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Thread: Need feedback on sink P-trap below floor level

  1. #1
    DIY Junior Member Buttonsrtoys's Avatar
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    Default Need feedback on sink P-trap below floor level

    I'm a homeowner taking on my first DIY plumbing job. I need to move a sink about 5 feet from one wall to another. Due to cabinetry and tight spacing, there's not much space to run the arm from the P-trap back to the existing stack at the conventional elevation. I've been reading up on the code online. I'm in Canada (Nova Scotia) and understand that the building code allows for a tailpiece up to 36" (longer than the 24" inches in The States). 36" is enough to put the P-trap in the floor space and run the arm between the joists back to the original stack. It would be a 4' run and I'd install an access panel in the basement ceiling below.

    Below is a sketch of what I have in mind. I've been reading up on siphoning, which I understand is a concern with a long tailpiece. I'm thinking that as long as my arm is a pipe-size higher than the P-trap, I should be OK. Someone online with a similar configuration considered adding two 45-degree bends to the tailpiece so slow down the water. Is that a good idea? Also, should I add a clean-out? Any feedback as to whether my design would work from a code perspective and otherwise would be greatly appreciated.

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    DIY Senior Member dlarrivee's Avatar
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    Why would you want a 3 foot long tail piece though?

    We try to keep tail pieces as short as physically/legally possible for sanitary reasons.

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    Forum Admin, Expert Plumber Terry's Avatar
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    A long tail piece allows the trap to siphon. That's why the code restricts the length now, and why you no longer see p-traps below the floor.

    You would be better off keeping the p-trap in the cabinet and adding an AAV.

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    DIY Junior Member Buttonsrtoys's Avatar
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    Quote Originally Posted by dlarrivee View Post
    Why would you want a 3 foot long tail piece though?

    We try to keep tail pieces as short as physically/legally possible for sanitary reasons.
    It wouldn't have to be 36". I was just saying that's what Canadian code allows. I just measured it. It would be more like 30", which is still long.

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    DIY Junior Member Buttonsrtoys's Avatar
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    Quote Originally Posted by Terry View Post
    A long tail piece allows the trap to siphon. That's why the code restricts the length now, and why you no longer see p-traps below the floor.

    You would be better off keeping the p-trap in the cabinet and adding an AAV.
    Thanks Terry. So you would go with an AAV over a Chicago Loop? (Not sure that's the universal term. Below is a pic from Wikipedia.)

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  6. #6

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    a 'c"chicago loop" doesn't meet our code requirements in Canada, and an AAV only meets code if installed above flood level rim of fixture and accessible

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    DIY Junior Member Buttonsrtoys's Avatar
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    Quote Originally Posted by mikeplummer View Post
    a 'c"chicago loop" doesn't meet our code requirements in Canada, and an AAV only meets code if installed above flood level rim of fixture and accessible
    Thanks Mike. So my original design meets Canadian code? Do you recommend any tweaks to my design? I'm hoping to tackle this this weekend.

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    DIY Junior Member Buttonsrtoys's Avatar
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    I've been reading up on my configuration some more and came across the sketch below. I would have thought a P-trap above the floor but vented below was effectively an S-trap. If not, I could install the P-trap in the cabinet space with a short arm then run the 2" pipe between the joists to the existing stack. If someone could confirm that's a better approach than my original sketch I would greatly appreciate it.

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    DIY Senior Member dlarrivee's Avatar
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    You're not listening, or you're just hearing what you want to hear... not sure.

    You cannot use either of those, there is no vent on them.

  10. #10
    DIY Junior Member Buttonsrtoys's Avatar
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    Quote Originally Posted by dlarrivee View Post
    You're not listening, or you're just hearing what you want to hear... not sure.

    You cannot use either of those, there is no vent on them.
    Not sure why you say I'm not listening. As I said in my previous post, I'd run 2" pipe between the joists back to the existing stack (where it would vent). The sketch below shows illustrates my question. The configuration on the left is my original configuration, which my understanding is that it meets Canadian code provided the tail piece is less than 36" (mine would be 30"). My question is whether it makes sense to elevate the P-trap as shown on the right. I would think the configuration on the right would be effectively an S-trap and more prone to siphoning than the config on the left, but the website I lifted this figure from said that if the trap arm is 2X the trap diameter, you're good. I'm looking for an opinion as to which of these meet code. If both meet code, which would perform better.

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    Forum Admin, Expert Plumber Terry's Avatar
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    That has to be some strange Canadian Code.

    My experiences with installed plumbing tells me that both of those arrangements will siphon the trap.
    In the real world, it's pretty easy to get a siphon going. Again, I would add an AAV and be done with it.
    If it isn't code in Canada, at least it's going to work in your home. And isn't that work something?

    I have also installed plenty of the island sink plumbing installation, what you call a Chicago Loop. I've never heard it called that before. For us, it's just called plumbing in for an Island Sink. Only if the last ten years have I used an AAV. They now make some nice ones in the $20 -$30 range. I have never used a "cheater" vent.
    Last edited by Terry; 11-03-2012 at 11:42 AM.

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    DIY Senior Member dlarrivee's Avatar
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    Terry this is not some strange Canadian thing at all, this is a guy who is picking a choosing which parts of the code he wants to apply to his situation.

    I've never seen this type of work installed in a home, so why are you trying to re-invent the wheel? Lots of folks run their drain inside the back of the cabinet instead of inside the wall, I'm sure you can do that too.

    Since when is the trap arm the distance between the trap and a 90* drop? It's not.

    You need to have a VENT for that fixture before it turns down. If anything the elbow will ENSURE trap siphon.
    Last edited by dlarrivee; 11-03-2012 at 01:41 PM.

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    DIY Senior Member dlarrivee's Avatar
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    Maybe this will help you understand why you cannot have a very long trap arm, or a drop in your trap arm.

    http://www.ashireporter.org/HomeInsp...Talk-Back/1851

  14. #14

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    you could try oversizing that line.

  15. #15

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    Quote Originally Posted by Buttonsrtoys View Post
    I've been reading up on my configuration some more and came across the sketch below. I would have thought a P-trap above the floor but vented below was effectively an S-trap. If not, I could install the P-trap in the cabinet space with a short arm then run the 2" pipe between the joists to the existing stack. If someone could confirm that's a better approach than my original sketch I would greatly appreciate it.

    Name:  P trap conversion.JPG
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    these are both s-traps....thats simple

    i also agree with Terry regarding the AAV....if it is in the cabinet and above the bottom of the sink, then it is a - accessible and b - if the sink clogs potentially gunking up the AAV it can be removed and replaced or cleaned as necessary.
    does it meet code? - no...is it practical? - absolutely

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