Please review DVW scheme for basement

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?

W.C., 3"
Lav sink, 1-1/4
Tub/shower, 2"
Washer, 2"

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?

First, I am only a DIY plumber and not an expert here.

Personally, and even though the actual drain is smaller, I would use 2" for the lav sink also. And, I believe you need vents behind the lav and between the tub and washer.

The biggest problem I see here, however, is that the basement bathroom is going to overflow if your septic system ever backs up ... and think of that this way:

If you only had the basement bathroom going out into your septic system, you would immediately know there was a problem if the tub drain began running slow or the toilet would not flush properly ... and now go upstairs and consider what is going to happen when you flush there and the septic system backs up.

Even though your slab-out drainage seems quite convenient, I would nevertheless pump that basement bath out through a sump and pump with a check valve.

vent every wall, in the wall

gdavis said:

.... bathroom and an adjacent washer/dryer .... must the W.C. be vented behind the fixture? .... How is ... best vented ... with a framed wall behind?

Click to expand...

AFAIK, it looks like a good start. The questions about venting mean that you may not have a handle on the whole concept... The toughest question is about the WC: in a horizontal plane installation, you don't try to vent a WC per se, instead you route the sink drain over to connect as close as possible to the WC's 3" drain. AFAIK. As for all the other fixtures, you put a vent in the wall right at the point where the P trap arm goes into the wall, and with a SanTee you connect that arm to that vented pipe. This means you need a vent in the wall behind the Washer, at the tub, behind the lavatory, etc. The vent is a pipe that you connect to any other vent; only one of them needs to be connected to the outside. AFAIK. You don't need to upsize to 4"; staying with a 3" pipe is good for sweeping solids out of the pipe.

david