The attached plan shows an area of a walkout basement, slab on grade, in which there is a full bathroom and an adjacent washer/dryer closet. As can be seen, all fixtures on this floor drain through the slab to waste lines below.
We plan to drain with line sizes as follows. Does this sound OK?
Lav sink, 1-1/4
As for venting, must the W.C. be vented behind the fixture? If so, what size vent?
Can we get by without venting the W.C. behind, and consider the stack, seven feet down the discharge line, as the vent?
The W.C. waste line is shown in the drawing as going over to the common wall between the bath and the mechanical room adjoining, where it goes into a 3" stack that brings the waste down from the bath above.
The stack then turns at its base with a long turn 90, making the underslab main drain line, which runs under the slab at a pitch of 1/4" in 12, running out of the building to the septic tank.
If we need to go from 3 inch to a 4 inch main waste line, where do we best make the transitions?
One more question. It deals with the only other fixture drain that is in the basement slab, that for the tub/shower. How is a tub/shower best vented, when draining into a slab like shown, with a framed wall behind?