I want to install a toilet in my upstairs bedroom. I was told that I cannot connect to the 3" vent line for the downstairs toilets. the main reason being that a vacuum will be created when we flush the upstairs toilet and cause the water to be removed from the downstairs toilets. also that any home inspector will not pass the permits with that kind of configuration. He recommended that I run a drain/vent line using the exterior cleanout port, up the exterior of my home directly to the room in question.

has anyone ever done this before? I cant seem to find any information online about this. what should I be worried about with this "exterior" drain? Any information would be helpful.

  • 3
    In the bedroom?
    – iLikeDirt
    Sep 29, 2014 at 0:23
  • 4
    Location/climate will matter. IME, very common to see exterior drains on British houses, but the climate is milder there and many were retrofitted into masonry houses built before indoor plumbing was invented. This sort of thing is essentially never seen in the Northeastern USA, as freezing would be a major problem in the winter (I guess it may be seen sometimes on summer camps, though that's more common "under" them than "alongside" - in either case, not functional for winter use.)
    – Ecnerwal
    Nov 29, 2014 at 18:20

1 Answer 1


In Illinois this wouldn't be a problem provided it adheres to everything on this list and fits these numbers. Your concern may be that the stack is not a clear vertical run to the roof or that the toilet flange will be more than 24" from it.

My codes expect 4" stacks to begin with. A lot of people seem to get away with 3".

b) Minimum Size of Building Drain, Horizontal Branches, Drainage Piping

1) The minimum size of any gravity building drain shall be 4 inches in >diameter. -Code

  • 1
    "seem to get away with 3" until it blocks at 3am on Christmas day!
    – Walker
    Jul 28, 2015 at 16:14

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