Please see attached sketch for more details, but we want to avoid cutting through the joists below the bathroom and also avoid creating a bulkhead in the family room below. We are located in Toronto, Canada.

We are thinking of using a wall mount toilet with an in wall tank from Geberit. Their catalog shows an elbow connector that appears to allow you to run the toilet drain (I assume this is a 3 1/2" pipe) at an angle instead of straight down through the floor. Can we then run the drain horizontally through the bathroom wall (approx 6 ft) and then down the floor into the mud room? If 6ft is too long what is the maximum length we can run the pipe horizontally (not sure if it needs a slope along the run)?

We would also like to connect the vanity drain in the bathroom to this drain. Is that doable? There is another bathroom directly behind this one and can the drain for the vanity in that bathroom also connect to the same drain above?

Hoping the experts in the forum can provide some insights for us.


enter image description here


4 Answers 4


That 6’ run cannot be horizontal - it needs to slope, so either the wall end has to drop or the toilet end needs to be higher.

Your locality will specify the amount of slope in the regulations, however, some I have seen have a drop of about 6” across that distance. If the slope is too shallow, then things get stuck...

  • "Things" will get stuck......it is rather easy to envision what is likely to get stuck.
    – Michael Karas
    Aug 26, 2019 at 4:12
  • @MichaelKaras I was trying to avoid the “bad” words filter... :)
    – Solar Mike
    Aug 26, 2019 at 4:15

What kind of studs are you planning in that common wall where the intended sloping horizontal pipe will be placed?

If this is a typical wall with 2x4 studs (nominal 1.5 b 3.5 inch dimension) then there is no feasible way to run a 3.5" diameter pipe horizontal through the wall. There are several ways this situation is handled:

  1. Use of a wider studs in the wall. 2x6 studs may work but will be very marginal with very little material left on each side as a hole is cut to pass the 3.5 inch drain line.
  2. Use a double framed wall with a blind space left between the two walls to run plumbing through the blind space. Sometimes one of the two walls can be framed with the studs turned flat to the wall surface of the wall is one with minimal lateral loads placed upon it.

I'm not trying to be funny with this, but it sort of is:

enter image description here

If it's done here, I'm certain yours will be ok, as long as your local codes allow it.

  • The horizontal room will be sloped at 1/4" per ft. Is that sufficient? Also can the 2 vanities drain into the same horizontal drain?
    – user105375
    Aug 26, 2019 at 4:23
  • 1
    I imagine that for efficiency, the water you wash your hands with will be used for the next flush. Aug 27, 2019 at 20:44
  • Not certain about that 🙂 but a thumbs up goes to you for having the intuition 👍 Aug 27, 2019 at 23:19

1/4" per foot is recommended--so 1.5" drop across the six foot run. Steeper than than can cause issues-- the water drains too fast and the poo dries out and eventually the pipe is full of hardened poo.

Yes, the sink can tie into the same drain. Be sure to use a trap! Personally, I would make the sink connection to the drain as close as possible to the vertical.

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