Speedball1 wrote in a different thread:
"Let me tell you how a typical basement bathroom group rough in goes down. 4" under the cement from the toilet to the main sewer. 2" from the lavatory to connect to the 4" toilet drain line. From the top of the lavatory stubout tee a vent line will be run through the roof or revented back to a existing dry vent line in the attic. 2" from the shower to connect to the lav drain where it will be wet vented as will the toilet."

This sounds exactly as I have except the rough in has a 1.5" tub drain. The lavatory / wet vent is about 30" to the right of the toilet 4" and the tub drain is several feet to the left (located for a left drain tub). The lav drain comes up through the sill plate of the exterior wall.

I desire to cut this space in half 16" to the right of the toilet. The lav drain is actually within the future kitchenette space. The bath space would include a tub, toilet, and vanity. The kitchenette will have a sink. My plan would be to plumb the kitchenette sink into the 2" lav stubout, but this leaves the dilemma of where to drain the vanity.

Best from a layout standpoint is to drain the vanity into the same drain as the tub, but I'm concerned about the feasibility of draining both into a single 1.5" pipe. I presume if it were possible the sink would not need a trap since the trap already exists in the floor and I'm guessing there might be a tendency for the tub drain to siphon the water out of a 2nd trap.

A slightly less desirable, but probably slightly more feasible solution is to drain both sinks into the 2" lav drain via a 2" cross. It would run the bath vanity drain through the divider wall, then around to the existing lav stub. The kitchenette sink would also drain into the other side of the cross. Is it acceptable to wet vent with two sinks draining into the same location?

Also worth mentioning is that the top of the cross would be vented with a studor since there is no easy vent stack location from this area. The builder didn't have the foresight to run a vent stack up to the attic.

Please, I appreciate (especially Speedball1's) expert comments.

Thanks very much,
Larry

Hey Larry,

"My plan would be to plumb the kitchenette sink into the 2" lav stubout, but this leaves the dilemma of where to drain the vanity."

Since the lav stubout will also vent the bath room group out the roof and the kitchenette sink will be back to back with it I would simply pick up both with a 2" cross and use the lav vent as a common vent for everything since both would appear to fall within code. I would reduce the lav arm to 1 1/2" at the cross and keep the sink arm at 2".

"Best from a layout standpoint is to drain the vanity into the same drain as the tub, [Not the way I see it] but I'm concerned about the feasibility of draining both into a single 1.5" pipe. [ As would I] I presume if it were possible the sink would not need a trap since the trap already exists in the floor and I'm guessing there might be a tendency for the tub drain to siphon the water out of a 2nd trap."

The sink would indeed need it's own trap and vent. You may not use one trap for two fixtures

"A slightly less desirable, but probably slightly more feasible solution is to drain both sinks into the 2" lav drain via a 2" cross. It would run the bath vanity drain through the divider wall, then around to the existing lav stub. The kitchenette sink would also drain into the other side of the cross. Is it acceptable to wet vent with two sinks draining into the same location?"

Answered above.

"Also worth mentioning is that the top of the cross would be vented with a studor since there is no easy vent stack location from this area. The builder didn't have the foresight to run a vent stack up to the attic."

This is acceptable if local codes allow. Good luck, Tom

Tom,

Thanks very much for the reply. I will indeed follow the 2nd plan. I was considering use of 1.5" PVC for the lav drain already since it would be easier to run horizontally through the new wall.

The local codes do indeed allow a studor, the house already has two.

Larry