Direct answer: Most under-sink drain leaks come from one of three spots: the sink strainer at the basin, a loose or crooked slip-joint connection, or a cracked sink P-trap. First figure out whether it leaks only when water drains or even when the faucet is off.
Most likely: If the cabinet stays dry until you run water, start at the sink strainer and the slip-joint nuts on the drain assembly. If it drips all the time, you may be looking at a shutoff valve or supply line instead of the drain.
Put a dry paper towel under each connection and trace the highest wet spot, not the drip hanging at the bottom. Reality check: water often travels along the pipe and fools you. Common wrong move: tightening every nut hard enough to crack a plastic trap or distort a washer.
Don’t start with: Do not start by smearing sealant over every joint or buying a full drain kit before you know the first wet point.
The cabinet stays dry until you run water, then you see drips at the trap, tailpiece, or right below the sink bowl.
Start here: Dry everything completely, run a small stream, and watch the sink strainer and each slip-joint from top to bottom.
There is water under the sink even when nobody has used it recently.
Start here: Check the sink shutoff valves and supply tubes first, because a true drain leak usually needs water flowing through the drain to show up.
A slow trickle may not leak, but a full-basin drain causes a fast drip or spray.
Start here: Focus on the sink strainer seal and any joint that may be slightly loose or misaligned under heavier flow.
The puddle is not directly under the trap, and the source is hard to see.
Start here: Use a flashlight and dry paper towels to find the first wet point high up, especially around the sink strainer, wall connection, and shutoff valves.
This is the most common under-sink drain leak, especially after cleaning, bumping the trap, or storing items in the cabinet.
Quick check: Dry the nuts and joints, run water, and look for a bead of water forming right at one slip-joint nut.
If water appears high up under the sink right below the drain opening, the strainer seal is a strong suspect.
Quick check: Fill the sink partway, then release it and watch the underside of the drain opening for seepage around the sink strainer body.
Older plastic traps can split at the bend or near the nut threads, and thin metal tailpieces can pinhole or corrode.
Quick check: Wipe the pipe dry and look for a fine line crack, mineral trail, or water weeping through the pipe wall instead of from a joint.
A shutoff valve, faucet hose, or supply tube can drip down onto the drain and make the trap look guilty.
Quick check: With the sink dry and no water running, watch the shutoff valves and supply lines for fresh moisture.
You save time by figuring out whether the leak needs water pressure or just drainage. Under-sink leaks get misread all the time because water runs down whatever is below it.
Next move: You now know whether the leak happens only during drainage or even when the sink is idle. If everything gets wet too fast to tell, dry it again and test one condition at a time: idle, slow drain, then full-basin drain.
What to conclude: A leak that shows up only while draining points to the sink drain assembly. A leak that appears with no drainage points away from the drain and toward shutoff valves, supply lines, or faucet connections.
The highest fresh moisture is usually the real source. The drip at the bottom of the trap is often just where the water finally falls off.
Next move: You should be able to narrow the leak to one of three places: the sink strainer, a slip-joint connection, or a cracked pipe section. If you still cannot isolate it, repeat the test with only a slow stream first, then a full-basin dump. Some leaks only show under heavier flow.
What to conclude: A wet spot right under the sink bowl usually means the sink strainer is leaking. A wet ring at a nut points to a slip-joint issue. Moisture through the pipe wall points to a cracked or corroded part.
This is the most common fix, and it is usually simple if the washers and pipe alignment are still good.
Next move: If the joint stays dry through a full-basin drain, the repair is done. If the joint still leaks after proper alignment and reassembly, the washer may be worn or the sink P-trap section may be distorted or cracked.
A sink strainer leak often shows up only when a larger volume of water passes through the drain opening, and it can mimic a trap leak below.
Next move: If a slight snug-up stops the seepage, test several more drain cycles to make sure it stays dry. If water still appears at the sink strainer body, the proper fix is to reseal or replace the sink strainer.
Once you know the exact source, the right repair is usually straightforward. Guessing at multiple parts just wastes time and can create more leaks.
A good result: A dry test after both light and heavy drainage confirms the repair path was correct.
If not: If a new washer or trap still leaks, the drain pieces may be mismatched, the wall connection may be out of line, or the problem may be in the sink strainer above.
What to conclude: Most under-sink drain leaks are local assembly problems. If the leak involves the wall connection, hidden piping, or multiple corroded parts, it is time for a plumber.
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That usually points to the sink strainer or a drain joint that only opens up under heavier flow. A full-basin dump puts more water through the drain at once, so weak seals show themselves faster.
No. Start with the leaking joint only. Overtightening is a common way to crack a plastic sink P-trap or distort a slip-joint washer. Snug and aligned beats very tight.
Watch the highest wet spot. If moisture starts right under the sink bowl at the drain opening, suspect the sink strainer. If the first wet spot is at a lower nut or at the bend of the trap, suspect the sink P-trap or its washers.
That is often not a drain leak. Check the sink shutoff valves, supply tubes, and faucet hose connections first. A pressurized leak can drip down onto the drain and make it look like the drain is at fault.
Not for a normal slip-joint connection. Those joints seal with the washer and proper alignment. If a slip-joint leaks, fix the washer, alignment, or damaged pipe instead of smearing sealant on the outside.
Replace the whole sink P-trap if the trap is cracked, warped, badly stained from old seepage, or the joints will not line up and seal even with a fresh washer. If the pipe is sound and only one washer is worn, a washer is usually enough.