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Thread: Island Sink Plumbing Questions

  1. #1
    DIY Junior Member pnosko's Avatar
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    Default Island Sink Plumbing Questions

    Hi all. I've read a few threads on the subject and have a question not covered in those threads. Even though AAVs are allowed here, I'm doing this from scratch so I'd rather do it the better way, assuming this is better. Going by this common diagram...



    My floor joists run the other direction. I'm doing this inverted U in a 5.5" space behind the cabinet (I put a 2x6 flat on the floor between the sink base and the cabinet behind it just for plumbing/electrical). So it needs to be just as shown above the floor, but below the horizontal runs to the vent and drain will head towards the front on the sink base. How do I change this, assuming the 45 connected to the T in the drain below the floor is rotated 90?

    In case it is relevant, the horizontal drain/vent will run about 8.5', then make a 90 and run another 11' or so to reach the main vertical drain (stack?) and vent.

    Also, could someone name those Ts to make sure I get the right ones? I think the 3 upper (vertical) Ts are sanitary Tees, and the 2 lower horizontal ones are Wye Tees. Correct?

    Finally, I think the horizontal drain needs to slope 1/4" per foot, but how important is it that the vent be sloped? Since both are running to the same area, I thinking the best I may be able to do is near level.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    I don't know if I can describe so you would do it correctly, but the "vent" Y would be downstream a ways, with a "street 45'' into it pointing back to where the vent comes through the floor. Then just connect the two points together.

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    DIY Junior Member pnosko's Avatar
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    Thank you so much for your reply. I think I know what you mean. But what about the pitch in the horizontal vent line? How important is it that it run downhill (vs. level) towards the sink since it is then going straight up anyway.

  4. #4
    DIY Junior Member pnosko's Avatar
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    Also, are those Ts at the very bottom wye Ts? I assume the 45 part of it should be at 12:00 orientation when I use a street 45 to get to the vent.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    The Y just has to be above the horizontal, and might NOT fit if it were at the 12:00 position. You will not have that tee on the vent because it will become a long sweep elbow into a combo or Y in the horizontal line.

    Last edited by Terry; 01-26-2012 at 05:29 PM.

  6. #6
    DIY Senior Member jastori's Avatar
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    You want some pitch in the horizontal portion of the vent so that any condensation / rain, etc. that enters the vents will drain out. You don't want any standing water in your vents. If the pitch wasn't sloped towards the sink, you would have a low spot where standing water would accumulate.

  7. #7
    DIY Junior Member pnosko's Avatar
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    Thanks all for the replies. As I was filling a Homer bucket with all the fittings, I ran into a trusted plumbing guy, and he talked me into an AAV. He said the inspectors here even favor them. I bought the only one they carry-- the Oatey 39016. Any thoughts about this one (is a Studor better)?

    On another note, I saw in another thread that the proper rate of pitch for the drain line is a min of 1/4" per foot and a max of 3" per foot (2" and 1.5" pipes). Should I aim for the middle?

    Where the old line ran, they installed braces to I guess both support and set the pitch, but the two braces are 65" apart and only 3/8" difference in height, so maybe this is why I had repeated clogging over the years (excluding the time I put the peels from 11lbs of potatoes in the disposer-- never again!!!)

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