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Thread: Quick question with DWV pipe size

  1. #1
    DIY Junior Member jwater7's Avatar
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    Question Quick question with DWV pipe size

    I have a (hopefully) quick question that I haven't been able to find a straight answer on..

    When I put drain pipe in for a lav, code (UPC 2009) requires 1 1/4 in. I've heard you can go up one "nominal" size (so I think that would imply going to 1 1/2 in.). What if you want to grow to 2" or even 3", is there code preventing this? Without getting into if it's really a good idea or not, I'm just wondering if it would pass inspection.

    Thanks

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    Retired Defense Industry Engineer jadnashua's Avatar
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    You'd be hard pressed to find really big, pretty lav fittings, but, once it's in the wall, it can become any size larger.
    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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    DIY Junior Member jwater7's Avatar
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    So to clarify, I could have a 1.5" trap/arm going into wall with a 3x3x1.5 santee, even if the lav is the only thing on the branch?

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    Forum Admin, Expert Plumber Terry's Avatar
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    Most of the time in Washington, we're using a 1.5" trap arm and p-trap.
    1-1/4" is the smallest, but you can't find that in ABS.
    The 1.5" trap arm can tie into any size that is the same or larger.

    I often run 2" over to a lav. That gives me options later on.
    The only reason I would run 3" below a lav is if it were used as a cleanout for the nearby toilet.
    Last edited by Terry; 01-28-2013 at 05:55 PM.

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    DIY Junior Member jwater7's Avatar
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    Great, thanks for the clarification

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    Test, Don't Guess! cacher_chick's Avatar
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    Much of the reason behind the pipe sizing is with regard to the flow of material through the pipe. If you use a 3 inch pipe for a lavatory, the flow would not scour the pipe which would allow excessive buildup of solids.
    Last edited by cacher_chick; 01-29-2013 at 09:11 AM.

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    Moderator & Master Plumber hj's Avatar
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    I seldom, if ever, use anything smaller than 2" for any drain lines. You COULD use 3", but would have to install a 3" cleanout opening for use if it ever plugged up. There is a "fine line" between being "large enough" and being "oversized" to the point that flow velocity slowed down which can cause stoppages.
    Licensed residential and commercial plumber

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