Wet venting basement bathroom thru a portion of the upstairs vertical drain line?

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Mtl, QC
Hi there,

Currently in the midst of a bathroom remodeling project and for a few weeks now I have been looking up information on plumbing do's and don't and naturally since this forum came up a lot in the results, it's the first place that came to mind for some of the questions I am still having difficulty finding clear answers to.

For a little insight, the project is a full bathroom addition to the basement (well there was a half bath, but it's a whole redo since the existing layout didn't work at all with what's planned), as well as the remodeling of the 1st floor bathroom to my own house. Below is a diagram of the existing drains & vents as well as the additions that are being considered in the new configurations. Also attaching the PDF version of the image, since it seems a bit blurry in the preview.

Do note that it is not to scale and an isometric view is used to make it easier to distinguish between levels; not all lines run orthogonal to each other, no transitions shown, etc.

My first question is regarding the vertical drain pipe leading to the basement; would wet venting thru that line be allowed / proper practice given what it services on the floor above or would that just be asking for trouble?

On that the assumption it is allowed, is the vent as drawn even necessary for the basement sink?

What about the basement bathtub? Or is it's drain line to the Ø3" line at ±10'-0" away too far for wet venting?

For what it's worth, in the current configuration, the basement toilet is vented only by that drain pipe and seems to have worked well enough for the last decade; and that is without the aux. vent that is being considered (from the 1st floor to the main existing vent thru the attic), so it's essentially currently being serviced by a very long wet vent, the way I understand it.


Thanking you all in advance.
Do feel free to give feedback on anything that could be improved in the planned configuration.


  • BathroomsRemodelingDrains&VentsDiagram.pdf
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