I am planning the install of a new half bathroom in an existing space in my home. There is currently one full bathroom, that is on the second story of the house. The soil/vent stack runs vertically down from the second floor, has a small horizontal section around grade, and then drops down below grade into the basement. Once in the basement there is a final horizontal run before a 90 down to below the basement foundation, and tie in to the sewer service line.

I am trying to determine the the most efficient way to plumb the new half bathroom, and if additional vents are required. Below is an isometric drawing of the scenario. Red lines are existing plumbing, green dotted lines are just to represent the plane that is the 1st floor grade, and blue is the new plumbing. Since the new plumbing lines (blue) will be less than 6' in length, do they need a vent? I was hoping to make my ties in(s) for the new plumbing horizontal because of a lack of vertical distance between the horizontal run in the basement and 1st floor grade. Tie in fittings would be 45 and Wye, or combo wyes. Install is in the Allegheny County, PA.

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Red pipe (existing) is 3". 3D sketch proposed interpretation to Fresh Codemonger

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1 Answer 1


What size is the red pipe? The trick here is the weir. If your sink p trap arm can be on the same horizontal plane as the red pipe then you can wet vent otherwise where the trap arm hooks into a vertical it needs a connection to the vent above that can not have a horizontal run until it rises above the flood plane of the sink.

Probably the toilet is fine. The sink you'd need a vertical pipe running up and hooking into a non horizontal run of the main stack or an aav (if allowed in your area) or an independent vent.

Hope that makes sense without a diagram.

  • Thanks for the reply. The red pipe is 3". I think I understand what you are saying. I can do a direct vent style tie in with the sink to the horizontal leg (mimicking the diagram on the right side of the image, but with a horizontal tie in). If that isnt possible, I will need to bring the vent up above the sink basin before going horizontal. The toilet can just be tied in direct to the horizontal run in the stack below since it is within 6'. I attached a 3D sketch in the original post - am I understanding correctly?
    – Chris
    Commented Mar 17, 2020 at 15:36
  • Yes you can do that as long as you don't exceed the critical trap arm length. JLC has a good article on it. jlconline.com/how-to/plumbing/… Commented Mar 17, 2020 at 16:21

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