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Thread: Lavatory drain size

  1. #1
    DIY Junior Member mrd's Avatar
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    Aug 2010
    de, us

    Default Lavatory drain size

    Following the IPC, Lav = 1 DFU, and using Table 710.1(1): 1-1/2 drain @ 1/4in takes max 3 DFU, whereas 2in @ 1/4 slope takes 21 DFU max. I know greatly oversized pipes are more likely to clog.. but 2 isn't greatly oversized from 1-1/2. So which is statistically less likely to clog?

    The table reads 1-1/2 is plenty for a lav.. but does this translate practically to the real world? Wouldn't a 1-1/2 lav drain be more likely to clog than a 2in? Or would the scouring action in a 1-1/2 keep it cleaner than the 2?

    Drain clog statistics would be interesting to look at..

  2. #2
    In the Trades Gary Swart's Avatar
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    Sep 2004
    Yakima WA


    You think a 2" pipe isn't that much larger than a 1-1/2" pipe? Basic high school math taught us that the area of a circle is Pi (3.14) times radius squared. That means a 1-1/2" pipe has an area of 7.065 square inches. A 2" pipe has an area of 12.76 square inches...that's almost twice the size. I don't know if that really answers your question, but it's a stat that may be relevant. Apply the same formula to water pipe and you may be amazed at how much the area is increased by going for 1/2" to 3/4" for example. We tend to think that it's only 1/4" inch difference or in your case, 1/2" difference, but when the radius is squared the difference is quite apparent. Yeah, I know, you probably thought you'd never have a use for that math stuff when your teacher taught the lesson. I fought that sort of thinking for 30 years as a teacher. You'd be surprised how many times former students have said to me, "Why didn't you make us learn that?" Well, I tried everything but sever beatings, and even that was tempting at time. LOL.

  3. #3
    Forum Admin, Expert Plumber Terry's Avatar
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    Aug 2004
    Bothell, Washington
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    A two inch drain will be easier to take care of later.
    And if you ever want to add on, you will be able to do more with the 2" drain. Many plumbers will run 2" to the tee, with 1.5" for the vent and trap.

    And Gary, the area was very interesting when we had to start learning "cross sections" for the inspector. There is a whole lot of things taught in school that make a lot more sense now that I'm out of school. I had no idea how many cool things I would use math for.
    Who woudda thought?

    My favorite thing is sports stats, but I also make some fun spreadsheets for the business too.
    Last edited by Terry; 12-02-2010 at 02:59 PM.

  4. #4
    Master Plumber-Gas Fitter shacko's Avatar
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    Jan 2006
    Rosedale, Md


    From the people I've run into in the trades I don't think they teach anything that pertains to math.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    Aug 2004
    Cave Creek, Arizona


    1. it is easier to clean a 2" pipe than a 1 1/2" one.
    2. it would take TWICE as much "debris" to clog a 2" pipe.
    3. The water flowing from a lavatory sink is NOT enough to create major velocity changes in the two sizes.

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