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Thread: Hide trap below floor for Pedestal sink

  1. #1

    Default Hide trap below floor for Pedestal sink

    I am installing a new bathroom with a pedestal sink. There is not enough room in the sink column (base) to hide the trap and AAV. My wife was hoping to just see the sink and not the plumbing. Is it possible to mount the p-trap and AAV directly below the sink in the floor? There is plenty of room to do this and still get enough slope for the drain water. Putting the drain in the wall is not an option in this case.

  2. #2
    Licensed Grump GrumpyPlumber's Avatar
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    The stand pipe above the trap would exceed 24"...potentially causing it to siphon..and smell.
    The correct way to pipe a pedastil is inside the wall with a stub out height at about 20-21" off finish floor.
    Not easy, but if you want it done right...thats the way.
    Also, the AAV (not legal in my state, but legal in many) should be 6" above the flood rim of the basin if possible to prevent water bringing debris into it when it drains.
    "The biggest regrets we have in life are the chances we never took."

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    Master Plumber Redwood's Avatar
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    Putting the drain in the wall is not an option in this case.
    Depending on where you live it may be your only option.

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    Licensed Grump GrumpyPlumber's Avatar
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    Apparently I stand corrected by Redwood, in WI at least...I'm told code WI allows it.
    Further fuel for the UPC, IPC, national uniformity debate.
    "The biggest regrets we have in life are the chances we never took."

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    Master Plumber Redwood's Avatar
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    In wisconsin they have a special lite water they use when they make lite beer. It doesn't carry the same momentum through the trap so on pedestal sinks only they allow traps below the floor.

    All other places I know of do not allow it!

  6. #6
    Plumber canton's Avatar
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    Default Massachusetts

    Not allowed here.

  7. #7

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    Thanks for the timely advice! I will reposition the sink to make use of the back wall for the drain. Does it matter if it is an exterior wall for the drain? The supply lines will still come from the floor. Your help is very appreciated. This forum is the best!

  8. #8
    Master Plumber Redwood's Avatar
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    A drain in an outside wall is okay, Supplies through the floor is highly recommended. How are you venting this sink? An AAV cannot be stashed in a wall.

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    Forum Admin, Expert Plumber Terry's Avatar
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    An AAV cannot be stashed in a wall.
    I've put them in walls with venilated access covers. (grills)

    Or you could run a vent up to the ceiling and tie into the other plumbing vent that the other fixtures are using.
    Or if they are on the same wall, that would even be easier.

    In Washington, the drain must be in the wall too.

  10. #10
    DIY Senior Member Nate R's Avatar
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    Looked it up. Amazing that they (WI) allow this much fudge factor with a pedestal sink:

    82.32 (4)(b) b The vertical distance between the top of the fixture drain outlet of a pedestal fixture or a cuspidor and the horizontal center line of the trap outlet shall not exceed 60".

    The Wisconsin code specifies 15" as the max vertical drop for most other fixtures. (It notes exceptions.)

    If I have a 2nd floor pedestal sink, (I'm in Wisconsin) I read this as the trap could be below the floor level, or does the trap need to be accessible so it couldn't be enclosed in between floors unless there was an access panel to it?

  11. #11
    DIY Senior Member construct30's Avatar
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    If you can put the trap below the floor, not allowed where I live either, but if you can then what about the AAV, would it still not have to be 6" above the flood rim. The only exception I know of for that is under sinks and they still say as high as possible. So if they allow the trap below the floor then is as high as possible under the floor, I can't see you getting two pipes up the pedestal.

  12. #12

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    In Pennsylvania, I was told by a licensed plumber, we can put the AAV in a wall or ceiling, if there is an access panel for maintenance. I didn't ask about installing it in the floor, but based on what I have read here, it would not be a good idea. I am planning on putting the drain and AAV in a pipe chase to be built behind the sink, and with an access panel. The trap arm will extend from below the sink drain horizontally into the chase. From what I have gathered, the problem with the trap under the floor is the distance the waste travels before going into the trap. That's it, I just need a pedestal sink with a very short pedestal (LOL).

  13. #13
    DIY Senior Member construct30's Avatar
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    Quote Originally Posted by gorman View Post
    In Pennsylvania, I was told by a licensed plumber, we can put the AAV in a wall or ceiling, if there is an access panel for maintenance. I didn't ask about installing it in the floor, but based on what I have read here, it would not be a good idea. I am planning on putting the drain and AAV in a pipe chase to be built behind the sink, and with an access panel. The trap arm will extend from below the sink drain horizontally into the chase. From what I have gathered, the problem with the trap under the floor is the distance the waste travels before going into the trap. That's it, I just need a pedestal sink with a very short pedestal (LOL).
    If you live in PA as I do it is illegal to put the AAV under the floor, it is also illegal to make a stand pipe that long too for a pedisatal or anything else. Water flows the same rate down a drain as it does in the rest of the world excep where this guy lives. I was just wanting to know if they are allowed to put the trap under the floor because they can't get up the wall, then what about the AAV?

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