First of all, I just want to say this forum is great. I got a lot of input from other posts and examples. Thanks for hosting this site.
We are looking to add a bathroom to the second story of our house. We are turning a large game room into a new master bedroom suite. House is on a raised foundation, so we can access to the existing 4" main drain through the crawl space. Currently there is some plumbing that may have been for a wet bar or something like that, but it was capped off when we bought the house. Below the new 2nd story bathroom is a jack & jill bathroom, but we are removing one toilet and lavatory sink, and changing the current shower to a tub (in the same location). Because the downstairs bathroom is about 6' wide and standard tubs are 5' long, we have about 1' we can use to help route plumbing downstairs. The 2nd story has exposed beam vaulted ceiling, and all of the plumbing from the J&J bathroom vented horizontal to the 1st story part of the house (that had attic space). The current vent line runs between 2x10 floor joists.
For the new upstairs bathroom, we are adding a double vanity on the side wall. I'm thinking of running the wet drain between the floor joists back towards the upstairs shower & toilet drain. The vent we will just take through the roof above the vanities. I think I'm going to have to remove the existing 1st story venting between the floor joists since that will cross where the toilet & shower will need to drain. The other challenge is that both the toilet and shower drains will need to go through a floor joist. Even though they are short runs (toilet ~3', shower ~4'), I can't combine them in the bay the drains are located in and can't combine them until they are in the first bay where I'll add a new vertical stack, so it looks like I'll have to cut a 2+" hole and 3+" hole through the 2nd joist. I'm vertically challenged between floors (2x10 joists).
What I've come up with is attached. I can combine the upstairs shower and toilet in a double combination Y-1/8 (3" with reducers for the shower closet bend and vent). I'll have to make a slight joggle to get the vent pipe into the 2x6 wall (a couple of 22.5 or 45 degree bends). If the drain pipes are in the first bay, I can run them down outside of the wall in the space between the tub and original wall. With that I can tie in the wet drain from the upstairs double vanity below/downstream of the shower/toilet. I would also tie in the downstairs tub into this new vertical stack. The existing downstairs 2" vent would have to get rerouted through the 2x6 framed wall into the 2nd story where it will have to get tied into the new 2nd story vent from the shower/toilet.
Does this approach seem reasonable? Any other suggestions?