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When I turn on the water to my double-vanity bathroom sinks, water begins pooling within the bowl after only a few seconds. The drain lines below the sinks appear to be clear (there is no debris in any of the pipes I could access when I took apart the P-traps) and the problem occurs even if I remove the drain stoppers completely.

Each sink bowl has its own 1.25-inch P-trap, both of which tie into a 1.5-inch vertical PVC pipe that goes into the floor. There is also a vertical vent line going up through the wall, although I can't see where it connects to the drain line going into the floor (in fact, I don't know for sure that they even are connected, but that is the only reason the vent stack would be there, so I'm assuming they are).

I installed this double vanity myself in place of a single vanity. I didn't make any real modifications to the drain line when I did that; I just installed P-traps below my two new sinks and tied them into the vertical line that goes into the floor, which was already there.

I'm wondering if I needed to install a separate vent stack for each sink when I converted to a double vanity. I'm also wondering if there could be some kind of issue where each sink in the vanity wants to vent out of the other sink instead of using the vent pipe in the wall. Basically, I don't understand enough about how venting works to figure out if this is indeed the problem, and if so, what specifically is not right.

I'll hire a plumber if I can't figure this out, but I'm hoping to get insight here that will save me that expense.

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  • Can you add pictures of the plumbing under the vanity?
    – UnhandledExcepSean
    Commented Feb 4, 2019 at 20:58
  • What is the distance of the horizontal trap arm, measured from the end of the P-trap to the point where it TEES into the vertical drain line? A 1.5" common drain pipe should be adequate for your two 1.25" drain fixture trap arms.
    – Chris Taylor
    Commented Feb 4, 2019 at 21:02
  • You COULD test with a large bucket(s) under the final pipe point where it exits just to make sure that you do not have obstructions prior to that point.
    – Mark Schultheiss
    Commented Mar 5 at 16:30

2 Answers 2

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Going from a single sink to a double should not cause any problems. With the vent being close less than 6' from the drain that should also be ok. When I plumb double sinks I only use 1 trap, you mentioned 2 traps so the sink that is further offset from the drain may not drain as well. I would reduce down to 1 trap just prior to the wall and see if this solves your problem.

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Maximum trap arm distances, from trap to vent

Try cleaning the drain pipe with a snake. If it is clear then it may be a venting problem.

Refer to image, above. If the vent is too far away from the trap you can increase the trap arm size from 1.25" dia. to 1.5" dia. This may help.

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