This pipe has a very slow leak. What is the pipe for and what is the best way to stop the leak?

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  • 2
    If you haven't already unscrewed that cap, note that you'll probably have a gusher of waste water when you do. While the cause of the drip is very probably a bad cap, it probably wouldn't be an issue if you didn't have water backing up to that point. So consider using some drain cleaner. (But then, of course, if it doesn't fix it and you unscrew and have a gusher, then you have a caustic gusher, which is very bad.) Oct 4, 2020 at 22:49
  • 6
    it might be better to try mechanical means of clearing the drain frist, eg: plunger or snake,
    – Jasen
    Oct 5, 2020 at 1:13
  • Given the presence of an electrical outlet under the sink, the odds are pretty good that the lower outlet was for a since removed, in-sink garbage disposal unit (aka garburator / insinkerator) installation. A lot of municipalities have banned them in favor of green waste collection programs. Take @JACK 's advice re:bucket, wrench, Teflon tape, but only a few rounds is sufficient.
    – Ian W
    Oct 6, 2020 at 8:46

3 Answers 3


It is a drain cleanout. You can try turning it clockwise to see if it tightens enough to stop the leak. If not, have a bucket or pan to catch any spillage, turn it counter clockwise, remove the plug, clean it off and the inside of the pipe. Wrap some Teflon tape around it and screw it back on.

  • What does the teflon tape do?
    – stevec
    Oct 5, 2020 at 15:52
  • 12
    @stevec Teflon tape fills in the gaps on threaded connections so that nothing leaks out. Anytime you are attaching or reattaching any kind of threaded connections in your plumbing, you will need to apply new teflon tape to prevent leaking.
    – BlackThorn
    Oct 5, 2020 at 15:57
  • 3
    Also don't skimp on Teflon tape. They are designed to compress and fill in the gaps of the thread. If you apply too little, you'll have to take it out, clean off the old tape, and re-apply. On something this size I would go around the cap at least 10 times.
    – Nelson
    Oct 6, 2020 at 6:34

There's no way for us to know; it is unusual. Possibly your drain was re-configured at some point, and that closed-off pipe is an old drain which had some trouble.

I guess your real question is how to stop the leak? Unfortunately, if that pipe is full of waste water, opening it may cause a mess; so be prepared for that -- have a bucket that can fit under it, shop vac ready, towels, a pair of waterproof gloves, etc.

I'd open the seal using a pipe wrench, drain it, then apply pipe joint compound, often called pipe dope to the threads of the plug, then screw it back in. That should stop it from leaking. The pipe joint compound is a waterproof compound that forms a gasket-like seal to stop water from leaking out of clean-outs and similar seals.

If the above doesn't fix the leak, the threads of the plug or pipe could be damaged; but that's unlikely, the fresh pipe dope will probably fix the problem.

  • If it's an old drain that was re-run, there shouldn't be water inside it. It's much more likely that it's still connected to the current . Although it's also possible that it's not actually leaking, and being cast iron it's just a case of condensation that's causing the damage to the drywall around it.
    – Joe
    Oct 5, 2020 at 17:20
  • Given the presence of an electrical outlet under the sink, the odds are pretty good that the lower outlet was for a since removed, in-sink garbage disposal unit (aka garburator / insinkerator) installation
    – Ian W
    Oct 6, 2020 at 8:47
  • I have the same suspicions; but I also used to live in a house with really screwed up kitchen plumbing. Someone had re-routed the drains to a nearby river and left all the old drains disconnected but still in place. I figured it best to caution the person and make suggestions about how to prepare in case it's something wacky! Oct 6, 2020 at 16:03

From the perspective of your photo it looks to be perfectly in line with your drain pipe. So it appears that this is another tap into that line. Perhaps it wasn't in the right place and rather than remove it they just stacked another one on top of it.

If it's leaking out of the cap, you should be able to remove that, use some thread sealant on the threads and reinsert it. If it's leaking out of the joint behind the cap, then you may need to do some additional work to cut off the bad joint and fix whatever is going on there. You may need to open up the wall to get to what you need to get to in order to fix this properly.

If this were mine, I'd likely get rid of this mess entirely. There is no purpose for this extra drain now and it's, as you can see, causing issues.

  • 1
    Looks like a standard cleanout to me, possibly for a line that serves multiple locations. No need to reconfigure anything.
    – isherwood
    Oct 6, 2020 at 13:05

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