Pipe Leaking Under Sink: Causes, Diagnosis & Fixes
Under-sink leaks are usually caused by loose compression fittings, worn rubber washers, or corroded supply lines — and most are fixable without a plumber. Parts cost between $5 and $25, and the repair typically takes under an hour. You'll need a bucket, adjustable wrench, and possibly replacement washers or a new P-trap to stop the drip.
Step-by-Step Fix
- 1
Turn off the water supply
Locate the shut-off valves under your sink (usually oval knobs on the hot and cold lines) and turn both clockwise until they stop. Open the faucet briefly to release remaining water pressure in the lines. Place a bucket underneath the leaking area to catch any residual water.
- 2
Identify the leak source
Dry the pipes thoroughly with a towel and visually inspect all connection points: the compression fittings where supply lines meet the shutoff valves, where supply lines connect to the faucet body, and the P-trap joints. The leak location tells you what part to fix or replace.
- 3
Tighten loose compression fittings
Using an adjustable wrench, turn the nut at the base of each leaking fitting clockwise about one-quarter turn. Do not over-tighten, as this can crack the fitting. Turn the water back on and check if the leak has stopped; if it persists, proceed to the next step.
- 4
Replace the rubber washers
Turn the water off again. Unscrew the compression fitting nut completely with your wrench and pull out the old brass ferrule and rubber washer inside. Take the old washer to a hardware store to match the size (usually 3/8 or 1/2 inch), install the new one, and reassemble the fitting hand-tight, then snug with the wrench.
- 5
Reseal or replace the P-trap if needed
If the leak is coming from the P-trap (the U-shaped pipe below the sink), unscrew both slip nuts with your wrench and remove the trap. Inspect the rubber washers inside the slip nuts and replace them if cracked or hardened. Reinstall the trap, hand-tighten the nuts, then snug with the wrench — do not over-tighten.
- 6
Test and monitor
Turn the water supply back on fully and run the sink for two minutes while watching all connections and the trap for drips. Leave a towel under the sink for one hour and check it afterward to confirm the leak is gone. If water still drips, the supply line itself may be corroded and need replacement.
⚠️ Safety Notes
- Turn off the water supply before loosening any fittings to prevent flooding and water waste.
- Do not use a pipe wrench on chrome-plated compression fittings, as it will leave visible marks; use an adjustable wrench instead.
- If you smell gas or see corrosion on brass fittings, do not attempt the repair — contact a licensed plumber immediately.
When to Call a Pro
If tightening fittings and replacing washers does not stop the leak within 30 minutes, or if the leak is coming from inside the supply line itself (visible pinhole or cracked tubing), the line needs professional replacement. Also call a plumber if you find mold, soft wood, or soft drywall around the leak, as this indicates water damage requiring inspection.
Frequently Asked Questions
- Why is water dripping under my sink?
- Under-sink leaks are most commonly caused by loose or corroded compression fittings, worn rubber washers that have hardened or cracked, or a deteriorated P-trap. Tightening fittings or replacing washers fixes approximately 80 percent of leaks.
- How much does it cost to fix a leaking under-sink pipe?
- Rubber washers cost $2 to $5, slip-nut washers cost $3 to $8, and a replacement P-trap costs $15 to $25 at any hardware store. Most DIY repairs cost under $30 in parts.
- Can I fix a leaking pipe under the sink myself?
- Yes, tightening fittings and replacing washers are easy DIY tasks that require only a wrench and 20 to 45 minutes. If the leak is from a corroded supply line, professional replacement is required.
- What is the P-trap and why does it leak?
- The P-trap is the U-shaped pipe below the sink that holds water to create a seal blocking sewer gases. It leaks when its slip-nut washers harden, crack, or compress over time — replacement washers cost $3 to $8.
- How tight should compression fittings be?
- Hand-tighten compression fittings first, then use an adjustable wrench to turn the nut one-quarter to one-half turn clockwise. Stop immediately if you feel resistance; over-tightening cracks the fitting and makes the leak worse.
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