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?

Mark