Correcting/improving current back-to-back bathroom drainage/venting

[Tom Gabriele]

Hey all, I am a DIY homeowner with a strong motivation to do things correctly (and zero credentials). In RI, 2015 IPC, non inspected, ABS allowed.

On the main floor, we have mirror image bathroom groups with a horizontal stack for the sinks, toilets, and tub/shower in that order. Sinks are 1.5" dry vented with a double sani tee, toilets join in with a double wye with 45's, followed by a standalone 3" vent on the stack, then the turn vertical, then the tub/shower combo and shower enter on the vertical with another double wye. The tub doesn't have its own vent, and the shower has an attempt at a vent that comes off after the drop.

Pictures worth 1,000 words, so here's a photo album: https://imgur.com/a/C8bZZ4B

There aren't any current issues the I'm aware of, but from what I can tell, this violates both best practices and IPC if it were new construction today, for a few reasons:

1. Tub/shower can't be wet-vented past the toilets like that

2. The 1.5" vent on the sinks should be 2" if it's a wet vent

3. Back to back toilets even on a "proper" wye like that isn't best practice for when we replace the original toilets with higher-velocity ones

So while I have the one bathroom gutted and plenty of access through the open laundry ceiling below, I'd like to get this up to snuff.

Here are the changes I think would be appropriate, which I could use some conformation on:

1. Replace the 90 turn to vertical and the vertical wye for the tub and shower with a sanitary tee with left and right inlets, 3x3x3x2x2 (https://www.supplyhouse.com/Nibco-I...-Sanitary-Tee-with-90-Right-Left-Inlets-5870R) which would make the turn, upgrade to 2" traps for the tub and shower, and add a dry 3" vent for both of them (going up to the attic and tying in with the rest to go through the roof).

2. Replace the double wye for the toilets with sequential single wyes so they're not entering at the same fitting.

I believe those two changes will make the system functional and passable if it were to be inspected, but could use some help considering I have no credentials. I have done a bunch of reading and research about what the IPC requires and how I can meet it, but I am starting from zero and it's been a real uphill climb.

An additional layer is that the laundry room below has the washing machine and utility sink, both of which currently vent through the existing 3" which is up and around on the horizontal stack (as far as I can tell). My proposed new vent right at the turn to vertical ought to improve their ventilation too, and more ventilation couldn't hurt, right?

 

[James Henry]

Two things I noticed. The lav vent is OK but the lav drain has to be 2" and it can't have a vertical offset in it.

 

[Sponsor]

 

[Tom Gabriele]

Two things I noticed. The lav vent is OK but the lav drain has to be 2" and it can't have a vertical offset in it.

Vertical offset meaning that little jog down with the 45's before the reducer?

Do you have any thoughts on the venting for the tub and shower and my proposed solution with the sanitee with inlets?

 

[hj]

WHY "no offsets"? Not all jurisdictions would approve that double inlet tee, so check first.

 

[Tom Gabriele]

Not all jurisdictions would approve that double inlet tee, so check first.

Gotcha, thank you. Looking at it more, the inlets aren't swept at all, so it seems like a bad idea whether it would be allowed or not.

Instead of that, would a regular sani tee to turn horizontal to vertical while adding a new 3" vent going up from it, with a double sani tee just below to catch the tub and shower be properly vented? Or is that still technically a wet vent downstream of toilets?

 

[James Henry]

WHY "no offsets"? Not all jurisdictions would approve that double inlet tee, so check first.

I thought it was a wet vent but I was wrong.