Adding another sink to dual sink drain

[froddan]

I have a double sink in a bathroom set up as in the picture below.
Another bathroom is behind the visible wall, and a new sink will go up against this wall (so the dual sinks will be back-to-back) with the new single sink.

The drain and vent-pipe is 2" and goes to a stack of 4".
Can I add this new single 1.5" drain to the existing wye and have all 3 sinks go down the same drain?
Would I add the new sink above the wye, or just below?
I believe code is that 3 lavatory sinks are allowed for the 2" drain.

[Basement_Lurker]

If I understand you correctly, you have an existing dual sink setup with a double wye or perhaps a double tee (dwv), and you want to add in a why or tee (dwv) to add in a sink sink on the other side of the wall in a new bathroom.

If you do this, you will make all three sinks a wet vent installation. You say you have a 2" stack to work with, and if your code allows that many fixture units on a 2" stack, then it should be fine to do so. Now if your dual sinks are already plumbed, it would be easier to add in the wye or tee (dwv) underneath the double wye or double tee (dwv) so that you don't end up placing the new p-trap too high.

Another option would be to simply to cut out and reverse on of the 90s on one of the trap arms that you have and turn it around and point it out backwards for the new lav sink, then use the remaining trap arm in the existing bathroom to drain both sinks from a single p-trap....like you would for a kitchen sink.

Make sense?

[hj]

Putting the new drain above the double fixture fitting will normally make it too high for the new sink, so you have to put it beneath it.

[froddan]

Thank you!
Yes, the dual sink is already plumbed and being used, just as in the picture.
So it seems like both of you suggest that I add the new sink from the other bathroom just below the existing fixture arm on the drain.
That was exactly what I was hoping for, should be an easy solution as the common wall is already torn up from completing the plumbing for the dual sink.

If I understand you correctly, you have an existing dual sink setup with a double wye or perhaps a double tee (dwv), and you want to add in a why or tee (dwv) to add in a sink sink on the other side of the wall in a new bathroom.

If you do this, you will make all three sinks a wet vent installation. You say you have a 2" stack to work with, and if your code allows that many fixture units on a 2" stack, then it should be fine to do so. Now if your dual sinks are already plumbed, it would be easier to add in the wye or tee (dwv) underneath the double wye or double tee (dwv) so that you don't end up placing the new p-trap too high.

Another option would be to simply to cut out and reverse on of the 90s on one of the trap arms that you have and turn it around and point it out backwards for the new lav sink, then use the remaining trap arm in the existing bathroom to drain both sinks from a single p-trap....like you would for a kitchen sink.

Make sense?

[hj]

Knowing what to do and actually doing it can be vastly different things if you cannot separate the pipes far enough to insert the tee.