My wife and I just replaced the vanity in our bathroom. To fit the new sink, we had to cut some new PVC. Here's a picture.


All the plumbing works, without leaks, except that it will not drain completely out of the sink. After the sink stops draining, there is still about 1/2 inch of water standing in the sink.

All of the pipes that look horizontal are almost exactly horizontal. Did we not include enough of a downward slope? If so, how would we do that?

  • Where does the drain go inside the wall? E.g. how far from the stack is it, and does it go uphill at any point? Also, do you have an adjustable stopper in the sink, and if so, does the problem still happen if you remove it?
    – BMitch
    Jul 25, 2011 at 17:58
  • The old drain went into the wall and I don't know where it goes -- we just used what was there and was working. No, there is not an adjustable stopper. It's just a straight drain. Jul 25, 2011 at 18:00
  • Is this new vanity shorter than the old one? Or is it deaper than the old one?
    – auujay
    Jul 25, 2011 at 18:02
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    What's with the large amount of plumbers putty, is there a junction there being held with putty?
    – Tester101
    Jul 25, 2011 at 19:11
  • 3
    There should be a compression fitting that screws onto the threaded pipe to secure the non-threaded pipe. It should be similar to the one below that where the pipe connects to the top of the trap.
    – Tester101
    Jul 25, 2011 at 19:47

2 Answers 2


This might not be the answer, but for reference here is a problem that can occur if the plumbing is not installed properly.

If you look at Fig. 3, this is what a proper drain looks like. You'll notice the orange line represents the water level in the system, the water levels out in the trap preventing sewer gases from entering the sink drain.

In Fig. 1, you can see what will happen if the drain line has to go up before meeting the main drain line. Again the orange line represents the water level in the system, and as you can see the water level is much higher in the drain.

Fig. 2 shows what could happen if the sink was installed lower than the rise in the drain line. The water would never fully drain from the sink, since it would require extra pressure in the system to push the water up the drain.

enter image description here

I wouldn't guess this is the issue in this case; since as you can see, if you opened the trap in this case you would have more water drain than what is typically in the trap.

In a perfect world Fig.1 and 2 would never pass inspection, but as we all know not all plumbing is properly installed and/or inspected. So this could possibly be the case in this situation (given the plumbing was installed by an amateur plumber), but it is doubtful. It would be more likely caused by a clog in the line.

  • Wow, this is a really detailed answer. Thank you for taking the time to write it out and draw those diagrams. My setup is like figure 3, so theoretically, it should work. Jul 26, 2011 at 1:59
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    @michaelkoss: I figured from your comments that the plumbing was probably correct, that's why I suspect the problem is a clog. Also for future reference, problems like this is why I never glue trap assemblies. I usually use compression fittings and threaded connectors, so the whole thing can be disassembled if there is a problem (or a ring or something falls down the drain).
    – Tester101
    Jul 26, 2011 at 2:10
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    @michaelkoss: Also, please get a proper fitting so you can remove that plumbers putty. That's a "works for now" solution, not a proper installation technique ;)
    – Tester101
    Jul 26, 2011 at 2:14
  • I assume Home Depot or Lowe's would have something to thread on there? How would I add it post-glue? I'm not opposed to cutting this out and doing it again if I need to. I'd rather it be right. Jul 26, 2011 at 3:07
  • +1 for the pics @Tester. I was thinking the same thing, hence my original comment. If it was a clog, I'd expect it to always be slow. The only thing left I can think of is an air bubble in the drain, hence my question about the adjustable stopper.
    – BMitch
    Jul 26, 2011 at 11:53

I see one issue that doesn't meet the plumbing code. Before you drain down in the vertical there should be a vent. An air admittance valve will take care of that and probably your dining issue too

  • 2
    The vent pipe may be in the wall. Jul 29, 2020 at 13:25

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