How to Repair a Leaking P-Trap
That puddle under your sink is almost always coming from the P-trap — the curved pipe section that holds a small pool of water to block sewer gases. P-trap leaks are among the easiest plumbing repairs because the connections are designed to come apart by hand. In most cases, the fix is simply tightening a slip nut or replacing a cheap rubber washer. The entire job takes 15-20 minutes and requires no special tools beyond a pair of slip-joint pliers.
Time Required
15-20 minutes
Difficulty
Easy
When to Call a Pro
Corroded metal trap or pipes inside the wall
Step-by-Step: Fixing a Leaking P-Trap
Place a bucket under the P-trap
The P-trap always holds water — that's its job. Before you touch anything, position a bucket or large bowl directly under the curved section to catch the water when you disconnect it. Clear out cleaning supplies and anything else under the sink to give yourself room to work. A headlamp or flashlight is helpful since the area is usually dark.
Identify the leak location
Dry the entire P-trap with a paper towel, then run water slowly and watch exactly where the drip appears. The leak will be at one of three places: the top slip nut (where the trap connects to the tailpiece coming down from the sink), the bottom slip nut (where the trap connects to the wall pipe), or from a crack or hole in the trap itself. Knowing the exact location determines your repair approach.
Try tightening the slip nuts first
Many P-trap leaks are simply loose connections. Try hand-tightening both slip nuts by turning them clockwise (when viewed from below). If they're already hand-tight, use slip-joint pliers for an additional quarter turn. Do not use pipe wrenches or channel locks aggressively — plastic slip nuts crack easily, and overtightening chrome nuts can strip the threads.
Disassemble and replace washers
If tightening doesn't stop the drip, unscrew both slip nuts and lower the P-trap curve into your bucket. Pour out the standing water (it will be dirty — that's normal). Slide the slip nuts up the pipe to expose the washers. The old washers will be flat, compressed nylon or rubber rings. Replace them with new slip-joint washers from a hardware store — make sure the beveled (tapered) side faces up, toward the slip nut. This is the number one mistake people make during reassembly.
Reassemble and align the trap
Slide the P-trap back into position, making sure the tailpiece and wall pipe are properly aligned so the trap sits level. A misaligned trap puts uneven pressure on the washers and will leak again. Hand-tighten both slip nuts first, then snug each with pliers — about a quarter turn past hand-tight. The connection should be firm but not forced.
Test with running water
Run water for a full 30 seconds while watching the connections closely. Then turn the water off, dry the outside of the entire P-trap assembly with a paper towel, and wait 3-5 minutes. Check for any dampness — some leaks only appear under pressure or take time to seep through. If all is dry, you're done.
When to Replace the Entire P-Trap
- Cracked or corroded trap: If the P-trap itself is cracked, badly corroded, or has a pinhole leak, no amount of tightening will fix it. Replace the entire trap assembly — they cost $5-12 for plastic or $12-25 for chrome.
- Cross-threaded slip nuts: If the slip nut threads are stripped or cross-threaded, the nut can't seal properly. Don't force it — replace the damaged piece.
- Metal-to-plastic transitions: Old chrome traps connected to new plastic tailpieces often leak because the diameters don't match precisely. Replace with an all-plastic or all-chrome assembly for consistent sizing.
- Persistent odor: If sewer gases are coming through despite the trap being full of water, the trap may have an internal crack above the water line that isn't visible. Replace it.
Understanding P-Trap Anatomy
- Tailpiece: The straight pipe that drops down from the sink drain. The P-trap connects to the bottom of this.
- Trap curve (J-bend): The U-shaped section that holds standing water. This is the actual "trap" that blocks sewer gases.
- Trap arm: The horizontal pipe that runs from the P-trap into the wall drain. Building codes regulate its length and angle.
- Slip nuts: The threaded nuts that hold the trap in place at both ends. They compress washers against the pipe to create a watertight seal.
- Slip-joint washers: Beveled nylon or rubber rings that deform when the slip nut compresses them, filling the gap between the pipe and the fitting.
Pro Tips
- •Bevel side up — always: Slip-joint washers have a flat side and a beveled (angled) side. The beveled side must face up toward the slip nut. If installed upside down, the washer won't compress into the joint properly and will leak no matter how tight you make it.
- •Clean the pipe surfaces: Old pipe dope, mineral deposits, or corrosion on the pipe surface will prevent the new washer from sealing. Wipe the insertion areas clean with a rag before reassembling.
- •Check the trap for clogs while you have it apart: Since you're already disassembled, take 30 seconds to clean the inside of the P-trap. Hair, soap scum, and grease accumulate in the curve and slow drainage.
- •Keep spare washers on hand: A pack of assorted slip-joint washers costs $2-3 and covers every sink in your house. Buy one and keep it with your plumbing supplies — you'll use them more often than you think.
Frequently Asked Questions
Why does my P-trap keep leaking even after I tighten it?
The slip-joint washers are likely worn, compressed, or installed incorrectly. The beveled side of the washer must face up (toward the slip nut). Also check that the pipes are aligned — if the tailpiece and wall pipe are not lined up, the P-trap sits at an angle and the washers cannot seal evenly.
Can I use plumber's putty or silicone on P-trap connections?
No. P-trap connections are designed to seal with slip-joint washers and compression only. Adding putty or silicone can actually prevent a proper seal and makes future disassembly messy and difficult. If the connection won't seal with new washers, the pipes or fittings need replacement.
What is the P-trap for and why does it hold water?
The P-trap's curved shape holds a small amount of water at all times, creating a seal that blocks sewer gases from entering your home through the drain. If you smell sewer odor from a sink, the P-trap may have dried out from disuse — just run water for 30 seconds to refill it.