Under-Sink Leak: Where to Look First
Why is the pipe under my sink leaking?
Most under-sink leaks come from a loose slip nut, worn washer, cracked P-trap, leaking supply line, or shutoff valve connection.
Can you fix an under-sink leak yourself?
Often yes. Tightening slip joints, replacing washers, or swapping a plastic P-trap are beginner-friendly repairs when the water is turned off.
When should you call a plumber?
Call a plumber if the shutoff valve leaks, copper pipe is corroded, water damage is spreading, or you cannot identify the leak point.
Quick Answer
A leaking pipe under the sink usually leaks from one of three places: a slip-joint connection on the P-trap (tighten by hand or 1/4 turn with pliers), the P-trap itself (disassemble, clean, replace washers), or the supply line connections (re-wrap PTFE tape, replace flexible supply lines). Place a bucket under the work, then test each connection with a flashlight while running water.
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Save to PinterestA leaking pipe under the sink is one of the most common plumbing problems, and one of the most satisfying to fix yourself. That puddle in the cabinet or steady drip beneath the basin usually comes from a loose connection or a worn washer, not a burst pipe, so the repair is often quick, cheap, and tool-light.
This guide walks you through finding exactly where the under-sink leak is coming from and fixing the usual culprits, the trap, the compression fittings, and the supply connections, step by step.
Why Pipes Leak Under the Sink
The pipework under a sink has several joints and seals, and a leak often starts at one of them:
- Loose slip-nut on the trap: The plastic nuts holding the U-bend (P-trap) work loose, the single most common under-sink leak.
- Worn washers or seals: The rubber washers inside the trap and compression joints perish and let water seep through.
- A blocked trap: A clogged trap can cause water to back up and force its way out of the joints.
- Failed supply connections: The connections on the supply lines feeding the taps can loosen or corrode and drip.
Tools and Materials You Will Need
- Bucket or shallow tray
- Adjustable wrench or pliers
- Replacement washers and seals
- PTFE (plumber’s) tape
- Old toothbrush and cloth
- Torch
- Rubber gloves
Step-by-Step: How to Fix a Leaking Pipe Under the Sink
Step 1: Find the Source of the Leak
Place a tray underneath, dry everything with a cloth, then run the tap and watch closely with a torch. Pinpoint whether the water appears at the trap, a compression joint, or a supply connection, this tells you exactly what to fix.
Step 2: Tighten the Connections
Often the fix is simply to hand-tighten, then gently snug up, the slip-nuts on the trap or the compression nuts. Do not overtighten plastic fittings, as that can crack them.
Step 3: Check and Clean the Trap
If tightening does not stop it, unscrew the trap (with the tray underneath to catch water), clean out any debris, and inspect the washers inside. Clearing a blocked trap often cures a leak caused by backing-up water.
Step 4: Replace Worn Washers and Re-tape Threads
Fit new washers where they are worn or flattened, and wrap fresh PTFE tape around any threaded metal connections to ensure a watertight seal before reassembling.
Step 5: Reassemble and Test
Put everything back together, run the tap, and fill and drain the sink while watching the joints. Check again after a few minutes to be sure the leak is fully stopped.
Pro Tips to Prevent This Problem
- Hand-tighten plastic fittings and only gently snug them with a wrench, overtightening cracks them.
- Keep a tray and torch handy, as some water always escapes when you open the trap.
- Take the old washer to the store to match the exact size and type.
When to Call a Professional
Call a plumber if the leak comes from a corroded or solvent-welded pipe rather than a joint, if you cannot isolate or stop the water, or if water has already damaged the cabinet or floor. A persistent leak you cannot trace, or corroded pipework, is best handled professionally.
Frequently Asked Questions
Why is my sink leaking underneath only when I run the water?
A leak that appears only when water flows usually comes from the trap or a drain joint rather than the supply lines. Tighten the slip-nuts first, then check the washers inside the trap.
How tight should I make the nuts under the sink?
Hand-tight plus a gentle snug with a wrench is usually enough, especially for plastic fittings. Overtightening can crack plastic nuts and damage washers, making the leak worse.
Can a blocked trap cause a leak?
Yes. A clogged trap makes water back up and force its way out of the joints. Clearing the trap often stops a leak that tightening alone would not fix.
Final Thoughts
A leaking pipe under the sink is usually a quick, low-cost fix. Trace exactly where the water appears, tighten the connections, clean the trap, and replace any worn washers with fresh PTFE tape on threaded joints. Avoid overtightening plastic fittings, and you will have a dry cabinet again in no time, leaving only corroded pipes or untraceable leaks to a plumber.