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Thread: P-Trap below floor?

  1. #1
    DIY Junior Member Sinatra68's Avatar
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    Default P-Trap below floor?

    OK, so I've done a lot of research on this but can't find a definitive answer. What is the best way to tie in a bath sink (on the second story) to an existing waste pipe that sits under the floor?

    I was originally going to use an S-Trap, but am reading those are illegal, so my only other options are:
    (1) to tie in the P-Trap directly under the sink and then run straight down (is this a modified S-Trap?), or
    (2) run straight down through the floor first and put the P-Trap under the floor.

    Neither seems ideal, but was wondering which is better and the pros and cons to each. Are there other options? FYI, either option will be vented below the floor.

    Thanks in advance!

    Jim

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    Neither solution...

    While the suggested maximum drop is 24" before a trap, you still need a vent, so you'd likely have problems just running it down. Keep in mind, some crud will accumulate in the pipe before the p-trap, and a long run can really start to stink after awhile, so that isn't a great idea even if you could make it right.

    The correct way to do this is connect to that line (well, where that goes and what it serves really needs to be determined first), and run it up the wall behind the sink and continue up for a vent. Then, put a P-trap beneath the sink and run a trap arm into that new vertical line you installed in the wall.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member Sinatra68's Avatar
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    Default

    Thanks for the input Jim, didn't realize those long runs through the floor would get smelly over time - definitely not something I want to deal with.

    Shouldn't be too tough to run a pipe behind the wall (I actually have to connect two sinks) and tie them into it using the P-Traps. I'll tie the vent out at the front of the line so it can support both sinks and then drop the line through the floor after the second sink and tie to the existing waste line.

  4. #4
    Moderator & Master Plumber hj's Avatar
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    Default sink

    Are you trying to test us? First you have one sink that you intend to connect somewhat illegally, so we tell you how to do that. Then you surprise us with the information that it will really be TWO sinks, and now we are not sure, given how you intend to connect them, if THEY will be done properly.

  5. #5
    DIY Junior Member Sinatra68's Avatar
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    Default

    Quote Originally Posted by hj View Post
    Are you trying to test us? First you have one sink that you intend to connect somewhat illegally, so we tell you how to do that. Then you surprise us with the information that it will really be TWO sinks, and now we are not sure, given how you intend to connect them, if THEY will be done properly.
    No, hj, I was not testing anyone - far from it, just trying to get advice. Certainly did not mean to upset anyone.

  6. #6
    Forum Admin, Expert Plumber Terry's Avatar
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    If you have a double lav set up. You will need a 2" waste line into a 2x1.5x1.5x1.5 fixture cross.
    The vent and trap arms can be 1.5"

  7. #7
    Moderator & Master Plumber hj's Avatar
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    Default upset

    I was no "upset" I was resorting to hyperbole. But two sinks can change the entire parameters compared to a single sink.

  8. #8
    DIY Junior Member Sinatra68's Avatar
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    Thanks Terry - picture is great, exactly what I was intending to do after Jim replied.

    hj - understood. All I can say in my own defense is that if I knew what questions to ask and which info was important, I would not be here. It's a case of not knowing what you don't know.

    The reason I did not mention the two sinks originally was because there is a cabinet between them - so if it would have been OK to drop the traps under the floor I was going to treat them separately. Obviously, that is not the case now and I will simply tie them together behind the wall and then tie in the existing vent to support both (as shown in Terry's pic).

    Bottom line is I want to do things the right way the first time and I appreciate everyone's time and help in getting me to that end.

    Cheers!

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