Need advice with weird vent layout

[Andypetrina]

I am in need of advice on how to go about changing the lavatory drain in my upstairs guest bathroom.

Previously the old lav drained though the vanity (which limits its functionality) through the back of the shower and into the wet vent for the shower. I'm trying to have the lav drain into the back wall, the problem is there is a joist running parallel a few inches in front of this wall and there is a a dry vent running parallel through the joist bay which limits where and how I can move the lav drain to connect. This horizontal vent ties into the waste stack and runs also vents the main floor powder room on the opposite side of the house.

The easiest solution would be to connect the lav and have it empty into the horizonal vent, but I'm not sure if this is safe or advisable.

 

[wwhitney]

The dry vents from the story below need to stay dry, you can't turn them into wet vents. And right now the lav drain is wet venting the tub; if you divert the lav drain, your tub would end up with no vent.

The simplest change you could make would be to turn the san-tee that the lav trap arm drain into currently by 45 degrees, and then use a 45 elbow so the trap arm is tight to wall behind the lav. That would minimize the intrusion of the trap arm on the vanity drawers and storage. Or you could stick the trap arm in the wall behind by drilling some studs--the trap arm would still interfere with the back left corner of the vanity. If that wall between the tub and lav is a 2x6 wall, you could even have the lav trap arm pass across the vent stack to get most or all of the trap arm in the wall framing (turning the corner would be tricky so it still might stick out).

If you put the lav's san-tee in the wall behind the vanity, then you'll still need to run a dry vent from there up and over through the wall framing to the existing vent stack, so you'll face the same difficulties. Plus then you'll also have to figure out how to run the drain down through that joist and over/under that power room dry vent, so it can still connect up with the tub drain to wet vent the tub.

Cheers, Wayne