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I'm in the process of remodeling a basement bathroom. It used to have a big shower in it and a vanity on the wall opposite the shower; the shower used a floor drain (obviously) and the vanity sink had a long sloping pipe to a vented drain pipe in the adjoining wall. The new layout puts the vanity and new tub/shower on the same wall right next to each other. The tub will be using the same drain that the shower used before, and I am trying to figure out what to do with the vanity sink drain.

If I route it to the same pipe as before, it's about 10 feet with a right-angle around the corner. This isn't hard to do with PVC pipe alone, but I have to notch out a whole bunch of studs to make room for the 1.5" drain pipe to run the whole length (the studs are right up against the concrete surrounding the underground basement). I would really prefer not to have to notch out all of those studs--though, at least, it's not a load-bearing wall.

However, if I route the sink drain into the existing drain pipe, it's much shorter and more direct, even if I stay in the wall and don't go straight through the side wall. Here is what I am thinking:

Joined drain diagram

This arrangement will require notching out exactly three studs, two of which have permanent open access via the storage area (i.e. no drywall on that side). Since we already have to trench into the concrete a bit to get the tub drain in correctly, we'll have a span of horizontal drain pipe that should allow an extra "T" junction for the drain pipe to join below the height of the tub drain itself, which should negate the possibility of any sink water flowing back up into the tub (unless the whole underground drain is clogged, but that's a bigger problem).

Is there any reason not to do this? Even if I have to re-vent the sink drain, it would be far easier to run a new 1.5" pipe up the open wall and across above the ceiling (big crawlspace available) to join the existing vent. I'd do that any day over notching out all those studs.

Note: I'm primarily interested in physical limitations that would make this a dumb idea, but also interested in code compliance. The home is in Virginia, USA.

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A couple considerations:

You cannot "double trap" any fixture. To utilize the trap in the floor for your lavatory, you need to ensure that the length of the trap arm does not exceed maximum allowable distance (42" for 1 1/2" trap arm, in CA USA, I think). It looks like you are pretty close to max distance but you did not say in your question.

Also, max trap arm change of direction is 90°, but you would have two 90° bends. You AHJ might allow though if you install an accessible clean-out.

  • I wasn't thinking about double traps. We already have to chisel out some of the concrete in order to allow the tub drain to connect properly without raising the tub many inches off the floor, so the simplest solution may be to provide a second tap into the in-ground drain pipe past the existing drain trap so the sink can have its own normal trap and we can quite easily re-plumb and vent the whole thing. I'll check with our contractor. – Jeff Rowberg Dec 24 '16 at 4:37
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    Should work. If you trap it below the lav per normal practice you wouldn't have to worry about the distance, other than vent distance considerations which it sounds like you have a plan for. BTW the "double-trap" issue is both a code and a physical limitation due to siphoning potential. – Jimmy Fix-it Dec 24 '16 at 5:41
  • Thanks for the clarification. I assumed the double-trap requirement was more than just a pencil-pushing bureaucrat arbitrarily making it hard on plumbers. That was one of the things I hoped to find out. Some code requirements seem pointless, but this isn't one of them. – Jeff Rowberg Dec 24 '16 at 20:44

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