Question about using a double WYE vertically to connect to stack

[casechopper]

Hello,
I'm working on adding a bathroom to a third floor attic room.
I have a question about whether a certain layout for the drains will work as the room setup is a bit odd (the main stack runs right through the center of the room). My question is whether I can use a 3" double WYE vertically on the stack and have the toilet run into one side of the double wye and then have the lavatory and shower drain to the other side of the wye. The shower and lavatory would be dry vented separately, tying back into the stack at a higher point and the toilet would vent through the stack (there's nothing draining into the stack above this room). I understand that it would be more normal to use a sanitary tee for these to connect but because of the soil stack being in the center of the room I'm trying to find a way to connect from both sides of the stack. I considered a double sanitary tee but my understanding is that this may not be permissible or could cause issues with snaking the lines.

Any guidance would be appreciated? I've attached a rough drawing to try to show what I'm considering.

 

[tvier]

You can't use a wye to join into a vertical pipe period.
Not sure about the double tee. I think that's normal where I am, but I'm not an expert, so I can't say for certain.

 

[Sponsor]

 

[GReynolds929]

You can't use a wye to join into a vertical pipe period.
Not sure about the double tee. I think that's normal where I am, but I'm not an expert, so I can't say for certain.

You most certainly can on a drain, but not on a trap arm

 

[John Gayewski]

Your drawing is fine. Not sure what post #2 is about about. A toikey's vent doesn't matter if it's below the trap or not so it's vented correctly by the stack.

You likley don't need a separate vent for the shower drain as long as the distance from your (2") lav drain to the shower trap is shorter than 8'. The lav drain is up sized to accommodate venting for the shower trap.

 

[tvier]

You most certainly can on a drain, but not on a trap arm

TIL on the drain vs trap arm distinction. Doesn't the drawing put the toilet's trap arm into the stack using a wye though?
Maybe I'm interpreting the drawing wrong.

 

[wwhitney]

TIL on the drain vs trap arm distinction. Doesn't the drawing put the toilet's trap arm into the stack using a wye though?

The requirement for the use of a san-tee (and for the vent connection to be at an elevation no more than one pipe diameter below the trap outlet) does not apply to a WC, as it is a fixture that intentionally siphons. Also, the outlet of the WC itself is already well below the trap weir.

Cheers, Wayne

 

[casechopper]

Thanks for the help guys. This post got held up for a long time before being made public so I had already moved ahead. Due to a lack of vertical accessible space to put the double wye I switched to a double sanitary tee. Here's a photo of how it ended up.