I plan on adding another bath into an apartment I have attached to a hangar building. It has an existing septic system with a kitchen / bath and laundry on it now.

I want to add a shower, toilet and lav to the new bath.

This needs to function well, but there is no permit or inspection worries

I made some drawings in hopes they would help.

1. Should I run 3" into the new area, then Y it off and have the shower continue on 2" and feed the toilet on the y'd off 3 inch?

2. Where should the Lav tie into? The toilet run, or Y'd off the shower run?

3. Can I vent back to the main vent by going overhead? (It would be about 10.5 feet from the lav wall to the current vent) It has a metal roof and I'm trying to avoid another penetration.

4. Do I P trap it under the shower?

I'm also converting the bathtub in the existing bathroom to a samller shower. I know I can get a 2" offset drain, but would rather just move the drain closer to the center.

5. Would I assume that the bathtub had a P trap and just run my moved pipe to the center, or do I need to add a P?

Since your file failed to open up I'll answer your questions as best I can.

1. Should I run 3" into the new area, then Y it off and have the shower continue on 2" and feed the toilet on the y'd off 3 inch?

In my area we mandate 4" under the slab for the main and toilets with 3" on upper levels. Pick the rest up with 2".


2. Where should the Lav tie into? The toilet run, or Y'd off the shower run?

Most bathroom groups are roughed in like this.
Toilet connects to sewer main. The toilet wet vents through the lavatory vent. Lavatory connects to toilet drain and runs a vent off the top the stubout tee out the roof or revents back into a dry vent in the attic.. The shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes but also The Standard Plumbing Code Book.

3. Can I vent back to the main vent by going overhead? (It would be about 10.5 feet from the lav wall to the current vent) It has a metal roof and I'm trying to avoid another penetration.

Yes if what you're venting back to is a dry vent. This is called "reventing".

4. Do I P trap it under the shower?

Yes.

5. Would I assume that the bathtub had a P trap and just run my moved pipe to the center, or do I need to add a P?

The only thing that you may "assume" in older plumbing is that you can not assume anything. There may be a drum trap under there or no trap at all. You just don't know until you check.
Would this shower you're converting to be a custom tile shower or a manufactured base and enclosure?

Good luck, Tom

Sorry about the attachments, the word file was too big. I made jpeg's and re-attached them.

2. Seems like what you laid out was almost what I was planning, but the lav wasn't tying to the toilet in my attempt. I guess I should find a way to run lav to shower to toilet?

3. Yes, it would be tying back to the dry vent which is one of 2 currently penetrating the roof (the other being by the sink)

5. Both showers will be a mud bed tile, not prefab.

I always worry when I dig around in this structure. It was built and owned by a local builder, but after finding numerous pieces of romex just twisted together and taped splices stuffing into the wall insullation, I wonder about the rest of his work. I gutted the whole place and replaced the wiring. Also ripped out the CPVC supply system he had and put in Pex.

Your lay out looks good to me. I would tighten the lav drain a tad and connect it downstream of the shower trap where it would be wet vented by it. The shower will have a "P" trap under the pour and the raiser brought up a few inches above it. Got a little problem with the 3 X 3 X 2" wye. You want a 3" branch on that wye so ask for a 3 X 2 X 3" wye instead or reduce the top end of a 3" wye down to 2". Good luck, Tom