Venting back to back toilet and shower

[CallmeJake]

Hi,

I am attempting to install a full bath on the second floor above a half bath. I think I have a plan, but not sure it if it works. The project is in Michigan near Ann Arbor.


I have attached a couple of sketches, still trying to get my head around isometric drawings. The non-iso drawing showing a "y" with a leg pointing down is actually supposed to coming out toward you.


The first question I have is around sharing the drain for the first and second floor WC. My understanding is a fixture cannot drain into a toilet vent. To avoid doing that, I was thinking about wet venting the first floor bathroom toilet through the lav and up through the roof. Does that work?

The second question is around the second floor bathroom. The WC and tub are back to back with only about 23” between drain centers and the stack being about center of the two. My plan was to use 3” sanitary tee with a 2” side inlet. Then into the 2” inlet I would ‘y’ the tub and lav drain. Out of the top of the tee I need to bush to 2” and immediately run two vent 90s to get into a wall to get up to the roof. Is the side inlet sanitary tee the right fitting for this? Is there a problem with the two vent 90s to offset the stack?

Is there a smarter way to do all of this?

Appreciate the help.

 

[Plumber69]

Vent the top bathroom with the lav. Wye the toilet in below the tub and lav. The horizontal dry vent won't work

 

[Sponsor]

 

[Terry]

Yikes! A san cross with a toilet and shower? That shower is going to get plugged with poo.
Like mentioned above, use the lav to vent the toilet and the tub. Bring the toilet in below the tub trap arm, not at the same level.
You can't have a horizontal dry vent below the flood level, which is why they are at 42" or higher.

 

[Plumber69]

Yikes! A san cross with a toilet and shower? That shower is going to get plugged with poo.
Like mentioned above, use the lav to vent the toilet and the tub. Bring the toilet in below the tub trap arm, not at the same level.
You can't have a horizontal dry vent below the flood level, which is why they are at 42" or higher.

They did that lots here in town with cast along time ago. The side outlet is higher and off to the side. But was vented off the top. In his situation that ter is not needed

 

[Terry]

They did that lots here in town with cast along time ago. The side outlet is higher and off to the side. But was vented off the top. In his situation that ter is not needed

I've seen the side outlet, but with the straight across action going on, there's nothing to bend it down. It already skips across a fixture cross pushing a surge of air along, and making the trap loose water.

 

[CallmeJake]

Thanks for the responses, saved me a big headache. Didn't think about the dry vent below flood level.

To clarify on the tee, my drawing isn't great, I was planning to use the side inlet fitting not a cross to avoid the poo in the shower. Or does that fitting have the same problems? Sounds like it's unnecessary anyway, but I am curious when it would be used.

The part I was having trouble with was keeping the stack going up and getting everything to fit in the joist bay. If I understand right, by venting through the lav I won't need to jog the vent in the way I thought. I had it in my head I had to keep going up. It would be a wye for the toilet with just a 90 over to pick up the tub and lav.

Do I not have to worry about the toilet siphoning the tub trap with the vent being further away?

Thanks again