Leaks only when the faucet is running
Water appears under the sink during use, then stops after the faucet is off.
Start here: Start with the supply lines, faucet tailpiece connections, and shutoff valves.
Bathroom sink leak help
Bathroom Sink Leaking
Direct answer: A bathroom sink usually leaks from one of four places: the faucet supply connections, the shutoff valves, the drain assembly under the bowl, or the P-trap. The fastest way to find the right fix is to dry everything first, then see whether water appears only when the faucet is on, only when the sink drains, or all the time.
Most likely: The most common causes are a loose bathroom sink supply line connection, a dripping bathroom sink shutoff valve, a leaking bathroom sink pop-up drain connection, or a slip-joint leak at the bathroom sink P-trap.
Start with containment and the first wet point. A drip at the bottom of the cabinet may have started much higher up, so dry the area completely, run a short test, and watch the highest place that gets wet first.
Don’t start with: Do not start by replacing the whole faucet or smearing sealant around every joint. That often misses the real source and makes the next repair messier.
Leaks only with faucet onCheck the bathroom sink supply lines, faucet connections, and shutoff valves first.
Leaks only while drainingCheck the bathroom sink drain assembly, pop-up pivot area, and P-trap joints first.
Last reviewed: 2026-03-23
What kind of bathroom sink leak do you have?
Leaks only when the sink is draining
The cabinet stays dry until water goes down the drain.
Start here: Start with the drain flange, pop-up pivot ball area, tailpiece, and P-trap joints.
Leaks all the time
You find a steady drip or dampness even when the sink has not been used recently.
Start here: Start with the shutoff valves and supply lines because those stay under pressure.
Leak seems to come from the back of the cabinet or wall
The floor or cabinet back is wet, but the sink parts are hard to blame at first glance.
Start here: Dry everything and trace the first wet point carefully before assuming the sink itself is the source.
Most likely causes
1. Loose or worn bathroom sink supply line connection
These joints leak only when pressurized or may seep slowly all the time if the connection or hose has failed.
Quick check: Dry the supply lines and valves, place a dry paper towel around each connection, then run the faucet for 30 seconds.
2. Bathroom sink shutoff valve leaking at the stem or body
A shutoff valve can drip even when the sink is not being used because it is always under house pressure.
Quick check: Feel for moisture around the valve handle stem, packing nut, and where the valve connects to the wall pipe and supply line.
3. Bathroom sink pop-up drain or tailpiece leak
Leaks at the drain assembly usually show up only while water is draining from the bowl.
Quick check: Fill the sink halfway, dry the drain parts, then release the stopper and watch around the drain nut, pivot ball area, and tailpiece.
4. Bathroom sink P-trap slip-joint leak
A trap joint often drips during drainage, especially after cleaning, bumping, or partial clogging.
Quick check: Run water steadily for a minute and look at the top of each trap nut and the underside of the trap bend for the first forming drop.
Step-by-step fix
Step 1: Contain the leak and identify when it happens
You can narrow the problem quickly by learning whether the leak is pressure-side, drain-side, or unrelated to sink use.
- Place a towel or shallow container under the leak area.
- Empty the cabinet enough to see the shutoff valves, supply lines, drain assembly, and P-trap clearly.
- Dry all visible plumbing parts and the cabinet floor with a towel.
- Wait a few minutes without using the sink.
- If water returns without using the sink, focus on the bathroom sink shutoff valves or supply lines.
- If the area stays dry, run the faucet briefly without filling the bowl, then test again by filling and draining the sink.
Next move: You now know whether to follow a supply-side path, a drain-side path, or a possible non-sink leak path. If everything looks wet at once, dry it again and test one condition at a time: faucet on, faucet off, then bowl drain.
What to conclude: Leaks that happen with no sink use are usually on the pressurized side. Leaks that happen only during drainage are usually in the drain assembly or trap.
Stop if:
- Water is entering from the wall, floor, or cabinet back instead of from sink plumbing.
- A valve will not turn gently and feels seized.
- The leak is heavy enough to risk cabinet swelling or floor damage before you can isolate it.
Step 2: Check the bathroom sink supply lines and shutoff valves
These are the most common sources when the leak appears during faucet use or even when the sink is idle.
- Use a flashlight to inspect both bathroom sink shutoff valves and both bathroom sink supply lines from valve to faucet.
- Wrap a dry paper towel around each connection point one at a time to find the first damp spot.
- If a connection is slightly loose, snug the compression nut or coupling gently with the correct wrench while supporting the valve body so it does not twist.
- Look for corrosion, mineral crust, bulging braided hose, or a drip from the valve handle stem.
- If the valve stem is the only wet point, try a very small tightening of the packing nut if accessible.
Next move: If the drip stops after a gentle snugging, dry the area and recheck after several faucet cycles. If the hose itself is wet, the braided line is damaged, or the valve body or stem keeps dripping, plan on replacing the failed bathroom sink supply line or bathroom sink shutoff valve.
What to conclude: A connection leak may only need tightening, but a wet hose, cracked nut, or leaking valve stem/body usually means the part itself has failed.
Step 3: Check the bathroom sink drain assembly and pop-up area
If the leak shows up only while the sink drains, the drain body under the bowl is a likely source.
- Dry the underside of the sink around the bathroom sink drain assembly.
- Close the stopper, fill the bowl halfway, and look underneath before draining to see whether the leak starts while the bowl is holding water.
- Then release the stopper and watch the drain nut, tailpiece connection, and the small pivot ball opening where the pop-up rod enters the drain body.
- If the leak is at the pivot ball area, confirm whether the retaining nut is loose or the pivot ball seal has failed.
- If the leak is from the drain flange area directly under the bowl, the drain assembly may need to be reseated or replaced.
Step 4: Check the bathroom sink P-trap and slip-joint connections
Trap leaks are common after a clog, cleaning, or accidental bumping under the vanity, and they usually show up only during drainage.
- Dry the bathroom sink P-trap completely, including the trap bend, trap arm, and both slip-joint nuts.
- Run water steadily for 30 to 60 seconds while watching the highest joint first.
- If a slip-joint nut is visibly loose, hand-tighten it first, then snug it slightly more if needed without overtightening.
- Look for a crooked washer, cracked trap nut, or a split in the trap itself.
- If the trap leaks after proper alignment and gentle tightening, replace the damaged bathroom sink P-trap or the affected drain section.
Next move: If the trap stays dry during a long drain test, the leak was likely a loose or misaligned slip-joint. If the trap still drips, remove and inspect the washers and trap pieces, or replace the damaged bathroom sink P-trap assembly.
Step 5: Repair the confirmed part, then retest before closing the cabinet
Once you know the first wet point, you can fix the right part and make sure the leak is truly gone.
- Replace the failed part that matched your test results: bathroom sink supply line, bathroom sink shutoff valve, bathroom sink pop up drain rod and pivot ball kit, or bathroom sink P-trap/drain section.
- If your confirmed repair is a supply line, use the correct length and connection style and avoid twisting the hose during installation.
- If your confirmed repair is a shutoff valve and the local valve cannot isolate cleanly, stop and arrange a plumber before removing it.
- After the repair, dry everything completely and run three tests: faucet on for one minute, bowl filled and held, then full drain.
- Leave a dry paper towel under each repaired joint for 10 to 15 minutes and check again before putting items back in the vanity.
Repair guide: How to Replace a Bathroom Sink Supply Line
Related repair guide: How to Replace a Bathroom Sink Shutoff Valve
A good result: If all joints stay dry through each test, the bathroom sink leak is fixed.
If not: If the leak source changes or water appears from the wall, overflow area, or cabinet back, stop chasing sink parts and move to a drain-in-wall or supply-in-wall diagnosis.
What to conclude: A dry retest confirms the repair. A new wet point means the original drip may have masked a second leak or the sink was not the true source.
Replacement Parts
Bathroom sink supply line
Bathroom sink shutoff valve
Bathroom sink pop up drain rod and pivot ball kit
How to Replace a Bathroom Sink Pop Up Drain Rod And Pivot Ball Kit
Bathroom sink P trap
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FAQ
Why does my bathroom sink leak only when I turn the faucet on?
That usually points to the pressurized side of the sink: the bathroom sink supply lines, faucet connections, or shutoff valves. Dry those parts first and look for the highest point that gets wet when the faucet runs.
Why does my bathroom sink leak only when it drains?
That usually means the leak is in the bathroom sink drain assembly, pop-up pivot area, tailpiece, or P-trap. Fill the bowl, then drain it while watching underneath to see where the first drip forms.
Can I just tighten everything under the sink?
No. Gentle tightening can help a loose joint, but overtightening can crack plastic nuts, distort washers, or damage a shutoff valve. Tighten only the joint that testing shows is leaking.
Is a leaking bathroom sink shutoff valve a DIY repair?
Sometimes, but only if you can shut water off reliably and the valve connection is in good condition. If the valve is badly corroded, the wall pipe moves, or the valve will not isolate water, call a plumber.
Should I replace the whole faucet if my bathroom sink is leaking underneath?
Not unless testing points to the faucet body or faucet tailpiece connections. Many under-sink leaks come from the supply lines, shutoff valves, drain assembly, or P-trap, not the faucet itself.
What if the cabinet floor is wet but I cannot see the leak start?
Dry everything completely, then test one condition at a time: no use, faucet running, bowl holding water, and bowl draining. If water appears from the wall or cabinet back first, the problem may be outside the sink assembly.