How to Find a Leak Under the Kitchen Sink

basket strainer leaking

If you’ve opened up your kitchen cabinet and found soggy paper towels or drops of water on your cleaning supplies, you might wonder if the garbage disposal, kitchen faucet, or pipes underneath your sink are leaking. With a large water leak, this is usually obvious due to the amount of water or signs of corrosion. If it is a small leak, though, it may be hard to tell. There is often water from the leak on many different parts that aren’t leaking. 

At Legacy Plumbing, we deal with these sorts of scenarios all the time, so in this article is a systematic approach that you can use to test the plumbing underneath your kitchen sink. Once you find out where the leak is, you can then find out what is involved in getting it fixed.

Establishing a Baseline

When diagnosing a difficult leak, it is always helpful to properly prepare the area and establish a baseline. First, take everything out from underneath the sink to give you enough room to look at everything. In order to see behind the sink basin, you will probably need to lay down underneath it in the cabinet. You will then need to get a flashlight and a few dry towels.

Next, take a small, dry dish rag or a couple of paper towels and dry off everything very thoroughly. Dry off the garbage disposal; dry off all of the drain pipes; dry off the shut off valves and faucet/dishwasher supply lines. Use the corner of your rag or paper towel to absorb any water in the nooks and crannies.

During this process, you may get more evidence of where the leak is. If the leak is coming from one of the water lines or from up inside the faucet, you may notice a spot where water comes right back as soon as you dry it off. If that is the case, then you found a leak! If not, proceed to the next step.

The Flow Test

After everything is dried off, you then want to systematically test the drain pipes. Most kitchen sink leaks are on the drain lines (very frequently on loose or improperly connected slip joint connections). These won’t show themselves unless you’re running water. Start by running water in both of the sink basins. Let it run for two or three minutes in each basin while carefully checking for leaks with your flashlight and a dry piece of paper towel/tissue paper.

Start at the highest points where the pipes/disposal connect to the sink basin(s). You need to pay close attention in order to catch the leak as soon as it starts happening. If you wait, it may be hard to tell where the water is coming from. Use your hands to feel for water in areas that are difficult to see.

The Basin Test

If you didn’t find anything during the regular flow test, it is time for a stress test on the drain pipes! Put a stopper in each basin and fill them up with water. We call this the “basin test”. Once the basins are full, you remove the stoppers and let them both drain at the same time. Run the garbage disposal while the sinks are draining. This puts all of the drain pipes under the most stress they will receive during normal use. Monitor all of these pipes and the garbage disposal closely while this is happening.

If you still don’t see anything, next start the dishwasher. Let the dishwasher run for a few minutes – allowing it to fill up with water. Then press the “cancel/drain” button (each dishwasher has a different way of manually initiating the drain cycle). Your dishwasher will probably drain into the side of your garbage disposal at the top through a black or white pipe. You should hear the water trickling into the disposal. In some setups, the water flows through a device called an “air gap” which is mounted on the countertop next to the faucet. Check this as well.

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The Faucet Stress Test

If you still haven’t found where the leak is coming from after performing the basin test and draining the dishwasher, then get your flashlight and carefully check the shut off valves and faucet supply lines another time. The leak may be slow enough that you didn’t catch it on the first pass while you were drying everything off. Look for signs of white mineral buildup or rust on the hardware.

Put the faucet through a rigorous test in the same way you tested the drain pipes. Turn it on and off a few times, moving the handle in various directions. Move the spout back-and-forth as you are doing this. Pull out the sprayer several times and run it through different modes. As you’re doing this, keep checking for signs of fresh moisture under the cabinet. Some faucets will only show the leak as they are being operated in a certain way or if the spout is in a certain position.

What’s Next

If these tests have not revealed anything, then it is possible you have an intermittent leak. Instead of guessing and replacing parts unnecessarily, it is better to observe it over a period of time. In order to do this without risking damage to the cabinets, you can take two or three baking trays and put them underneath all of the plumbing under the sink. They also sell rubber trays specifically designed for this purpose.

Put all of your cleaning supplies back underneath the sink and use it normally. Check it occasionally after heavy usage. If your cleaning supplies are wet or there is water in the baking tray, then you can repeat the troubleshooting process above. The catch pan will help prevent your cabinet from being damaged by any water that does leak.

Along with this, another thing you can do is buy a water alarm. This is a battery-powered device that will sense any water present in the cabinet and sound an alarm. There are even some that connect to your Wi-Fi and send a notification to your phone if they get wet. It’s not a bad idea to have one of these under the kitchen sink and one of these in the water heater drain pan as well.


Hopefully, this process helps you narrow down where the leak is coming from. If it does happen to be the garbage disposal, check out the garbage disposals we carry for more information. If it is the kitchen faucet, read this article on choosing the right kitchen faucet. We do carry a standard kitchen faucet, but you’ll want to purchase your own if you would like special features/finish.

If you are unable to locate the leak, feel free to give us a call. Because we do this every day, there are many things we watch for in order to quickly diagnose most leaks under the kitchen sink. We carry everything on our trucks to take care of any leak you may have: whether it is the faucet/supply lines, garbage disposal, drain pipes, or shut off valves. We will cover everything on the first visit so you don’t have to worry about it anymore.



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