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Thread: main drain line sizing?

  1. #1

    Default main drain line sizing?

    I live in georgia and I am trying to plumb my own house, what is the min. size drain line under the house (crawl space). I know that from the house clean out to the septic tank should be 4", is the entire horizonal drain require to be 3" or 4"?

  2. #2
    Master Plumber Redwood's Avatar
    Join Date
    Dec 2007


    Either 3 or 4" is commonly used. Check with your local authorities.

  3. #3
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default pipe

    It depends on your local regulations and the number of toilets in the house. Here if 3" is adequate it can run all the way to the septic tank.

  4. #4


    Here, we're required to run 4" out under the footing, but other parts of the state have overlooked it and allowed 3" depending on toilets. 2 toilets = 3" required pipe size, and if one toilet is more than 15' horizontally from the others, 3" is allowed. But the pipe through the footing still has to be 4".

    Codes vary greatly on this, I'm sure. Personally, I'd want that third toilet to be piped into 4" even if I didn't have to, but I'm old-fashioned. :-)

  5. #5
    Plumber BAPlumber's Avatar
    Join Date
    Jul 2007
    Vashon, Washington


    What kind of house are we talking about. We could be looking at anywhere from a 3" building drain to 6" plus. Is this a 1 bath rambler or a 12 bath mansion.


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