red is 2” and 1 1/2” ABS. Also attached a photo of the DWV system. The thick black line is the building exterior and attic floor/bathroom ceiling.


DWV mechanical layout Photo of the DWV layout

  • I hope you posted that photo upside down... in which case I don't see the problem. Everything seems to be running into tees pointed the right way, and at least some of it may even be sloped correctly. The way you have originally posted the image, with multiple downward branches, it makes no sense to me at all
    – keshlam
    Jan 12, 2023 at 7:34
  • 2
    The photo (experience speaking) is posted right side up. Vents carry air UP, not down,
    – Edwin Buck
    Jan 12, 2023 at 13:06
  • ...ok. I'm not used to seeing multiple vent pipes. I think in my place the vent stack just continues straight up from the main vertical drain pipe and it provides venting for everything that drains into it. One vent stack for the original house, one at the far end which vents for the back bathroom and laundry, no stack at all for the basement slop sink. Of course the newest plumbing in this place is 1960s; code may be different elsewhere/elsewhere. ;
    – keshlam
    Jan 13, 2023 at 1:25

1 Answer 1


The current venting is not correct, and very likely forbidden in your jurisdiction.

Your observation that it has been fine so far reflects the reality of normal use, and most likely your lower bathroom has remained vented. A vertical 3in stack can still vent as long as the discharge is minimal, but one should not rely on it.

Now is your chance to run a proper 1.5in venting pipe to the upstairs.

You can run a new 1.5in parallel to the 3in through the new bathroom ceiling to the attic bathroom, and join it to the attic's 3in before that stack exits through the roof.

The dry vent Tee must be at least 6in above the attic sink's flood level.

There's no need to run a new additional vent through the rough.

  • Thanks for the reply! My house is 3 stories and the roof pitch is such that I'm not...particularly inclined to head up to the roof to install a new vent + flashing if I can avoid it, can I hook a new vent pipe to the existing 3" line as long as it's above the outlet fitting for the attic bathroom? (i.e. run a new 1.5in parallel to the 3in, and join it to the 3in before it exits the roof line). I can hire someone to add a new vent to the roof of course, if that's the only option, but contractors out here in western WA are pricy :-)
    – Adam
    Jan 13, 2023 at 0:05
  • Yes, run parallel and tie vents together, at least 6in above the attic sink's flood level.
    – P2000
    Jan 13, 2023 at 0:41
  • 2
    Thanks – you prompted me to do this properly!
    – Adam
    Jan 14, 2023 at 18:04
  • @Adam great, that's the kind of nudging I also need from time to time! I edited the answer to include the gist of our comments.
    – P2000
    Jan 14, 2023 at 18:31

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