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Thread: Plumbing drainage question

  1. #1

    Smile Plumbing drainage question

    Hi all. I was always told never to plumb in a minor fixture under a major fixture(2nd floor toilet over first floor lav) If the first floor lav is seperatly vented up to and above the higest fixture is it ok to do. What exactly do they mean by this?? Thanks for your help.


  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    A pipe should be only one thing, waste or vent. So, if you want to use what is now a vent for waste, you need to provide a new vent for those lower fixtures. This would be a new pipe branch that goes above where those pipes are waste to where they become vents for the items above. typically, you can either run that new vent all the way to the roof, or you can rejoin the vents above...that needs to be done typicall 42" above where they are a waste line (basically it says 6" above where waste could accumulate if the drain was plugged, i.e., the flood plane - but, a typical kitchen counter is 36", so they often want 42" even if it is a vanity which could be lower).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default fixture

    In the first place you could not connect a lavatory on the first floor to the toilet drain from the second floor WITHOUT having its own vent. The ONLY purpose for a vent is to relieve positive or negative pressures. Once you do that, you can connect the fixture to any drain line you so desire.

  4. #4


    OK Thank for the info. Not to try and grossly over simplify drain and venting, but to help me better understand it. Given that you use the proper size drain line in (FU) and grade properly. If you are running a horizonal or vertical soil line in residential work, can you tie in all your fixtures as you go as long as each fixtue is seperatly vented up above the highest flood rim by 6". (also not running your vent horizonal before this).


    Last edited by Terry; 11-29-2008 at 08:54 AM.


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