Bathroom Sink Drain Pipe Is Leaking – How To Fix – What To Check?

I installed a new bathroom faucet and it came with a new drain pipe and sink drain outlet flange. The faucet works great but the drain pipe is leaking under the sink. It is leaking down into my bathroom cabinet and getting everything wet. The leak is coming from where the plastic nut on the drain pipe meets the bottom of the bathroom sink. I installed the sink drain pipe with the included rubber sealing washer and gasket but without any plumbers tape or plumbers putty. Is that why it is leaking?

Bathroom sink drain leaking at bottomBathroom sink drain leaking at bottom

If the drain pipe under your bathroom sink is leaking, you may need to simply re-tighten the nut. If no Plumbers Putty was used when installing the drain outlet flange in the sink, this is another reason it may be leaking. You may also have the gasket or rubber sealing washer that goes between the nut and the sink overtightened and this is causing the leak. The gasket or rubber sealing washer may not be completely flat and therefore not sealing. Check to see if you are able to tighten the nut with another half turn or so and check for leaks. If no matter how tight the nut is and it still leaks, take everything apart including the drain pipe and the drain outlet flange in the sink.

After taking the bathroom sink drain outlet flange and drain pipe out, inspect all parts to see if it may be cracked or if the plastic or metal threads are stripped. Is the gasket, rubber sealing washer, drain outlet flange, drain pipe, or nut damaged? If the drain pipe and the components are not damaged, you will NOT need any new parts but you will need a few extra items to make sure it does not leak before reinstalling.

2 items that will help stop a leaking sink drain pipe:
Plumbers Tape (Works with any fitting needing a high quality seal – Ensures a secure and tight seal)
Plumbers Putty (Flexible stretchable compound – Molds easily – Will not harden, crumble, crack or shrink)
NOTE: These items are optional but are used in professional plumbing to avoid leaking pipes. Use Plumbers Tape on all threads to create a tight water seal.

Pipe Sealant Tape - Plumbers TapePipe Sealant Tape – Plumbers Tape

Plumbers PuttyPlumbers Putty

What could be the cause of the water leak under the sink?
1 – Water may leak in between the threads on the nut and the drain pipe if no plumbers tape was applied.
2 – The rubber sealing washer or o-ring may be squeezed excessively tight causing leakage.
3 – The nut may not be tight enough and water is simply coming through.
4 – The drain pipe is off center and therefore a tight seal is not possible.
5 – The drain pipe is cracked and water is coming through the crack.
6 – The fitting for the pop-up sink assembly is not tight.
7 – The sink drain may leak if no Plumbers Putty was used.

Bathroom sink drain parts identification 1Bathroom sink drain parts identification 1


Bathroom sink drain parts identification 2Bathroom sink drain parts identification 2


Bathroom sink drain parts identification 3Bathroom sink drain parts identification 3

How to reinstall the sink drain WITHOUT any leaks… (Be sure all parts are removed, clean, and not damaged)
1 – Add a 1/2″ thick ring or rope of Plumbers Putty to the underside of the top flange of the sink drain. (Warm the putty up with your hands first to make it easier to use – Be sure the Plumbers Putty is in a continuous circle)
2 – Apply some Plumbers Tape to the threads of the drain.
3 – Use enough Plumbers tape to cover the threads near the area where the nut will be positioned when fully tightened.
4 – Spin the nut all the way down or off of the threads.
5 – Insert the drain back into the sink hole and be sure it is centered.
6 – Install the nut and the sealing washer and tighten. (The excess putty will ooze out under the top flange until it is fully seated to the sink)
7 – Remove the excess putty from in the sink.
8 – Add Plumbers Tape to any areas of the drain pipe that has threads.
9 – Reinstall the drain pipe making sure it is centered and not overtightened.
10 – Run the bathroom faucet and check for water leaks under the sink.

Have questions on fixing or repairing a leaking sink drain? Leave your question below and we will assist you to stop your drain from leaking.

ALLEN VETTER - DIY Repair Assistant
Allen is a Home Maintenance/Appliance tech and the author/creator of this website. He has 30 years of experience troubleshooting and repairing all types of machinery. Contact here

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{ 6 comments… read them below or add one }

1 joe cancemi November 26, 2018 at 1:23 pm

I notice the threads are slightly stripped where the tail pipe goes into the flange, I covered it with teflon tape but I still get a drip from under the nut and I used plumbers putty under the flange and above the rubber washer that screws up the tail pipe to the sink and then tightened by the plastic nut. I put teflon tape about 3 rounds where the nut and black rubber washer screw up the tail pipe also. I am not sure about using the pipe dope as it may be hard to undo if I still have a drip?,or is there a type of sealant that can be undone if there is still a drip? do you think buying a new pop up assembly with good threads is a good step? Thanks


2 Larry Folkers September 19, 2018 at 3:21 pm

Can you confirm for me how the overflow holes work on a bathroom sink?
1. the holes are situated on the pipe higher than the gasket.
2. therefore there will be a pool of water sitting on the gasket most all the time due to the fact it has no where to drain until the water level reaches to the overflow holes.
3. therefore if you do not have tape on the threads for a tight seal it will inevitably leak.


3 TS October 6, 2017 at 3:10 pm

Looked around & haven’t seen my situation addressed. – I replaced 3 sinks & the fixtures with them. First sink & fixtures went in beautifully. The other two I worked on simultaneously & have the same issue with both. Neither of the flanges for the drains will seat entirely into the sink – the “sides” of the flanges are flush to the sink, but the front & back of both have gaps, which I sealed with extra plumbers putty for now. Is this a flaw in the sink, or is it something I did during installation? I’ve replaced a few sink fixtures & drains; never had this happen before.


4 Amy August 9, 2017 at 6:00 am

I’m thinking that 1) I didn’t put quite enough plumber’s putty on, or 2) tightened to much and it squished out most of the putty. Also, what do you mean by “threading” the rubber gasket? Do I put plumber’s tape on that area, too?


5 JC April 12, 2017 at 4:04 pm

Thank you CJ for mentioning that, I have took this thing apart 3 times now, done everything you are suppose to and it is still leaking and I think this is exactly what might be happening. I did not know that the rubber gasket had to be threaded. I will try one more time, thank you, this makes sense.


6 CJ April 3, 2017 at 3:13 pm

You need to mention that the rubber gaskets are meant to “threaded” onto the tailpiece, for years I did not realize this and improperly “slid” the gasket up the threads, which does not allow the rubber to form a tight seal against the threaded tailpiece and allows water to travel down the threads and out from under the nut.

Screw/don’t slide the rubber gasket up to the sink bottom, and apply some vinyl tape or pipe dope to the threads beforehand for good measure.


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