I just discovered the faucet connection inside the vanity is leaking. The leak appears to happen at the top of the connection (where the black pipe meets the white of the vanity. What could be causing it, and how can I fix it? Photos: enter image description here

  • FYI. I believe It is not the faucet connection that is leaking, it is the drain connection. Did you install this ?, if so did you use plumbers putty on the top side under the drain flange ? Have you tried tightening the big black nut that compresses the rubber gasket to the underside ??
    – Alaska Man
    Jul 4 '19 at 19:16
  • This happened after I hired someone to replace the old vanity top with new. I don't know what they used. I tried turning the nut, but it appears to be tight all the way. Should I turn off the water if I want to experiment with opening the nut?
    – laconicdev
    Jul 4 '19 at 19:42
  • No need to turn off the water because the drain connection is not part of the water supply to the faucet. If the nut is tight then it is most likely that there is no plumbers putty on the other side of that connection, if you look in the top of the sink you’ll see the chrome metal ring down against the bottom of the sink, the installer should have put plumbers putty underneath that silver or chrome ring.
    – Alaska Man
    Jul 4 '19 at 20:28
  • My answer below addresses what has to be done, but now that you shared you hired someone, they should come back and do it right. It wasn;t done right. Also, since the vanity top is new, this emphasizes my point about the basin & top being level. You can get it leak free even if not level, but its far better in every way to get it level, then reconnect the drain.
    – noybman
    Jul 4 '19 at 21:05

First observation I see is that the basin is most likley not level as the the black drainpipe is not entering the trap flush. (it is cocked sideways and this is most likely because the basin isn't level). So to this note, is the cabinet firm and secure? Is anyone (small child or a teen) climbing up onto the countertop or basin? If so, put an end to it. If water is being allowed to sit on the back of the counter, it will over time rot the back portion of the wood which will weaken it, allowing the same thing to occur. You should clean out under the sink, get a bright flashlight and check for any water damage or areas where the counter is separated from the backsplash or wood supports. *(sometimes these are just wooden blocks glued on).

Once you've fixed or confirmed the stability and levelness of the basin, remove the sink drain and black pipe. Check the drain/pipe for any cracks, replace if necessary. Check the seal/quality of the black rubber/foam gasket, replace it if necessary, and clean all surfaces. Also, pickup some fresh plumbers putty at the local hardware store and apply fresh new putty graciously under the drain where the drain and portion that penetrates into the basin. This will help tremendously.

Reassemble everything, firmly, (tight but not overtightened), and clean off any excess. Make sure putty does not impede or protrude fromthe botton of the basin before you re-secure the washer and black nut. If you overtighten it, the drain, the nut, the tubing, may crack and you'll have to replace it.

Once reassembled, everything should be square, and you should be leak free. If not, inspect your work and redo it.

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