When I run the water in my bathroom faucet it sometimes (but not all the time) slowly starts to creep up the overflow hole and pour back into the sink. Included with the water is black and grey gunk.

The thing that puzzles me is that even when this happens, the main drain functions fine and the sink itself doesn't back up or start to fill with water (or at least it doesn't do so noticeably). From my limited understanding of how the sink is laid out under the surface (including the overflow) I can't for the life of me figure out where the clog is and exactly what is going on.

Should I pour bleach or draino down the main drain and overflow drain? Should I buy a snake and try to snake out the main drain and overflow drain?

3 Answers 3


I would start by removing the trap, putting a bucket underneath the tailpiece (below the sink) and just running some water and seeing what happens. Clean out the trap while you're at it and make sure there isn't a mess of hair stuck in there. If it all comes through without a problem and doesn't back up, then I would suggest buying a snake and going through the wall pipe and seeing what you can pull out. I've always been able to get a slow drain going with a snake. It might take a few tries, but it can usually work out. Make sure that you push the snake right up against the pipe opening, and then just keep cranking until something changes.

You want to be careful with chemicals because they can discolor the chrome around the sink if you're not very careful, and they can also react with the pipes in a unsafe manner. They make some gels that are more organic in nature that can eat away at organic clogs (hair, etc) that are much safer to use.

  • If the overflow is above the water level, why would it back up though running the water through the drain?
    – Steven
    Apr 11, 2013 at 15:11
  • Hey, my apologies for taking so long to mark this as resolved, I took your advice right away but blanked on coming back and taking care of business, my apologies! Worked like a charm, thank you so much for the help!
    – Michael
    Jun 26, 2013 at 3:35

I'm at a loss to explain how water is getting into the overflow area such that it runs out the inlet. Perhaps your basin has an unusual configuration that allows leakage from the faucet to collect in the overflow area. Not only is this an unusual configuration, but faucets should not be leaking anywhere as well.

Even lacking a reasonable explanation, one thing is clear. The overflow outlet into the main drain is obstructed. This is usually through small slots in the top of the tailpiece that aligns with the overflow area in an interstitial space between the inner and outer basin surfaces. Grease, hair, and soap scum often build up around the stopper assembly that occurs in the same area. Some of this will end up getting through these slots. Attempts to clear the stopper area of debris can push more material into the slots, compacting the material into an impenetrable blockage. The main drain will work fine, but the overflow will only collect so much before it fills and stops working.

The best way to clear overflow blockage is to disassemble the drain assembly. Short of that, the slots can often be mechanically cleared somewhat with a stiff hooked wire or similar tool. Once loosened, the debris can often be flushed out by running water into the overflow. Drain cleaner may also clear the blockage, but I would be concerned the heat generated could damage the basin. Bleach will kill off the black mold that grows on this stuff, but will not help actually clear the blockage.

Once the overflow outlets are functioning properly, nasty stuff will no longer come out of the overflow inlet, but this does not solve the issue of how water was getting in there in the first place. It will still be flowing in there, but now it will pass into the main drain unnoticed.


Check your pop-up and clean it. You might need to replace the pop-up. Snake the drain and clean the sink before installing a new pop-up.

  • 1
    Why would this result in water backing up through the overflow inlet instead of back up the drain?
    – BMitch
    Apr 21, 2013 at 0:27

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