How to Fix a Leaking Bathroom Vanity Drain

Drips under a bathroom vanity are a nuisance that often signal a failed seal or a loosened connection. Ignoring that small puddle leads to warped cabinet wood, mold growth, and eventual damage to your bathroom flooring, so addressing it quickly is the only way to protect your investment. Successfully fixing this project relies on isolating the source. Most leaks happen because the plastic slip-joint washers have hardened over time or the connections have vibrated loose. Once you pinpoint the drip, it is almost always a straightforward matter of cleaning threads, swapping a gasket, or tightening a nut by hand to get things bone-dry again.

1. Spot the Leak Source. Empty all items from under the vanity and wipe the pipes completely dry with a rag. Place a dry paper towel under the P-trap and supply lines, then wait a few minutes to see exactly where the moisture appears.
2. Hand-Tighten First. If the leak is at a joint, try tightening the plastic nut by hand. If it is already tight but still leaking, you will need to unscrew it completely to inspect the washer.
3. Check the Washer. Once the nut is unscrewed, pull the washer out and check it for cracks, mineral buildup, or flattening. If it feels brittle or shows signs of wear, replace it with a new washer of the exact same size.
4. Clean All Threads. Wipe the threads on the pipe and the inside of the nut to remove any grit or soap residue. A clean surface allows the new washer to seat flush and create an airtight seal.
5. Reseat the Washer. Slide the nut onto the pipe, seat the new washer, and hand-tighten the nut back onto the drain. Ensure the pipe is properly aligned so you do not cross-thread the plastic connection.
6. Confirm the Fix. Turn on the faucet and run water for at least two minutes, checking the joint with a fresh dry paper towel. If the towel remains dry, the repair is successful.