Adding Bathroom in Attic-Wet Vent?

I am adding a bathroom in my attic. Currently no plumbing. I have consulted with licensed plumber who suggested I drop a 3 in. line down the wall over an existing downstairs bathroom. Currently there exists a 2 in. vent for downstairs toilet. He suggested replacing 2 in. vent with the 3 in. line. It would then be used as a wet vent (drain for upstairs, vent of downstairs toilet). I have cut in to the wall behind the downstairs toilet and it appears the 2 in. vent goes below the concrete slab foundation where it ties into the main drain. My question is how deep will I have to jackhammer the foundation to expose pipes enough to tie in the 3 in. wet vent, and how will I tie it in? Can plumbers cut out a 2 in. vent, then enlarge the opening to fit a 3 in. pipe? Or, will the entire intersection of piping have to be cut out and replaced (meaning a considerable more material removal and more work).

Thanks.

I don't know where your plumber works, but that would never be allowed where they use the UPC plumbing code.


Here they would leave the 2" vent in place, and bring a 3" down and tie that in downstream of any vents for the downstairs plumbing.

What type of fittings will be needed?

Ok. If I excavate and expose the pipe junction from toilet to vent to main drain, then look toward tying the upstairs 3 in. line to this junction, what type of fitting or fittings shoule I use?

I have enough room to bring the 3 in. line down the wall next to the 2 in. vent. Do I use a sweep 90 to go from verticle to horizontal before tying into the junction? The junction will have to have 3 in. to toilet, or front, 3 in. to upstairs drain or left side, 3 in. to 2 in. vent on top, and 3 in. on bottom for main house drain. If there is such a fitting what is it called.

It also sounds like major excavation to be able to remove existing fitting. Sound right? How far down do you think I will have to go?

Thanks.

Dave