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Thread: drain pipe sizes 3" or 4"

  1. #1
    DIY Junior Member filipi's Avatar
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    Default drain pipe sizes 3" or 4"

    Hello

    I am new here and have been reading different thread and learned a lot.
    Great forum and very useful.
    I am hoping to get some opinion from some of your experiences so I can make the right decision in the plumbing makeover that my house is going to get.
    Basically my main concern at this time is to make sure the right size pipe is being used on the toilets and main drain.
    They will be low flow (Toto) and I understand that 3" pipe is enough and may be even better although you can use 3" or 4" according to codes.
    This remodel is for a one story house so several line are going to connect to the main drain underneath the house, which I was going to make a 4"(about 10 feet before exiting the house crawl space) but I am not sure if I even need to do that until I actually connect to the sewer line since it seems fine to run 3" all the way.
    I will check with the plumbing inspector but was hoping to get some input from anybody willing to help.

    Thanks.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Some of it depends on where you live and the codes that apply. Each type of drain has a value called a fixture unit. The total fixture units determines the minimum drain line required.
    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 filipi's Avatar
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    Thanks, and I did calculate my total fixture units and came out to 24.
    According to the California code a maximum of 35 is allowed on a 3" drain, but there is a note that I have a hard time interpreting,quote :
    " and not to exceed 3 water closets or 6 unit-traps on any horizontal branch or drain "

    If I read this correctly I need to go 4" on the last 10 feet before exiting the house because pretty much all fixture come together at that point.

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    Forum Admin, Expert Plumber Terry's Avatar
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    With UPC, it goes to 4" when the forth toilet is added.

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    Moderator & Master Plumber hj's Avatar
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    Unless you have 4 or more toilets, or their equivalent, (and a house would almost NEVER have an "equivalent"), you only need a 3" pipe.
    Licensed residential and commercial plumber

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    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    If one were to use a 4" coming off the WC, might that allow for less slope and therefore give you more flexibility in placement of the closet bend?

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    DIY Junior Member filipi's Avatar
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    I did read somewhere , and I think it was a code , that you can go 1/8 of an inch slope on bigger pipe , but from what I've gathered from all the pros here and other references, it seems that 1/4 is the ideal slope no matter what size the pipe is , but obviously I am no plumber.

    Thanks for the answer regarding my question.
    I will go with a 3" drain pipe on both toilets , those will drain into another 3" main drain branch , 4 other 2" branch will drain into the main drain , and 3 other 2" branch will drain into one of the 3" toilet branch. All correctly vented of course.

  8. #8
    Moderator & Master Plumber hj's Avatar
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    some areas require a 4" line outside the house, even if 3" is adequate inside. Here we can run 3" all the way to the city connection.
    Licensed residential and commercial plumber

  9. #9
    DIY Junior Member filipi's Avatar
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    The main sewer pipe I will be connecting to outside the house is 4".
    But I thought it would be slightly easier to work with 3" underneath the house , and I was also under the impression after reading some quotes on this forum that 3" works as well or better than 4" specially with low flow toilet , ( "scourging effect") and that it might be even better in the long run to have 3".

  10. #10
    Forum Admin, Expert Plumber Terry's Avatar
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    A 3" line does have a better drain line carry. If it's my home, I'm running 3"

    4" handles more, but the average home has no need for it. Lines outside the home are typically 4", mainly I believe because it covers all expansion in the future. Once you hit "4" four toilets, it's required.

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    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Believe it or not, Terry, here in Columbus (at least the older parts) the line outside is 6 inches. Typically for even an older small one bath home to have a 4" and then it transitions to 6" just outside the footing. Overkill, no?

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