Kitchen Sink Leaking Underneath? Why & How to Fix It

Kitchen sinks leak for a variety of reasons and finding the source of the leak is not always easy. The good thing is that fixing a leaking kitchen sink is usually easy and in most cases you don’t need to buy replacement parts.

A leaking kitchen sink is often caused by depletion of plumber’s putty the under strainer body’s rim, worn out washers, loose drain connections, faucets leaking under the sink or faulty shut off valves. If you have a garbage disposal, it could also be the source of your leak.

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If your kitchen sink is leaking from the edges, it is a sign that the caulk which creates a seal between it and the countertop has cracked and is not sealing anymore. To fix this problem you will need to replace the old caulk with new one.

In order to fix a leaking kitchen sink, you will first need to find the source of the leak. The solution will most often involve replacing worn out washers, putting fresh plumber’s putty on the restrainer body, replacing faucet cartridges and/or O-rings, or replacing shut off valves. Sometimes all you need to do is tighten a loose connection.

How to Find a Leak Underneath the Kitchen Sink

Does your kitchen sink leak only when using the sink or it leaks throughout? This is the first step in determining the source of the leak.

If the kitchen sink leaks only when you are using the sink, most likely the problem is with sink strainer body, often known as a sink basket. A kitchen sink strainer body is the finished part at the bottom of the sink drain. It links the sink with the drainpipe.

When the sink restrainer is being installed, plumber’s putty is usually applied under its rim to create a watertight seal and therefore prevent leaks. With time however the plumber’s putty is eroded and the sink starts to leak from underneath.

The restrainer body is held in place underneath the kitchen sink using a locknut backed up by a rubber washer and a cardboard washer, also known as a friction washer. If the locknut is loose or the washers are worn out, the sink will leak as well.

To understand the anatomy of of the kitchen sink drain, let us look at the image below courtesy of Signature Hardware.

While running the sink, water can also leak out from the P-trap. The P-trap is the U-shaped piece of the drainpipe under the kitchen sink.

It holds a little amount of water which acts as a barrier preventing sewer gases from coming up through your kitchen drain. P-traps also trap potential drain clogs preventing them from clogging the drainpipe further away where it would be harder to unclog.

When water is leaking from your kitchen sink P-trap, it is a sign that the connections are loose, the pipes are not properly aligned or the washer is not sealing properly. Aligning and tightening the connections will most of the time fix the problem.

If you have a dishwasher connected to the drainpipe or garbage disposal, check if the connection is the source of the leak. Start a dishwasher cycle and check if the leak is coming from its connection to the drainpipe or garbage disposal.

If water is leaking from underneath your kitchen sink all the time, you could be having worn out faucet cartridges or O-rings. When that is the case, the faucets leak from the base of the handle or spout and drip underneath the kitchen sink.

Another source of leaks underneath your kitchen sink is faulty shut off valves. The packing nuts on the cold and hot water shut off valves could be loose, or the washers could be worn out. Tightening the packing nut or replacing the washers is enough to fix the problem.

If you have a double bowl kitchen sink and it only leaks when running the garbage disposal, the garbage disposal is surely the cause of the leak. This is usually caused by a loose sink flange or worn out internal seals. While the loose flange can be fixed, worn out internal seals is usually a sign of needed replacement.

How to Fix a Leaky Kitchen Sink

There are several ways of fixing a leaking kitchen sink depending on what is causing the leak in the first place. Once you have already inspected the area under the kitchen sink and figured out the source of the leak then fixing it is easy.

Before starting to work on fixing the leak, duck under the kitchen sink and clear the area. If you use the space to store stuff get them out in order to get yourself enough room to work freely.

Here are the different methods of fixing a leaking kitchen sink:

1. Tighten Loose Connections

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The are many drain connections under your kitchen sink especially if you have a double bowl kitchen sink. A garbage disposal and dishwasher connections also make the job even more complex.

  • Start with the P-trap connections. If you have a plastic drainpipe, ideally all the connections should be hand tight. Make sure that all the pipes are properly aligned and that the connections are tight. You can use a wrench to slightly tighten the connections, being careful not to deform them or strip the threads.
  • If the dishwasher connection is leaking, grab a screwdriver and tighten the metal clamp. Run a short cycle and check if the leak has stopped.
  • Tighten the strainer body locknut with a wrench. If it is already tight don’t force it. Probably the washers are just worn out, or the plumber’s putty needs a replacement.
  • If the leak is coming from one of the 2 shut off valves, turn the packing nuts ½ turn clockwise to tighten them. Check if the leak has stopped.

For more information on how to fix a leaking sink trap check out this post.

2. Replace Kitchen Sink Strainer O-rings and Plumber’s Putty

Although you can buy the O-rings only, it is usually a better idea to buy the whole kit which includes the strainer body, O-rings and locknut. A decent replacement kit costs slightly above $10. Plumbers putty is also quite cheap.

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Once you have gathered all the materials together. Its time to get down to work. Here is how:

  • Disconnect the P-trap. Once again, remember that the connections are usually hand tight. Only use a wrench when you cannot loosen them with your bare hands. Before disconnecting the P-trap, place a small bucket or bowl under it to catch the water and gunk inside it.
  • Once the P-trap is out of the way, disconnect the tailpiece (vertical pipe connecting P-trap and sink drain) and place it away as well.
  • Remove the locknut. Grab a wrench or channel locks and loosen the locknut from the strainer body. Sometimes the locknut is so tight and corroded, and won’t just come off. In that case cut it off with a hacksaw blade. This is why it is a good idea to buy a complete kit.
  • With the locknut out, the strainer body is now free. Push it up and remove it from the top of the sink.
  • Clean the sink drain opening. Use a putty knife to scrape off old plumber’s putty from the drain opening as well as from strainer body. Clean the area with a paper towel and make sure that it is dry. Water is undesirable if you want to have a nice seal.
  • Scoop a generous amount of plumber’s putty and roll it between your palms into a pencil-size bead. Make sure that the length of the bead is slightly longer than the circumference of the strainer body.
  • Apply the plumber’s putty under the rim of the strainer body and cut off the excess amount.
  • Push the strainer body down the sink drain opening.
  • Align the rubber and friction washers on top of the strainer cup (rubber washer should be at the top and push them over the strainer body’s threads underneath the sink.
  • Holding the washers and strainer cup up with one hand, screw in the lock nut with the hand until hand tight. Grab a wrench or channel locks and tighten the locknut further.
  • Check the condition of the strainer body from above the kitchen sink. As a sign that you have a good seal, plumber’s putty should squeeze out from all around the edges of the strainer body. Trim off the excess plumber’s putty.
  • Connect the tailpiece and the P-trap and tighten the connections.
  • Turn on the water to the sink and let it run for about a minute. Check for leaks under the sink, at the very top where the new strainer sits.
  • Plug off the sink drain and let the sink fill with water. Continue monitoring for leaks. Drain the water and again check for leaks especially around the P-trap. Tighten the connections a little bit more if you notice any leaks.

3. Replace the Caulk

This is the solution for a kitchen sink leaking from around the edges. You will however need to remove the old caulk in order to put in the fresh caulk. Follow the steps below:

  • Use a straight-edged blade or knife to cut through the old caulk. Position the knife between the countertop and sink and remove as much of the old caulk as possible. Wipe it off with a rag or rubbing alcohol.
  • Apply the caulk round the edge of the sink. It is very important that you have a consistent and uniform bead. To do this, don’t lift the tube from the time you start until you finish. If you can’t help it and lift the tube, start the application about ¼ inch back from where you stopped to create an overlap.
  • For uniformity, run your finger round the caulk bead and wipe off the excess caulk.
  • Wait for about 1 hour for the caulk to dry, though it could take up to a day to completely cure.

4. Replace Faucet Cartridges and O-rings

If the source of the leak is worn out faucet O-rings and cartridges, you will need to replace the parts. If you have a Moen kitchen faucet you are lucky since they have lifetime warranty, and all you have to do is call them and they will send you the parts for free.

I have written a detailed guide on how to fix a leaking Moen kitchen faucet. Read it here. Replacing a faucet cartridge is usually an easy and straightforward task as the video below will demonstrate.

If you have a pullout kitchen faucet, the hose running back to the underside of the kitchen can develop a leak and mess your floor. In that case you will need to replace the hose.

5. Fix the Garbage Disposal

The kitchen sink flange which connects the garbage disposal to the sink can be loose, or the plumber’s putty under its rim eroded causing leaks. The drain hose connection can also be loose, or the washer warped or worn out.

To fix a leaking garbage disposal you will first need to know exactly where it is leaking from. If the garbage disposal is leaking from the bottom, most of the time it is as a result of worn out internal seals. In that case the garbage disposal would need to be replaced.

I have written a detailed post on how to replace a leaking garbage disposal. Read it here.

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