Common vented toilet and tub/shower using sanitary tee?
Posted by Chris on January 09, 2004 at 19:37:10:
I have bathrooms back-to-back with a nice large stack wall in between. I want to rebuild one of the bathrooms, swapping the toilet and the tub (so that the toilet in one bath will be back-to-back with the tub in the other, and vice versa).

I am going to add a second 4" stack teed into the main 4" vent in the attic, and wyed into the main 4" drain in the basement, so there will be a 4" vent behind each toilet/tub combo.

Can I have the fixture drain from a toilet, and the fixture drain from a tub/shower, connect back-to-back into a 4" stack as a "common vent" using a double sanitary tee (using a 4"-to-2" bushing in the side for the tub/shower)? All the examples I've seen of common venting involve identical fixtures, not a tub and a toilet. And I've seen comments to the effect that back-to-back toilets should use a double wye instead of a double sanitary tee, but obviously a wye won't work for the shower since the split in the wye would be below the trap weir.