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Thread: Required drain size for double sink and tub

  1. #1

    Default Required drain size for double sink and tub

    What is the required drain diameter for a double sink (back-to-back) with a tub downstream?

    Thanks for your help.

  2. #2
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default pipe

    It depends on how you do the piping, but normally 2".

  3. #3


    On the second floor of a two-story house, two lavs are at the beginning of a drain line. They are connected with a double sanitary tee and are branch vented with 1 1/2" pipe to the stack vent. The 1 1/2" drain runs 2-3 feet to the soil stack that serves the second floor. Note that a watercloset drains directly into this soil stack as well and that the soil stack is also branch vented to the same stack vent that the lavs are vented to.

    A tub is being added, and the drain needs to connect to the soil stack without taking too many bends. Can the tub drain connect to the existing 1 1/2" drain line coming from the double lavs?


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