we are in the process of modifying some rooms and we have an existing 3" pipe with a 1 1/5" drain pipe as shown here:

enter image description here enter image description here

We're trying to add a sink on the side currently not serviced by the drain pipe and as far as I can tell we have a few options:

  1. The most difficult option, but probably best, would be to cut it all out and at the bottom, use a wye with a 2" coming off it do service two sinks but that would be really difficult at this stage of the project.

  2. Outlined in red below: 1 1/2" pipe for all connections, add a double sanitary tee at the top of the pipe, optional mechanical valve.

  3. Outlined in blue below: 1 1/2" pipe for all connections, replace the top tee with a double sanitary tee.

We're in Michigan, is any of this is good or up to code.

enter image description here enter image description here

additional diagram from comment: enter image description here

  • Do NOT attempt to cut into that T in your 3" stack as you've shown in your "Blue" drawing. You'll ruin the T and end up having to replace even more pipe. What is the pipe sticking up out of the floor in the bottom left corner of that picture? Is that another drain? If so, feed the new sink into that.
    – FreeMan
    Oct 31, 2019 at 20:12
  • Thanks for the comment! It's not shown, but it's a very short run of pipe in an open wall for the blue drawing - I thought it might be preferable to do that since it's vented right there and it seemed like the correct place. I added a comment below with the picture here based on red - i.imgur.com/adVlOhz.jpg - does that seem reasonable?
    – James F
    Oct 31, 2019 at 21:48
  • I just noticed... the flanges of the fittings in the 3" stack are all ground flat. I presume that's to make them fit within a 2x4 wall. This is all waste, so there's no pressure on a potentially weakened fitting, so it probably won't burst, but I'm not sure that's to code. Was it like that when you opened the wall?
    – FreeMan
    Nov 1, 2019 at 18:02
  • It was not like that, that's all new fittings, it was unfortunately done by drywall folks who have since been let go. We haven't gone the rounds with an inspector yet, since this was done post inspection.
    – James F
    Nov 2, 2019 at 0:15

1 Answer 1


I can't speak to code, but what you'll want is a sanitary cross, or sanitary tee to do this. It should be legal in most areas.

The real trick here is you really need some way to allow air in. The best solution is to cut a wye or tee into the main stack further up and run pipe up back down into the new sanitary tee for your sink drains. If you can't do that, it should be acceptable to put an air admittance valve into the wall and add a vented access panel on one side (they should have had one on the sink you removed).

From this PDF

Sanitary Tee

  • Thank you, informative PDF. I've taken the red option and showed where it can connect to the 3". Does this seem correct? The pipe on the right is a vent, this is top floor, nothing above. Are 1 1/2" lines ok? I can't tell from DFU (bathroom sinks) i.imgur.com/adVlOhz.jpg
    – James F
    Oct 31, 2019 at 21:47
  • I think 1.5" drain lines are fine, but you need this small stack vented in some way. The main stack is undoubtedly vented.
    – Machavity
    Oct 31, 2019 at 22:06
  • "Both major plumbing codes in the U.S. permit a single basin lav to be roughed in with a 1.25” vent and a 1.25” trap. See IPC Table 709.1 or UPC Table 702.1." – hammerpedia.com
    – Mazura
    Nov 1, 2019 at 2:45
  • A fixture drain within 5' of a waste stack doesn't have to be vented. - Vanity sinks are 1 (maximum) fixture unit... 1-1/2" pipe is 3 MFU horizontal and 4 MFU vertical.
    – Mazura
    Nov 1, 2019 at 3:11
  • Thanks, that is clear, I'll see about that as I'm having a plumber come out just to give a final answer.
    – James F
    Nov 2, 2019 at 0:15

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