Water appears even when the sink is not being used
The cabinet floor gets wet with the faucet off, or you see a slow drip from a valve or supply connection.
Start here: Start with the pressurized side: shutoff valves and supply line connections.
Plumbing
Under Sink Leaking
Direct answer: An under-sink leak is usually either a pressure-side leak from a sink shutoff valve or supply connection, or a drain-side leak from the sink strainer or sink p-trap. The fastest way to narrow it down is to dry everything, then see whether water appears with the faucet off, only while the faucet is running, or only while the basin is draining.
Most likely: The most common branches are a loose or worn connection at a sink shutoff valve, a drip from a supply line connection, a leaking sink strainer seal, or a sink p-trap joint that seeps during drainage.
Most under-sink leaks can be traced without taking much apart. Start by emptying the cabinet, placing a towel or shallow pan underneath, and finding the first place that gets wet. Once you know whether the leak happens under pressure or only during drainage, the repair path gets much clearer.
Don’t start with: Do not start by replacing the whole faucet or buying random drain parts. The drip point under the cabinet is often lower than the actual leak source.
Leaks with the faucet offSuspect a sink shutoff valve, supply line, or a connection that stays pressurized.
Leaks only while drainingSuspect the sink strainer, sink p-trap, or another drain joint below the basin.
Last reviewed: 2026-03-17
What kind of under-sink leak do you have?
Leak starts when the faucet is turned on
Water sprays or drips while water is flowing, even before the basin fills enough to drain.
Start here: Watch both the supply lines and the faucet hose area under the sink while someone runs the water.
Leak happens only when the basin drains
Everything stays dry until water goes down the drain, then the trap or drain area gets wet.
Start here: Start with the sink strainer and sink p-trap branch.
The cabinet is wet but the source is unclear
You see pooled water, staining, or dampness, but not the exact starting point.
Start here: Dry all surfaces completely, then test one branch at a time with bright light and paper towels.
Most likely causes
1. Sink shutoff valve or valve packing leaking
These parts stay under pressure all the time, so they can drip even when the sink is not in use.
Quick check: Dry the valve body and the nut behind the handle, then watch for fresh moisture with the faucet off.
2. Sink supply line connection seeping
A loose or aging connection can drip slowly at the top or bottom of the supply line, especially when the faucet is running.
Quick check: Run the faucet and feel carefully with a dry paper towel around each supply line nut and hose connection.
3. Sink strainer seal leaking
If water appears around the drain opening underside or runs down from the basket area only during drainage, the strainer seal is a common cause.
Quick check: Fill the basin partway, then release the water and watch the underside of the drain opening before the trap gets wet.
4. Sink p-trap joint or trap body leaking
Trap joints often seep only when water is moving through them, and the drip may show up at the lowest bend rather than the real joint.
Quick check: Dry the trap completely, drain a full basin, and look for the first joint that becomes wet.
Step-by-step fix
Step 1: Dry the area and identify when the leak happens
You need the first wet point, not the final drip location on the cabinet floor.
- Remove stored items from under the sink so you can see all sides of the plumbing.
- Wipe the shutoff valves, supply lines, sink strainer underside, and sink p-trap completely dry.
- Place paper towels under each main area or wrap a dry paper towel lightly around suspected joints.
- Leave the faucet off for several minutes and check for new moisture.
- Then run the faucet briefly, and finally fill the basin partway and let it drain while watching closely with a flashlight.
Repair guide: How to Replace a Sink Strainer
What to conclude: A leak with the faucet off points to a pressurized connection. A leak only during drainage points to the drain assembly. A leak only while water is actively running can be either supply-side or a faucet hose/sprayer branch under the sink.
Stop if:
- Water is dripping into an electrical outlet, disposal wiring, or extension cord under the sink.
- The leak is heavy enough that you cannot keep up with towels or a pan.
- The source appears to be inside the wall, cabinet back, or floor rather than at an exposed sink component.
Step 2: Check the sink shutoff valves first
Shutoff valves are common under-sink leak points and are easy to separate from drain leaks.
- With the faucet off, inspect each sink shutoff valve body, the connection to the wall pipe, and the small nut behind the handle.
- Touch each area with a dry paper towel to find the exact wet spot.
- If the leak is at the handle area, gently snug the packing nut only a very small amount if accessible.
- If the leak is at the outlet connection to the supply line, try a careful slight tightening of that connection while supporting the valve so you do not twist the pipe.
- If the valve continues to seep, turn it off if it will close fully and confirm whether the dripping stops.
Repair guide: How to Replace a Sink Shutoff Valve
What to conclude: A leak at the handle usually points to valve packing or valve wear. A leak from the valve body or wall-side connection is a stronger sign the sink shutoff valve should be replaced or the branch connection needs a plumber.
Step 3: Check the sink supply lines and any sprayer hose branch
A supply line can drip only while water is flowing, and the water often runs down the hose before dropping somewhere else.
- Have someone run hot and cold water while you watch the supply lines from top to bottom.
- Use a dry paper towel at the top faucet connections and at the bottom connections on the sink shutoff valves.
- If the sink has a side sprayer or pull-out hose connection under the sink, inspect that hose and its connection points while the faucet is running.
- Look for a fine spray, bead of water, or a drip trail running along the hose.
- If a connection is obviously loose, make only a small adjustment and retest.
Repair guide: How to Replace a Sink Sprayer Hose
Step 4: Test the sink strainer before blaming the trap
Water from a leaking sink strainer often runs down onto the trap, making the trap look like the problem when it is not.
- Dry the underside of the drain opening and the top of the sink p-trap completely.
- Put the stopper in and fill the basin with a few inches of water.
- Release the water and watch the underside of the sink strainer immediately as the basin drains.
- If needed, wipe around the large locknut area and the seam just below the sink basin with a paper towel.
- Check whether water starts at the sink strainer and then travels downward onto the trap.
Next move: If the first wet point is at the sink strainer, you have isolated the leak above the trap. If the sink strainer stays dry but the trap joints get wet during drainage, move to the sink p-trap branch.
Step 5: Inspect the sink p-trap joints and trap body
Trap leaks are common, but the exact joint matters. Replacing the wrong section wastes time and may not stop the leak.
- Dry the entire sink p-trap, including the slip-joint nuts and the curved trap bend.
- Drain a full basin and watch each joint in order from the sink tailpiece down through the trap and into the wall-side drain arm.
- Use a paper towel on each joint to find the first fresh moisture.
- If a slip-joint nut is visibly loose and the pipes are aligned, try a small hand-tightening first, then a slight additional snug if needed.
- If the trap body is cracked, warped, or continues to leak after alignment and careful tightening, plan on replacing the sink p-trap assembly.
Repair guide: How to Replace a Sink P-Trap
A good result: If the leak stops after correcting a loose, aligned joint, run several basinfuls of water to confirm it stays dry.
If not: If the trap still leaks, the washer may be damaged, the trap may be misaligned, or the sink p-trap body may be cracked.
What to conclude: A leak at one trap joint usually points to a local trap issue, not the whole sink. A crack or distorted trap is a clear replacement branch.
Replacement Parts
Repair Riot may earn a commission from qualifying purchases, at no extra cost to you.
FAQ
Why is there water under my sink but I cannot see an active drip?
Water often travels along a pipe or hose before it drops. Dry everything fully, then test one condition at a time: faucet off, faucet running, and basin draining. Paper towels around each suspect area make the first wet point easier to spot.
How do I tell if the leak is from the drain or the water supply?
If water appears with the faucet off, suspect a pressurized part like a sink shutoff valve or supply line. If it stays dry until the basin drains, suspect the sink strainer or sink p-trap. If it leaks only while water is actively running, inspect both supply lines and any sprayer hose branch.
Can I just tighten the leaking connection under the sink?
Sometimes, but only slightly and only after you identify the exact joint. Over-tightening can crack plastic parts, distort washers, or twist a valve or wall pipe. If a small adjustment does not help, stop and reassess the branch instead of forcing it.
Is a leaking sink p-trap an emergency?
Usually not if the leak is small and only happens during drainage, but it should still be fixed soon to avoid cabinet damage and hidden moisture. A constant pressure-side leak from a valve or supply line is more urgent because it can drip continuously.
Should I replace the faucet if the area under the sink is leaking?
Not unless you confirm the leak is from a faucet hose branch or a faucet connection under the sink. Many under-sink leaks come from the sink shutoff valves, supply lines, sink strainer, or sink p-trap instead, so diagnosis should come first.