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Thread: shower to tub drain issue ( 1 1/2 to 2 p-trap)

  1. #1
    DIY Junior Member clapfc's Avatar
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    Default shower to tub drain issue ( 1 1/2 to 2 p-trap)

    I hope this is an easy one.
    I am remodeling my bathroom & switching the shower to a tub and the tub to a walk-in shower. In the shower to tub area I have a 2" drain pipe coming from the wall properly vented, I extended it by attaching more 2" PVC and a 2" P-trap. I lined up the 2" P-trap to where the tub tailpipe will come down. The tub (Whirlpool actually) has a 1 1/2 drain. So the question is this, can I have the 1 1/2 tub tailpipe (using a PVC adapter) go into the 2" P-trap right above the trap. Will this cause me any drain problems. The sub-floor is not yet nailed down so I can switch the 2" P-trap to 1 1/2" and move the 2" to 1 1/2" adapter to the vent side of the P-trap if it would make a difference.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    I'd leave the 2" trap. Code allows you to go from a smaller drainage pipe into a larger one, but not the other way around (a toilet is an exception if done right at the flange). You shouldn't need a bigger trap there, but the only bad thing is that it is deeper, and in some joists, that height is critical. Since it's already there, and you seem to have room for it, go for it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Forum Admin, Expert Plumber Terry's Avatar
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    The 2" trap is fine for the tub.

    For the shower, unless you change the trap arm back to the wye or tee, you will have 1.5"
    They used to do that in the 50's
    You can use a 1.5" trap there and increase with a reducing coupling below the shower drain. It becomes a funnel on the vertical.

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    DIY Junior Member clapfc's Avatar
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    Thank you for the reply to my issue. My fear was that the 1 1/2" pipe would have less force to push the waste water through the 2" p-trap causing a slow drain (or worse no drain). The last think I want is my wife telling...." see, I told...you should have called a plumber." Thank again, clapfc

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    DIY Junior Member clapfc's Avatar
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    Thanks for the advice on the tub to shower, which is a reverse of the shower to tub issue. On the tub to shower I will have to go from a 2" drain to a 1 1/2" p-trap. That part of the sub-floor has not been full taken up yet. I am trying to tackle the project one area at a time. My plan is to pull the sub floor to expose the tee and try to change the whole thing to 2". If I have problem I will be back. Thanks again

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