I am adding a half bath on the 2nd story. The toilet cannot be vented at the back wall of the toilet due to the roof configuration. Can I put in a wye fitting at the 3 inch waste stack just below the floor joist and run my 2 inch vent about 4 feet horizontally in the joist bay where I can then go vertically and tie into a new 2 inch dry vent. The vent would follow the new wall to the right.

Hi Joe.

I'm wondering if you can WET VENT the toilet using the drain/vent pipe for the lavatory sink...

Here, you run the 2" vent off the 3" horizontal drain pipe for the toilet but you run that 2" vent over to the sink, run that above the floor and pick up the sink waste and then run the 2" pipe up and through the roof or into attic and attach to a 2" vent or larger up there...

Of course, you would need to check with your local plumbing inspector and see if wet venting is allowed in your area.

A drawing or more information would really help us to advise best! For example, does the waste stack continue up as a vent stack and go through the roof? How is the sink getting piped in relation to all this? Are you adding onto an existing vent stack from the bathroom below or running a new waste stack from below the first floor? Etc...

Let me know what you can...

MARK