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Thread: Back to Back sink DWV fitting

  1. #1

    Default Back to Back sink DWV fitting

    What is the correct DWV (ABS) fitting to use to connect two sinks (Bath & Utility) back to back that share a common wall. I am assuming you cannot "stack" sanitees.

  2. #2
    One who lurks Basement_Lurker's Avatar
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    A double sanitary tee, or a double wye fitting would do the trick.
    Broken promises don't upset me. I just think, why did they believe me? -Jack Handy


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    DIY Senior Member construct30's Avatar
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    Doubles back to back sinkd are not allowed every where, check your code. The double wye is better as far as I'm concerned for back to back, but once again for venting reasons not allowed by every inspector.

  4. #4

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    The best way is with a double fixture fitting. It's not quite the same as a double sanitary tee.


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    Licensed Grump GrumpyPlumber's Avatar
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    If available, an "Owl eye", or "Frog eye" fitting is better...it's a double tee-wye with both inlets facing the same way, but they take more room.
    You can stack fittings as long as the lower is vented seperately & connected to 6" above the flood level of the highest fixture (usually the lav in a bathroom which means 40+ inches above finish floor will cover it)
    "The biggest regrets we have in life are the chances we never took."

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    DIY Senior Member construct30's Avatar
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    This is Terry's picture of a rough in for an island sink, why would it not work back to back if you just turned one and ran out the top with the vent stack?
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    DIY Junior Member davefoc's Avatar
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    This is the exact question I spent some time trying to get an answer to.

    My copy of plumbing codecheck says back to back sanitary tees are required by the UPC.

    But the UPC actually says:

    Back to back fixtures may be served by a single vertical drainage pipe if approved double fixture fittings are used. . . UPC 704.2

    So does that mean back to back sanitary tees are now precluded?

    As a practical matter it seems to me they should be. Snakes tend to pass right through them from one fixture to the next instead of down the drain.

    And if a double fixture fittings are used does that mean the length of the trap arm should be reduced from what it is if a sanitary tee is used because the weir is higher (I think)?

    I wrote to the author of the code check who very nicely returned my email. Unfortunately his answer didn't really go to my questions and he didn't respond to a follow up email.

  8. #8
    Moderator & Master Plumber hj's Avatar
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    Default tee

    A "back to back" sanitary tee is a sanitary cross and is seldom approved. The back to back fixture fitting shown in a previous picture and Terry's island vent is the approved fitting, and is the one specified in the first paragraph of the code section cited. Sanitary crosses, double "Y"s, and double combination Y-1/8 bends are all prohibited but for different reasons. And an "owl eye" fitting would only work if both sinks were on the same side of the wall, or if it were turned sideways with elbows for the sink connections, neither of which seems like a very sensible solution. Stacked tees can be used but then it makes one connection a lot lower than the other one.
    Last edited by hj; 01-11-2008 at 05:12 AM.

  9. #9
    DIY Junior Member davefoc's Avatar
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    Thanks hj. Your answer was clear.

    I was confused because standard hardware stores seem to only carry back to back sanitary tees and yet it doesn't seem like these are code compliant in the most likely way that they would be used. And in addition, the codecheck book seems to be incorrect with regards to back to back sanitary tees.

    Do you know the history of the back to back fixture fitting? The building I maintain was built in 1929 and back to back sanitary tees were used throughout. Did the back to back fixture fitting come along later to correct problems with the back to back sanitary tees? Why have the back to back sanitary tees stayed in production?

  10. #10
    DIY Senior Member MG's Avatar
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    Double wye fittings are fairly common even at the big box stores. You should have no trouble finding a 4" x 4" x 2" double wye or 3" x 3" x 2" double.
    Note: I am a DIY'er and not a professional. My posts here are observations / opinions and may or may not be in accordance with your local ordinances.

  11. #11
    DIY Senior Member construct30's Avatar
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    So what is the double fixture fitting? Something between a tee and wye? What about the distance from the trap on it? Should it always be close like the island roughin and can it be turned in the wall back to back or should it stay the way it is in the picture with 90's?

    I don't have much call for this and have usually stacked the tee's, but this works much better.

    Maybe a stupid question, but why do they make a double tee? Always seemed usless to me if you want t stay legal. Is there a legal use that you can think of or just for venting once and a while?

  12. #12
    Moderator & Master Plumber hj's Avatar
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    Default tee

    It is not a double tee, it is a cross, and it has application for vent systems.

  13. #13
    DIY Senior Member construct30's Avatar
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    Quote Originally Posted by hj View Post
    It is not a double tee, it is a cross, and it has application for vent systems.
    I do know what stuff is called, but unfortunately my hillbilly up bringing lets me use local terms for stuff even though it may be wrong.

    The old plumber at the supply house calls stuff what he wants and if you don't like it then get the _________ out of his space. I guess I feel the same at times.

    I guess I should start going to box stores and reading boxes.

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    DIY Senior Member Marlin336's Avatar
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    I know it's a tiny image but that's what I've always seen used, a threaded cross tee. I've never seen a back to back in plastic so I don't know what that fitting looks like. I've seen a bunch of these fittings get approved though.


    And why can't I link to any sites with the word "plumbing" or "plumber" in the URL?
    The same reason you can't link back to terrylove from those sites.
    Last edited by Terry; 01-12-2008 at 09:44 AM.

  15. #15
    Moderator & Master Plumber hj's Avatar
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    Default fitting

    Look at Terry's drain picture up above and you will see what a plastic back to back fitting looks like. It is similar to a double combination Y-1/8 bend, but modified to comply with the back to back limitations.

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