Is the cabinet floor visibly rotted, is there mold, OR has chemical drain cleaner been used recently in this drain?

Soft/rotted cabinet floors mean a chronic leak that may have spread into framing. Chemical cleaner residue can burn skin if a slip joint is opened.

Dry the trap completely with a paper towel, then run water for 30 seconds. Where does the first drip appear?

Marking the drip point precisely saves disassembly. Watch from below with a flashlight.

Place a bucket under the trap, unscrew the slip nut by hand, and slide it back. Inspect the nylon/rubber washer inside. What do you see?

The beveled (slanted) side of the washer faces the trap. A flat, deformed, or split washer will leak no matter how tight the nut.

Hand-thread the slip nut back on. Does it spin on smoothly for at least 3 full turns before snugging up?

If the nut catches, binds, or goes on crooked, it is cross-threaded. Cross-threaded plastic nuts cannot seal.

plumbing troubleshooting

Fix P-trap leak

Common symptoms: leaking p-trap; water under sink; drip from drain; cabinet floor wet; slip joint leak

Stop and call a pro if:

mold and rotted cabinet floor
raw sewage exposure
chemical drain cleaner risk

Step-by-step diagnostic flow

Step 1
Is the cabinet floor visibly rotted, is there mold, OR has chemical drain cleaner been used recently in this drain?
Soft/rotted cabinet floors mean a chronic leak that may have spread into framing. Chemical cleaner residue can burn skin if a slip joint is opened.
Step 2
Dry the trap completely with a paper towel, then run water for 30 seconds. Where does the first drip appear?
Marking the drip point precisely saves disassembly. Watch from below with a flashlight.
Step 3
Place a bucket under the trap, unscrew the slip nut by hand, and slide it back. Inspect the nylon/rubber washer inside. What do you see?
The beveled (slanted) side of the washer faces the trap. A flat, deformed, or split washer will leak no matter how tight the nut.
Step 4
Hand-thread the slip nut back on. Does it spin on smoothly for at least 3 full turns before snugging up?
If the nut catches, binds, or goes on crooked, it is cross-threaded. Cross-threaded plastic nuts cannot seal.

Possible outcomes

Stop — assess water damage or chemical exposure first

high confidence

Rotted cabinets or chemical-cleaner residue change this from a slip-nut job into a damage-mitigation or hazmat situation.

Safe next steps

Shut off the supply stops under the sink so the basin cannot be filled
Photograph the damage and any product labels
Ventilate the cabinet and avoid skin contact with standing water

What to document for a pro

How long the leak has been present
Brand of any drain cleaner used

Possible structural rot or mold remediation needed. Chemical drain cleaner residue in the trap

Replace the slip-joint washer

high confidence

A failed slip-joint washer is the most common cause of a P-trap leak and is a low-risk swap.

Safe next steps

Take the old washer to the hardware store to match size (typically 1-1/4" or 1-1/2")
Install the new washer with the beveled edge facing into the trap fitting
Hand-tighten the slip nut, then add at most a quarter turn with channel-locks wrapped in tape
Run water for 60 seconds with a dry paper towel under the joint to confirm dry

What to document for a pro

Trap pipe diameter
Whether trap is PVC or chromed brass

Cross-threaded slip nut — restart the threads

high confidence

Cross-threading prevents the washer from compressing evenly. Restarting threads by hand usually fixes it.

Safe next steps

Fully back off the nut and inspect threads for visible damage
Align the nut square to the pipe and turn counter-clockwise until you feel a click (threads dropping in)
Then turn clockwise — it should spin freely for several turns by hand
Snug with hands only; finish with a quarter turn from padded pliers
If it cross-threads again, the nut or pipe threads are stripped — replace the trap

What to document for a pro

Photo of nut threads
Trap material

Replace the P-trap assembly

medium confidence

Cracked traps, pinhole leaks, and stripped threads are not repairable — the trap kit is inexpensive and swaps in minutes.

Safe next steps

Buy a complete P-trap kit matching your pipe diameter (1-1/4" bath, 1-1/2" kitchen typical)
Bucket under the work area; remove the old trap by hand
Dry-fit the new trap before tightening anything to confirm alignment with wall stub-out and tailpiece
Install washers beveled-edge-into-fitting, hand-tighten, then a quarter turn with padded pliers
Fill the sink basin, then release — a full-volume test stresses joints more than a trickle

What to document for a pro

Pipe material and diameter
Photo of existing trap configuration before removal

Reseat and snug — retest before deciding

medium confidence

Threads and washer both check out — the leak may have been from an under-tightened or shifted joint.

Safe next steps

Hand-tighten all slip nuts, then add a quarter turn with padded channel-locks
Dry every joint completely with paper towels
Run water for 2 full minutes and watch each joint with a flashlight
If a drip returns, redo the washer-replacement workflow

What to document for a pro

Which joint reappeared as a leak
Photo of the trap configuration

Diagnostic guidance only. If unsure, stop and call a licensed professional — gas, electrical, and refrigerant work is hazardous to untrained users.

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