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Thread: Septic Connection for Half Bath in new outbuilding

  1. #1
    DIY Junior Member TomWS's Avatar
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    Default Septic Connection for Half Bath in new outbuilding

    I am building a workshop with a half bath (two sinks and a WC). The workshop is located about 75 from the existing house, which has a septic tank system. The septic tank and workshop are on opposite sides of the house and the workshop is about 5-6 feet in elevation above the house main floor and about 10 feet above the elevation of the ground level at the septic tank.

    Given these conditions, plan A was to trench and run the waste line from the workshop to the house foundation, cut through the foundation, and join up with the waste lines in the house crawlspace - using 3 inch waste line.

    AHJ says that we need to switch to 4 inch pipe once we're more than 10 feet from the workshop foundation forcing us to Plan B which would run 3 inch out of the workshop, join a 4 inch to run to the workshop side of the house, run the 4 inch line through the full length of the crawlspace, convert the lowest point of the waste lines in the house to four inch, including where it exits the house and joins the septic line, and join the workshop 4 inch line to the house 4 inch line near the house waste exit.

    This extra work is making me think that I might be better off installing a sewage ejector pump just outside the workshop, run a 2 inch flex line around the perimeter of the lawn (total length would be approximately 200 feet) and join the septic 4 inch line between the house and the septic tank. I'll call this Plan C.

    I've found no reference to plumbing code that would address this situation (two buildings joining to one septic tank, either by gravity feed or ejector pump). Plus, ejector pumps are typically used for uphill pumping, is downhill pumping allowed? Note that, as the 2 inch line nears the septic system, the ground levels off and may go slightly uphill so a pump is necessary. Is Plan C viable? Is it better or worse than Plan B?

    Tom

  2. #2
    DIYer, not in the trades LLigetfa's Avatar
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    I don't know where location 27312 is WRT frost, but that Plan C may need to consider it. Generally a line is pitched toward the tank so as not to trap sewage in sags.

  3. #3
    Consultant cwhyu2's Avatar
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    Pittsboro,NC

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    Consultant cwhyu2's Avatar
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    Quote Originally Posted by LLigetfa View Post
    I don't know where location 27312 is WRT frost, but that Plan C may need to consider it. Generally a line is pitched toward the tank so as not to trap sewage in sags.
    Pittdboro,NC

  5. #5
    Moderator & Master Plumber hj's Avatar
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    Most codes only require a 4" line AFTER you join a fourth toilet to the sewer line. Here, we can run 3" all the way to the sewer main, and do NOT have to transition to 4" once it leaves the building.
    Licensed residential and commercial plumber

  6. #6
    DIY Junior Member TomWS's Avatar
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    Would that be the case even if the fourth toilet was in the same building?

    I'm really trying to get a handle on the issues surrounding adding the half bath in another building. Will the ejector pump alternative be acceptable? Would "Plan D", where I run the Ejector pump into the main house crawlspace and join with the main 3 inch waste line work?

    Tom

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