Leak Under the Kitchen Sink? How to Find the Source in 5 Minutes and What Each Fix Involves

The cabinet under the kitchen sink is where every kind of plumbing meets: two pressurized supply lines, a faucet, a sprayer, a drain strainer, a P-trap, often a garbage disposal, a dishwasher hose, and sometimes a filter or a reverse osmosis unit. When something is wet down there, the puddle gives almost no clue about which of them is leaking, because water runs along pipes and drips off the lowest point. The good news is that each source leaks under a different condition, so a few minutes of testing will tell you exactly what is wrong before you buy a part or make a call.

If the leak is steady or the shutoff valves will not close, call or text (713) 409-7176. The visit to find it is free.

Before You Start: Shut It Off If It’s Flowing

If water is actively running rather than dripping, reach behind the pipes and turn the two small oval or lever handles clockwise. These are the angle stops that feed the faucet and dishwasher. If they are stuck or spin without stopping the flow, which is common on valves that have not been touched in years, go to the house main. Our guide on how to shut off your water main shows where it is. Do not force a stuck stop; they snap, and the leak gets worse.

The Dry-and-Test Method

1. Empty the cabinet completely. Cleaning bottles hide drips and soak up water.
2. Dry everything. Wipe every pipe, fitting, hose, and valve with a towel until there is no moisture anywhere. Dry the cabinet floor.
3. Lay down paper towels across the floor of the cabinet and under each fitting. A fresh drip shows instantly on a dry paper towel and tells you where it landed.
4. Wait five minutes with nothing running. Check. A new wet spot now means a supply-side leak, because that is the only water under pressure when the faucet is off.
5. Run the cold faucet for one minute, then the hot. Check the faucet base, the sprayer hose, and the supply connections at the faucet.
6. Plug the basin, fill it halfway, and pull the plug. Watch the strainer, the tailpiece, and every slip joint on the P-trap while it drains. A full basin puts far more water through the drain than a running tap does, which is why some leaks only appear after dishes.
7. Run the disposal with the water on. Check the top flange where it meets the sink, the side where the dishwasher hose attaches, and the bottom near the reset button.
8. Run a dishwasher cycle and check during the drain phase. Look at the drain hose, the clamp at the disposal or tailpiece, and the air gap on the counter if you have one.
9. Use the sprayer or pull-down head for 30 seconds and check the hose where it hangs under the faucet.

Source Table: What the Timing Tells You

Each Source, Explained

Supply lines and angle stops. Braided supply lines last 10 years or so; the rubber inside cracks and they weep at the crimp. Angle stops leak at the packing nut behind the handle, or at the compression nut where they meet the wall stub. A supply line is a simple swap once the stop is closed. A leaking stop is replaced, and the house water has to be off to do it.

Faucet. Houston hard water scales the O-rings and cartridge seats, so a faucet that leaks under the counter when it runs is usually a worn base seal or a loose supply nut at the faucet. If the faucet also drips from the spout, read our guide on dripping faucets and whether to repair or replace, because the two problems often arrive together.

Basket strainer. The strainer is sealed to the sink with plumber’s putty and held by a large nut under the basin. The putty dries out, or the nut loosens over years of vibration from the disposal. Tightening the nut helps for a while; resetting the strainer with fresh putty is the real fix. A corroded strainer body gets replaced.

P-trap. Plastic slip joints loosen, and the tapered washers crack with age. Hand tighten first. If that fails, reseat or replace the washer. A metal trap that has rusted through at the bottom of the bend is replaced. A leak at the wall, where the trap arm enters the drain stub, usually means a bad washer there or a trap arm that was never aligned properly.

Garbage disposal. Three places. The top mounting flange leaks when the putty fails or the mounting ring loosens; it is resealed. The side inlet for the dishwasher hose leaks when the clamp is loose or the hose has split; it is reclamped or replaced. The bottom of the unit leaks when the internal seal has failed; the disposal is replaced. Our guide on garbage disposal problems covers humming, jamming, and the other failures, and how long a disposal should last helps decide whether to repair or replace.

Dishwasher connection. The drain hose runs from the dishwasher up to a high loop or an air gap and then down to the disposal or tailpiece. The clamp at the end loosens, the hose cracks at the bend, or the air gap clogs and spits water onto the counter. All three are quick fixes once found.

Filters and ice maker lines. Push-fit connectors on reverse osmosis units and quarter-inch plastic ice maker lines weep when the tube is cut unevenly or not pushed fully home. Saddle valves on older ice maker lines corrode and drip. These are small, constant leaks that are easy to miss until the cabinet floor is soft.

Condensation. In Houston humidity, a cold supply line can sweat enough to form drips. The giveaway is a uniform film of moisture along the whole cold pipe with no single drip point, worse on hot afternoons. Insulating foam sleeves solve it.

What Is DIY and What Is Not

Reasonable to do yourself:

• Hand-tightening slip nuts and the strainer nut.
• Replacing a P-trap washer or the whole plastic trap.
• Swapping a braided supply line (close the stop first).
• Reclamping or replacing a dishwasher drain hose.
• Resetting a basket strainer with fresh plumber’s putty.
• Adding foam insulation to a sweating cold line.

Call a plumber for:

• Angle stops that will not close, spin freely, or leak when you touch them.
• Any leak that requires shutting off the house main.
• A corroded strainer or metal trap that needs cutting out.
• A disposal leaking from the bottom.
• A faucet leaking from the body or cartridge, especially on a brand with proprietary parts.
• A leak at the wall drain connection, which can mean the stub itself is damaged.
• Any leak you tested for and still cannot find; that is what leak detection equipment is for.

What the Water Does While You Wait

Most kitchen cabinets have a particleboard or MDF floor. It swells within a day of getting wet, crumbles within a week, and the moisture migrates into the toe kick and the sides of the cabinet. A slow drip also keeps the enclosed cabinet humid, which is how mold starts behind the cleaning supplies. If the leak is on the supply side, close the angle stops and use a bucket until it is fixed. If it is on the drain side, put a pan under it and stop using that sink. If the clog is also part of the picture, with water backing up into the basin, our guide on a clogged kitchen sink covers that side.

When to Call

Call Hugo Plumbing when:

• The angle stops will not close or they leak once you touch them.
• The drip is coming from the faucet body or under the deck and the faucet is more than a few years old.
• The disposal is leaking from the bottom.
• The leak is at the wall connection.
• The cabinet floor is already soft or stained.
• You tested everything and the paper towels are still getting wet.

The visit to find the leak is free, with no trip charge or diagnostic fee, and faucet repair and garbage disposal repair are priced on site as one flat written number before work starts. A replacement disposal supplied by us runs $495 to $625 by model with a 5- or 10-year parts-and-labor warranty and a $30 replacement coupon, and our work carries at least a one-year warranty. Licensed master plumbers (MPL 41319) answer 24/7 at (713) 409-7176.

Frequently Asked Questions

How do I find where the leak under my kitchen sink is coming from?

Empty the cabinet, dry every pipe and fitting with a towel, and lay paper towels across the cabinet floor. Then test one thing at a time: run cold water, run hot, fill the basin and let it drain, run the disposal, run a dishwasher cycle, and use the sprayer. Check after each test. A drip that appears with nothing running is a supply line or shutoff valve; one that appears only while draining is the strainer, P-trap, or disposal; one that appears only during a dishwasher cycle is the dishwasher hose or air gap.

Why is my kitchen sink leaking only when the water runs?

Water that appears only while the faucet runs is coming from the faucet itself or its sprayer hose, not the drain. On most faucets the hose connection under the deck or a worn O-ring at the base lets water run back along the underside of the counter and drip into the cabinet. If it only shows up while the sink is draining, the leak is on the drain side: the basket strainer, a P-trap slip joint, or the disposal.

Can I fix a leaking P-trap myself?

Usually, yes. Most under-sink drains are plastic slip-joint pipe. Hand-tighten the slip nuts a quarter turn first. If it still drips, loosen the nut, check that the tapered washer is seated with the thin edge facing the joint and not cracked, and reassemble. New washers are inexpensive and sold by size at any hardware store. If the trap is metal and corroded, or the leak is at the wall connection, that is a plumber’s job.

Is the garbage disposal leaking from the bottom repairable?

No. A drip from the bottom of the disposal body, near the reset button, means the internal seal has failed and water is passing through the motor housing. That seal is not a serviceable part, and the unit is replaced. Leaks from the top mounting flange or the side dishwasher inlet are repairable by resealing or replacing the hose and clamp.

How long can I wait to fix a leak under the sink?

Days, not weeks. Most kitchen cabinets have a particleboard floor that swells and crumbles once it gets wet, and the damage spreads to the toe kick and the cabinet sides. A slow drip also keeps the cabinet humid enough for mold. Close the shutoff valves under the sink if the leak is on the supply side, put a pan under a drain leak, and get it fixed before the cabinet floor is ruined.

If your cabinet won’t stay dry, call Hugo.