How to Fix a Leaking P-Trap Under the Sink
Updated September 18, 2026
Can I do this myself?
Safe to DIY
Yes, a leaking P-trap is one of the easiest plumbing repairs. It is a low-pressure drain part, so there is no need to shut off the water, just stop using the sink. Call a pro if the leak comes from the wall pipe or the cabinet floor is rotting.
Finding a puddle in the cabinet under your sink is unsettling, but in many cases the source is the P-trap: the U-shaped bend of pipe directly beneath the drain. The trap holds a small amount of water that blocks sewer gases from coming up into your home. It is joined to the rest of the drain with slip nuts and tapered washers, and these joints are exactly where most under-sink leaks start.
Before you buy anything, it is important to confirm the leak is actually from the trap. Water under a sink can also come from the faucet supply lines, the shut-off valves, the faucet base, or the sink drain's own seal (the basket strainer in a kitchen sink). Those are pressurised or different parts and need different fixes. A few minutes with a dry paper towel will tell you which one it is.
Once you know it is the trap, the fix is usually tightening a loose slip nut, re-seating a washer, or replacing the whole trap with a new plastic kit. No special skills or soldering are needed.
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Signs of the problem
- Water drips from the curved pipe or its nuts under the sink when water is running
- The cabinet floor is wet, warped or stained
- A musty smell or mould in the cabinet
- Drips only appear when the sink is drained, not when the faucet is off
- A sewer smell from the sink, which can mean the trap has dried out or is cracked
Common causes
Loose slip nuts
Most commonSlip nuts can work loose from vibration, knocking items in the cabinet, or plastic relaxing over time. A loose nut lets water seep out around the joint when the sink drains. Often a gentle hand-tightening is all it takes.
Worn, flattened or misaligned slip washers
Most commonEach joint has a tapered plastic or rubber washer that compresses to seal. Washers harden, crack, or sit crooked if the pipes were not aligned when assembled. Replacing them is quick and cheap.
Cracked or corroded trap
SometimesPlastic traps can crack if over-tightened or hit. Older metal (chrome-plated brass or steel) traps corrode from the inside and develop pinholes, especially along the bottom of the bend. The fix is to replace the trap.
Leak is from somewhere else
SometimesDrips can travel along pipes before falling, so the trap can look guilty when the real source is a supply line, shut-off valve, faucet base, or the sink strainer above it. Always trace the water to its highest wet point.
Safety first: before you start
- Critical:Put a bucket under the trap before loosening anything. The trap is always full of dirty water, and more can drain down from the sink.
- Critical:Do not open the trap if a chemical drain cleaner has been used recently. The water inside can cause burns. Flush the drain with plenty of cold water first if it drains, or call a plumber.
- If water has reached a plug socket, garbage disposal or water purifier in the cabinet, switch off that circuit at the breaker or fuse box before reaching in.
- Wear rubber gloves and safety glasses. Trap water and sludge carry bacteria.
- Empty the cabinet and keep cleaning products away from where you are working so they do not tip into the bucket.
Tools & materials
Tools
- Bucket and old towels
- Flashlight
- Dry paper towels (for finding the leak)
- Adjustable slip-joint pliers (groove-joint pliers)
- Rubber gloves and safety glasses
- Bottle brush or old toothbrush
- Tape measure and hacksaw (only if cutting a new trap arm to length)
Materials
- Replacement slip washers matched to your pipe size (usually 32 mm/1-1/4 inch for bathroom sinks, 38-40 mm/1-1/2 inch for kitchen sinks)
- Complete P-trap kit, if the trap is cracked or corroded
Step-by-step fix
- 1
Dry everything and find the source
Empty the cabinet and wipe every pipe, valve and connection completely dry. Then run water in the sink for a minute and hold a dry paper towel against each joint in turn, starting at the top: faucet supply connections, shut-off valves, the underside of the sink drain, then each trap nut. The first place the towel gets wet is the source.
Tip: If the leak only appears when you fill the sink and then drain it, it is on the drain side. If it drips even when nothing is running, it is a supply line or valve, not the trap.
- 2
Try hand-tightening the slip nuts
If a trap nut is leaking, turn it clockwise (as viewed from the direction you are screwing it on) by hand until snug. Test again. Plastic nuts often only need a small turn. If it still drips, give it a further quarter turn with slip-joint pliers.
Warning: Overtightening plastic nuts cracks them or distorts the washer and makes the leak worse. Stop at firm hand-tight plus a quarter turn.
- 3
Remove the trap
If tightening does not work, put the bucket directly under the trap. Loosen both slip nuts (the one at the sink tailpiece and the one at the trap arm going to the wall) counter-clockwise. Lower the U-bend and pour its water into the bucket.
- 4
Inspect and clean
Check the trap for cracks, especially at the bottom of the bend and around the threads. On metal traps, look for thin, flaking spots or pinholes. Clean out any debris with a bottle brush. Look at each washer: it should be firm, smooth and evenly tapered.
- 5
Replace the washers
Slide off the old washers and fit new ones with the tapered (narrow) side facing into the joint, toward the pipe it seals against, and the flat side toward the nut. A washer installed backwards is a very common reason a joint keeps leaking.
- 6
Fit a new trap if needed
If the trap is cracked or corroded, buy a trap kit of the same diameter. Plastic kits are easiest for beginners. Slide the nuts and washers onto the tailpiece and trap arm in the order shown on the package, then fit the U-bend. If the trap arm needs shortening, cut it squarely with a hacksaw and smooth the edge.
- 7
Align the pipes before tightening
The pipes should meet straight and square, without being forced sideways. A joint that is pulled at an angle will leak no matter how tight the nut is. Adjust the trap position until everything lines up, then hand-tighten each nut.
- 8
Test with a full sink
Close the stopper, fill the sink most of the way, then pull the plug so a large volume of water rushes through. Watch and feel each joint with a dry paper towel. Tighten a quarter turn more only where needed. Leave a dry paper towel under the trap overnight as a final check.
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How to check it's fixed
- A dry paper towel stays dry at every joint after draining a full sink
- No new water in the cabinet after 24 hours of normal use
- The sink drains freely with no gurgling
When to stop and call a professional
- The leak is from the pipe where it enters the wall or floor
- The leak is from a supply line or shut-off valve and you cannot stop it
- The cabinet floor or wall behind the sink is soft, rotten or mouldy
- The metal trap or drain pipes are so corroded they crumble when touched
- Water has damaged the ceiling below or is near electrical wiring
How to prevent it
- Store heavy or bulky items away from the trap so they do not knock it loose.
- Check under the sink with a flashlight every couple of months for drips or stains.
- Avoid chemical drain cleaners, which can soften plastic and corrode metal traps.
- Put a waterproof tray or mat in the cabinet so small leaks are visible early.
Frequently asked questions
Do I need to turn off the water to fix a P-trap?
No. The P-trap is on the drain side, which is not under pressure. Just stop anyone from using the sink while the trap is off, and keep a bucket underneath for the water it holds.
Which way do the washers go in a P-trap?
The tapered side of the slip washer faces into the joint, toward the pipe it seals against, with the flat side against the nut. Installed backwards, it cannot compress properly and the joint will drip.
Should I use plumber's tape or sealant on slip nuts?
Generally no. Slip joints seal with the washer, not the threads. Tape or putty on the threads can stop the nut seating properly. Some plumbers add a little tape on metal threads, but a correctly fitted washer is what matters.
Why does my sink smell like sewage?
The water in the trap blocks sewer gas. If a sink is rarely used, the water can evaporate. Run water for a minute to refill it. If the smell persists, the trap may be cracked, loose, or the drain may be poorly vented.
Can I replace a metal P-trap with a plastic one?
Usually, yes. Plastic trap kits come with adapters for connecting to metal tailpieces and trap arms of common sizes. Check local plumbing rules, and make sure the diameter matches your existing pipes.
This guide is general information, not professional advice. Homes differ, so follow your product manuals and local codes. If anything looks different from what's described, or you feel unsure, stop and call a qualified professional. In an emergency, leave the area and call your local emergency number.
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