I roughed in our basement for a full bath and the inspector is saying that I cannot tie in the shower/lavatory 2 inch line directly into the 3 inch under ground main drain line. He is saying that I must first branch off of the 3 inch line with a 3x3x3 wye for the toilet, then branch off of that line to the shower and lavatory. I don't follow the reasoning. I have a horizontal T that I know I need to change out, but the rest is stumping me a little. I was hoping someone could help. Here is a picture of what I have done.

[drive.google.com]

Any help would be much appreciated.

The way you have it, the toilet is not vented, and I would not approve a Studor vent in a basement.

The inspector is correct, you need to use the lavatory as a wet vent for the toilet. I'm surprised he didn't say anything about that sanitary tee on it's back downstream of the tub trap.



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If you have the depth, you can roll the wye up so that you can cross over the existing 3" run with the 2" piping. The drop for the tub spout needs to be out of copper. With CPVC you're likely to get a little water out of the shower head as the tub filler is being used.



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replace the 3 inch "Y" with a new one. cut the existing drain back under the wall for the tub. offset the upstairs drain towards the yellow drill. send it straight across and into the side of the new "Y".
now, come out of the back of the new "Y" with your 3 x 3 x 2 "Y" with the toilet draining into the end of it.
the 2 inch san tee should be a "Y" but if the inspector lets it go leave it..
i just noticed that the drain for the lav and tub is 1 1/2". 2 inch is the minimum size allowed underground.
you better make a sketch as to where the pipe will be 2 inch and where you will reduce to 1 1/2 inch. show it to the inspector so you don't have to rip it out again..
lastly, i would make the tub trap 2 inch and put a bushing in the inlet to make it 1 1/2 inch



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I might be wrong but I think the underground piping is 2", it appears he has used a 2" X 1 1/2" X 1 1/2" tee for the lavatory trap arm.

you are correct.