Venting for 16' underslab drain

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Mark Fleming, May 19, 2008.

  1. Mark Fleming

    Mark Fleming New Member

    Oct 25, 2004
    I'm in Washington (Uniform Plumbing Code), have my building permit, and am getting ready to pour the footings. My first floor will be slab on grade and has a bath and kitchen. The kitchen sink will be 16 feet from tying into the main line to the septic (4"). After looking at my plans, I realized that there would not be any vent in the line after leaving the sink wall. I planned on using 2 inch line.

    I called the county inspector and asked if that line would need to somehow have an additional vent in it (there are no walls between the sink and the connection to the main 4" line. He said no, one vent at the sink was enough, but that he wasn't a plumbing guru and that I would need to ask my inspector during the footing inspection. I can't wait till then. I always get the feeling that inspectors are used to wood frame construction, where almost always mistakes can be torn out and redone. Not so easy after reinforced concrete is poured.

    So my question is "If I go to three or four inch pipe for the run under the slab, can I get by with only one vent at the start of a 16 foot run for a kitchen sink?" I can't imagine the sink ever producing a flow in a 3" pipe that could evactuate a p-trap, but maybe there are other issues?

  2. Redwood

    Redwood Master Plumber

    Dec 15, 2007
    Service Plumber
    The only vent needed is at the sani-tee where the sink trap connects. No vent is needed on the line in the slab. The purpose of the vent is to protect the trap seal.

    If you do run 3" under the slab a full size 3" cleanout is required.

    A 2" line is sufficient for a kitchen sink.

    2" is a minimum pipe size underground.

    The sink should connect to the main on a horizontal plane using a Wye and 45 or, a combo.
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  4. Mark Fleming

    Mark Fleming New Member

    Oct 25, 2004
    If I understand this, I put in a vent T in the wall behind the sink and then the length of run under the slab doesn't matter. I thought that I had to have a vent every so many feet. I based this on my inspector for my first building making me put in another vent because my horizontal line ran more than 5 feet. I had a vent T right at the bathroom lav, the drain ran around the wall some distance and another vent where the drain dropped through the floor, but the final run to the stack/vent was more than 5 feet. He made me put in a third vent, which required building a chase wall and dropping the ceiling 6 inches. You're saying that one vent at the start of the drain's run is enough? It does make sense that one vent at the start precludes evacuation of the p trap, the whole purpose of the vent.

  5. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona

    We really don't know what you are asking because we cannot see where, or if, you have a vent at the sink. If you do then that is the only vent you need. If you do not, then it makes no difference where you put any other vent because it will not be a vent for the sink.
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