Adding 3rd bathroom sink drain/vent to double sink configuration

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by DanFL, Aug 24, 2021.

  1. DanFL

    DanFL Member

    Joined:
    Jun 13, 2014
    Occupation:
    Electrical Engineer
    Location:
    Orlando, Florida
    What is the correct way to add a 3rd sink to this double bathroom sink layout? Option A, Option B, neither?

    3 sink A.JPG

    3 sink B.JPG
     
  2. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Both are fine.

    Option A dry vents all the lavs; option B wet vents lav #2. With Option B the lav 3 stub out height is fixed by the 1/4" per foot slope, while Option A lets you set the lav 3 stub out at whatever height you need.

    So Option A has a slight advantage in my book.

    [BTW, on the right side you could use a wye and two 45s (trap arm comes into branch inlet of the wye) instead of a san-tee and a LT90, in case that's useful for some reason.]

    Cheers, Wayne
     
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  4. DanFL

    DanFL Member

    Joined:
    Jun 13, 2014
    Occupation:
    Electrical Engineer
    Location:
    Orlando, Florida
    For Option B, the center to center drain spacing on each sink is only about 24", so that would set #3 drain just about 1/2" higher than #2, which I'd be OK with. If it's OK to wet vent lav #2, then would I even need the vent up between drain #2 and #3?
     
  5. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    In option B, lav 2 is being wet vented via lav 3, at the horizontal combo you show for the lav 2 stub out.

    I'm a little unclear on whether you could just delete the vent between lav 2 and lav 3. If lav 1 were not there, then the IPC would let you do that; they call it common venting, and each lav trap arm (relative to the san-tee providing the vent connection) would have to comply with the trap arm rules individually.

    What I'm unclear on is whether the double fixture fitting that vents lav 1 and lav 2 is already a form of common venting, in which case you couldn't common vent lav 2 and lav 3, as common venting is limited to two fixtures. Or if the double fixture fitting counts as separate individual vents for the horizontal connections, in which case it would be OK to use common venting on one side.

    Cheers, Wayne
     
  6. DanFL

    DanFL Member

    Joined:
    Jun 13, 2014
    Occupation:
    Electrical Engineer
    Location:
    Orlando, Florida
    OK, I will probably just go with option A to be on the safe side. Thanks again.
     
  7. DanFL

    DanFL Member

    Joined:
    Jun 13, 2014
    Occupation:
    Electrical Engineer
    Location:
    Orlando, Florida
    One last thought... are these sections considered part of the trap arm and subject to the rule for minimum length of trap arm? That one for #2 will need to be shortened to get all the centers properly aligned.

    upload_2021-8-25_8-15-29.png
     
  8. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    The applicable distance is all the way to the trap in the cabinet, so it's basically impossible to not satisfy that rule for a lavatory.

    Even in the case of a laundry standpipe, where you might like a very short trap arm, you only need to have I think 1.5" of pipe showing between the two hubs. As the distance is measured from the horizontal inner surfaces of the pipe: the point in the trap where water first spills over into the trap arm to the inside face of the vent pipe.

    Cheers, Wayne
     
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