Fix a Leaking P-Trap Under the Sink

The P-trap under your sink is a simple U-shaped piece of pipe that holds standing water to block sewer gases from coming up through the drain. It's one of the few plumbing fixtures you can actually repair yourself without calling a plumber, and a leak here is almost never a structural problem—just a loose connection or a worn-out rubber washer that's lost its grip. The water pooling under your cabinet is annoying but fixable. This guide walks you through identifying where the leak is, taking apart the trap, and putting it back together tight. You don't need special tools or advanced skills. Most leaks stop the moment you tighten the slip nuts or swap in new washers.

1. Kill the Water Supply First. Locate the shut-off valve under the sink (usually on the hot and cold supply lines coming from the wall) and turn both clockwise until snug. You don't need to turn the house water off entirely unless the valve under the sink doesn't work. Remove everything from inside the cabinet—cleaning supplies, trash cans, anything that gets in your way. This gives you clear access to the P-trap and room to work.
2. Catch the Drain Water. Slide a bucket or shallow pan directly under the P-trap (the U-shaped section). The trap is always full of water, and when you loosen the nuts, that water will drain out. A standard 5-gallon bucket works, but any container at least 8 inches deep will catch the spill.
3. Find Both Slip Nuts. Look at the P-trap. You'll see a nut at the top (where it connects to the sink's tailpiece) and a nut at the bottom (where it connects to the wall drain pipe). These are slip nuts—hexagonal nuts that thread onto the outside of the pipes. The leak is almost always coming from one of these two connections. Loosen the nut at the top first by turning it counterclockwise by hand. If it won't budge, use an adjustable wrench. You need only to loosen it, not remove it completely.
4. Check the Top Washer. Once the top nut is loose enough by hand, unscrew it completely and set it aside. Behind that nut is a beveled rubber washer (it looks like a shallow cone). Slide the washer off. If it's cracked, flattened, or hard, it's not sealing anymore and needs to be replaced. If it looks intact and flexible, wipe it clean with a dry rag and set it on the bench—you might reuse it.
5. Loosen the Bottom Connection. Now loosen the slip nut where the P-trap connects to the wall drain pipe. This nut is often harder to reach and may be tighter. Use an adjustable wrench if your hand won't budge it. Once it's loose enough, unscrew it by hand and remove it completely. Behind this nut is another beveled washer. Slide it off and inspect it the same way. You'll now have the P-trap sitting free in your bucket—don't lift it yet, it still contains water.
6. Empty the Trap Completely. With both nuts loose and the washers removed, lift the P-trap gently and pour any remaining water into the bucket. You'll get about a quart to two quarts of water depending on how much the drain has been used. Set the empty trap on a clean surface—paper towels work fine. Let it drip for a minute, then inspect the interior with a flashlight. You're looking for mineral buildup, debris, or corrosion that might prevent a good seal.
7. Install Fresh Washers. If either washer was cracked, hard, or flattened, replace both washers as a pair—buy new beveled rubber washers that match the diameter of your pipes (typically 1.5 inches for standard sink traps). Slide the new washer onto each slip nut with the beveled side pointing downward. The slight cone shape should face into the fitting to create a tight seal.
8. Wipe All Contact Points. Using a dry rag or paper towel, wipe the inside of both slip nuts and the outside of the pipes where the washers will sit. Remove any mineral deposits, old rubber bits, or sediment. A clean surface seals much better than a dirty one. If deposits are heavy, use a small brush or old toothbrush to scrub them away, then wipe dry.
9. Reconnect the Top First. Slide the P-trap back into position, aligning it so the top connector sits flush against the sink's tailpiece. Thread the slip nut back onto the top connection by hand first—this ensures you're starting the threads straight. Once it's hand-tight, use an adjustable wrench to tighten it. Tighten until snug, but don't overtighten; you're aiming for the washer to compress just enough to seal, not to strip the fitting. A quarter-turn past hand-tight is usually right.
10. Secure the Bottom Connection. Now tighten the slip nut where the trap connects to the wall drain. Again, start by hand to ensure the threads catch, then use the wrench. Tighten until snug. Both connections should be equally tight—not hand-loose, but not cranked with all your strength either. A leak here comes from undertightening, not overtightening, so err on the firmer side.
11. Run the Water Test. Return to the shut-off valve and turn the water back on. Let both hot and cold water run for 10 to 15 seconds to fill the P-trap. Then stop the flow and wait 30 seconds. Look closely at both slip nuts and the seams of the P-trap. Water should not be dripping or pooling. If it is, turn the water off immediately and tighten the leaking nut another quarter-turn. Repeat until the leak stops.
12. Monitor for 24 Hours. Once the trap is dry and tight, put your cleaning supplies and trash can back into the cabinet. Leave the cabinet open or check it periodically over the next 24 hours. Watch for any water pooling under the sink or dripping from the trap. If everything stays dry after a full day of normal use, you're done.