I am installing double sink vanity replacing single sink. I need both drains to be on the left side of the stack. Which option is better? Option 1 or option 2? All the pipes are 1 1/2 inch. I would like to make sure I don't have drain problem and I am up to the code in Illinois. I saw that I have to maintain 42 inch distance from vent to trap. Assuming it is measured by trap arm length (3rd picture) I am going to be below 40inches so should be OK?

Update: I just found that around 5 inches below the hole there is actually another sanitary tee with cap and hole in the stud to the left (picture 4 below). It seems like builders started anoteher drain for 2nd vanity but never finished. I will open wall tomorrow and hopefully I can remove the cup and just run drain there.

Thanks enter image description here diagram with p trap enter image description here anoteher tee with cap few inches lower

  • "Better" under what metric? Option A looks much easier to execute. The 1-1/2" size caught my eye, but checking on it I discovered that it is sufficient.
    – popham
    Sep 29, 2023 at 20:58
  • Would like to make sure I don't have drain problems and I am code complaint (Illinois). Thank you
    – Arek
    Sep 29, 2023 at 21:05
  • Codes vary. I know that there's some goofiness in Chicago where electrical wire in the walls must be installed in conduit, so when you mention Illinois, alarm bells go off in my mind. The Uniform Plumbing Code says that a bathroom sink requires 1 drain fixture unit, and the UPC says that 1.5" pipe is sufficient for 3 drain fixture units.
    – popham
    Sep 29, 2023 at 21:13
  • You have to get the vent close enough to the furthest p-trap, though, where I think that number is 42" for 1.5" pipe. It looks like you'll be exceeding the 42" measured from the furthest P-trap. I don't know enough about plumbing to say for sure, though. hopefully somebody more knowledgeable will chime in.
    – popham
    Sep 29, 2023 at 21:14
  • where is the p-trap
    – Traveler
    Sep 29, 2023 at 21:20

1 Answer 1


Options 1 and 2 make no difference to the venting problem. Option 2 is likely easier.

With that setup, you probably need a separate vent pipe coming out to the cross-pipe. That would be a dry vent going up from the cross pipe and connecting across to the (hopefully also dry) vertical vent. If that vertical vent is a wet vent, you might well be in trouble trying to add another lavatory here.

You do appear to be limited to 3 feet, 6 inches trap weir to vent per Illinois code. And it appears doubtful that you can meet that with either setup for the distant lavatory trap.

You can't increase to 2" pipe unless you increase all the downstream drain pipe to 2" until it joins 2" or larger pipe. The vent itself could probably remain at 1-1/2." But it's almost certainly easier to add a dry vent arm into the cross drain.

  • thanks. I added in the original post distance from trap to vent and assuming it is correct I am going to be below 40inches. With that should be good to go with option 1? Thanks
    – Arek
    Sep 30, 2023 at 2:47
  • 1
    That shows 40" to an elbow to the trap adapter. That's not the same as 40" to the trap weir. Both the elbow and the trap tailpiece add additional distance to the trap weir. Looks like 3 or 4 inches from the turn to the trap weir on your presently installed pipe. So you'd be 2" over distance on the new one.
    – Ecnerwal
    Sep 30, 2023 at 2:55

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