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.