Add new toilet behind existing toilet drain picture please comment.

[tom in NC]

I have a one floor ranch and am adding a toilet. There is an existing toilet that drains into an existing drain line that is easy to access in the basement. I want to add another toilet about 30 inches away from it

Looking at the picture, the offset closet flange at the top of the pic is the existing toilet. The straight closet flange is the new toilet I am adding about 30 inches away from the existing toilet so they will be back to back toilets. The double fixture fitting is for the two lavs to drain into and also wet vent the two lavs, two toilets, and one tub. The vent size will be 2 inches. I have also shown at the right of the picture the p trap for the tub draining into this thing.

The biggest question I have is will flushing the new toilet siphon water out of the existing toilet as the waste passes thru the drain line underneath.

 

[Terry]

The shower needs venting before it enters the line with the toilets.
Also, to many changes of direction on it. I would have a wye for the shower and run near the main line, combo up at a 45 to pick up the lavs. The lavs can wet vent the shower. After the combo, you can then 90 over and pick up the shower.

 

[Sponsor]

 

[tom in NC]

I'm not sure I understand everything you said so here it goes. I now show a dry vent for the toilets and a wet vent for the shower via the lav double fixture fitting.

1. Is the 2 inch dry vent for the toilet necessary with the wet vented lav and shower? If the 2 inch dry vent is needed, a 1.5 inch vent will suffice for the two lavs and shower.
2. Will the 'downhill' toilet siphon any water out of the 'uphill' toilet?

Thanks so very much.