Quick question with DWV pipe size

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by jwater7, Jan 28, 2013.

  1. jwater7

    jwater7 New Member

    Messages:
    8
    Location:
    Washington
    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
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,261
    Location:
    New England
    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.
  3. jwater7

    jwater7 New Member

    Messages:
    8
    Location:
    Washington
    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?
  4. Terry

    Terry Administrator Staff Member

    Messages:
    15,283
    Location:
    Bothell, Washington
    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: Jan 28, 2013
  5. jwater7

    jwater7 New Member

    Messages:
    8
    Location:
    Washington
    Great, thanks for the clarification
  6. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,259
    Location:
    Land of Cheese
    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: Jan 29, 2013
  7. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,021
    Location:
    Cave Creek, Arizona
    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.
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