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Thread: Plumbing pedestal sink under floor

  1. #1

    Default Plumbing pedestal sink under floor

    I am ready to install a pedestal sink in my dpwnstairs bathroom. It seems to be easier if I were to plumb the main drain down through the floor and then install the p-trap and additional piping to the main line (unlimited access). Is this legal or should I try to pipe the p-trap above the floor?
    Last edited by llt01; 03-20-2006 at 07:38 AM. Reason: additional information

  2. #2
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    What you are proposing is an S-trap, which is disallowed by all plumbing codes I Know of.

  3. #3


    Please correct me if i'm wrong. Doesn't a P-trap come down vertical from the drain then form a 180 deg loop up then elbow 90 deg to a horizontal run? This is what I would like to do, if I can. Is there a specific measurement the P-trap needs to be from the sink's drain?
    Last edited by llt01; 03-20-2006 at 08:11 AM.

  4. #4
    Plumber plumber1's Avatar
    Join Date
    Aug 2005


    No it would then go through the floor and be re-plumbed to a different place.
    Yes it goes horizontal then.......

  5. #5


    I don't think you should do what you're proposing. I understand from your original post that you want to install a standard p trap - you just want that trap to be below the floor, instead of a few inches under the sink.
    Codes dictate that the trap be located no greater than a certain distance (I think it's 24").

    I haven't found a technical reason for this, but I *believe* it has to do with the fact that water falling from a greater height might push the water out of the trap.

  6. #6


    Please See Attached File
    Attached Images Attached Images  

  7. #7
    DIY Member casman's Avatar
    Join Date
    Sep 2004
    New York


    I believe prashter is correct there is a max length to the vertical drop and I think its to prevent water due to its drop from being pulled out of the trap.

  8. #8
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default trap

    you cannot do it with the trap under the floor. Even if you could you would still need a vent. Installing the "P" trap above the floor, the way you describe would create an "S" trap out of the "P" trap. You could create a serious drainage, and odor, problem if the pipe from the drain to the trap were that long.


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