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Thread: P trap requirements for bathroom sink

  1. #1
    DIY Junior Member larry528's Avatar
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    Default P trap requirements for bathroom sink

    I am replacing two sinks in the masterbath (many uses 24-7), but the sink drain location is 2 inches closer to the stub out wall than the previous.

    My question is does the bottom of the trap need to be lower than the stub out line, or can it be higher?
    Its not a design flaw, its an enhancement!

  2. #2
    In the Trades Gary Swart's Avatar
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    Can you make water run uphill? If so, then the outlet can be higher. In my area, water will only run down hill so the outlet would have to be lower.

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    Remodel Contractor GabeS's Avatar
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    Not a pro plumber. But I know that between the trap and the point where the pipe goes into the sanitary tee, there can be no bends. Otherwise it will not vent properly and you will have your traps being siphoned.
    Gabe

    Don't follow my advice, I only know a thing or two about a thing or two.

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    One who lurks Basement_Lurker's Avatar
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    larry528, your question doesn't make sense to me, you say your sink tailpiece is 2" further back towards the wall with the drain stubout than your previous sink?

    How would this affect your p-trap height unless you are trying to do some strange offset? If height is actually the real problem and you don't want to open the walls, then look into a vessel sink.
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    DIY Junior Member larry528's Avatar
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    The question is straight forward.

    what is the relationship of the BOTTOM of the trap for a sink to the line coming out of the wall.

    If its lower, then when in use, water coming down from the sink has a greater hydrostatic head than the short distance from the bottom of the trap to the bottom of the line going out. About 2 inches max.

    But I have an A/C line that also feeds into the column above the trap, and because of the way the stub out ends, with no way to cut off and reduce the distance from the trap to the thread on, I have to snake some extra pipe to make ends meet. I dont want to use that bellows type connector, other issues there.

    So, can the bottom of the trap be higher than the line going through the wall, equal, or is there NO difference, since the nature of the trap is secure with the offset that it has.

    My concern with it being higher is that the water coming down from the sink would create a siphon that would clear the trap, making it a vent and not a trap.

    As far as a vessel sink goes, I already have the right hand sink in, this is the left handed one.
    Last edited by larry528; 01-16-2009 at 06:47 PM.
    Its not a design flaw, its an enhancement!

  6. #6
    Jack of all trades frenchie's Avatar
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    No, the bottom of your trap shouldn't be above the wall inlet. That'd make it an S-trap.

    I still don't get it why/how the distance from the wall would affect the trap height. Pictures?

    FWIW... a two-piece trap (aka NYC trap), in combination with an offset on the tailpiece, allows a fair bit of adjustment forward & back.
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  7. #7
    Homeowner geniescience's Avatar
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    - keep the P trap at the same height it was at before.

    - you can adjust the rest of the geometry of the pipes to account for the loss of space in one direction or another, but not lower or raise the P trap.

    - your descriptions in both posts above are beginner-ish so bear with us no matter what you think about your clarity and our inability to get the picture. Search on "trap weir" if you want to learn more about P trap height.

    - install your P trap at the same height it was at before.

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