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Thread: P trap size

  1. #1
    Writing, constructionDIY Member Yersmay's Avatar
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    Default P trap size

    I am installing a new tub. The sewer line and hookup is new also... The waste and overflow items are joined together by a 1 1/2 inch San Tee. The trap is positioned directly beneath... The trap arm (horizontal piece after the P trap) will be connected to a nearby 2 inch comby which is part of the main 4 inch drainage line. I am assuming it would be a good idea to increase the drain size from 1 1/2 to 2 inches BEFORE the trap, rather than increasing the pipe size in the trap arm. Correct? Thought I'd better ask just in case there's some wrinkle to this I would never have anticipated. Thank you in advance!

  2. #2
    In the Trades Gary Swart's Avatar
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    I think you've got the right idea. One thing you should never do is to decrease the size of a drain, you should either stay the same or increase as you go toward the main drain.

  3. #3

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    In my area of NJ shower traps are required to have 2" drains and traps. Is yr tub drain 2"?
    (important note: I'm not a pro)

  4. #4
    Writing, constructionDIY Member Yersmay's Avatar
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    Prashster,

    I was sort of surprised to see that the waste, overflow, drain pop up mechanism came equipped with 1 1/2 inch fittings. Also included was a 1 1/2 inch santee. The brand is Geberit. This all fits onto a Kohler Villager. This is all fine, I guess, except that I configured the comby to be 2 inch... So it's just a matter of where I place the fitting that expands the pipe size. Sounds like my plan to expand the pipe size above the trap will work.

    Thanks for your input.

  5. #5
    Homeowner geniescience's Avatar
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    it is normal for Geberit to have made 1.5" pipe; this is the only size for a tub.

    Where you increase to 2" will not make a difference, in my opinion. It may be true that the drain will drain more quickly when 2".

    I have a Geberit drain assembly, and a 2" P-trap. The pipe gurgles more (when draining) than the previous tub did. Maybe it is because of the wider diameter pipe. Maybe it is the new shape / configuration. I'll never know.

    David

  6. #6
    In the Trades Gary Swart's Avatar
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    A shower requires 2", but your post said this was a tub, no mention of a shower. In the real world, there are many tub/shower combos that have only a 1-1/2" drain and work OK. I'd got to 2" as soon as possible, but you can't change the size of the tub's outlet.

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