How to Fix a Leaking Pipe Under the Sink

A leak under the sink is one of the more common plumbing problems homeowners deal with. The good news is that many under-sink leaks are relatively straightforward to fix without calling a plumber - as long as you can identify exactly where the water is coming from.

Find the Source First

Before you touch anything, dry the area under the sink completely with a towel or paper towels. Then run the water for a minute or two and watch closely. Look for where the dripping or wetness first appears.

Common sources include:

• The P-trap - the curved pipe section that looks like a U or P. This is the most frequent culprit.
• Slip joint connections - the plastic or metal nuts where pipe sections connect
• The drain basket - the fitting that connects the sink basin to the drain pipe below
• The supply lines - the hoses that bring hot and cold water up to the faucet

Each has a different fix.

Fixing a Leaking P-Trap

The P-trap is held in place by large plastic slip nuts. Over time, the washers inside wear out or the nuts loosen. To fix it:

1. Place a bucket under the P-trap to catch water still inside the pipe
2. Unscrew the slip nuts by hand - they usually do not require tools
3. Pull the trap section out and inspect the washers at each end
4. If the washers are cracked, flat, or missing, replace them (they are inexpensive at any hardware store)
5. Reassemble, hand-tighten the slip nuts, and test

If the P-trap itself is cracked or the plastic has become brittle, replace the whole section. A replacement P-trap typically costs a few dollars.

Fixing a Loose Slip Joint

If water is dripping from a connection point between pipe sections, try tightening the slip nut first. If that does not stop the leak, unscrew the nut, check the washer inside, and replace it if worn.

Do not overtighten plastic slip nuts - hand tight plus about a quarter turn is usually enough. Over-tightening cracks the nut and creates a new leak.

Fixing a Leaking Drain Basket

If water is dripping from where the drain meets the sink basin, the drain basket seal has likely failed. This is a slightly more involved fix:

1. From above the sink, remove the drain stopper or strainer
2. From below, unscrew the large nut that holds the basket in place (you may need slip-joint pliers)
3. Lift the basket out from above
4. Clean off the old plumber's putty from both the basket flange and the sink surface
5. Roll fresh plumber's putty into a rope, press it under the basket flange, and reinstall
6. Hand-tighten the locknut from below, then snug it with pliers

Run water and check for drips before reassembling the cabinet.

Fixing Leaking Supply Lines

Supply lines - the braided or ribbed hoses connecting the shutoff valves to the faucet - can develop pinhole leaks or fail at the fittings. If a supply line is visibly dripping:

1. Turn off the shutoff valves under the sink
2. Unscrew the supply line from both ends (the shutoff valve and the faucet tail piece)
3. Take the old supply line to the hardware store to match the size and connection type
4. Install the new line and hand-tighten the fittings
5. Turn the water back on slowly and check for leaks

Supply lines are inexpensive. When in doubt, replace them even if they look fine - they typically last around ten years and failures tend to happen suddenly.

When to Call a Plumber

DIY fixes work well for the parts listed above. Call a plumber if:

• The leak is coming from a copper or galvanized steel pipe (not a slip-joint connection)
• The pipe itself is cracked or corroded, not just a fitting
• You trace the water back to inside the wall
• You find soft or rotted wood under or behind the cabinet, suggesting the leak has been ongoing

Preventing Under-Sink Leaks

Check under your sinks every few months. Look for moisture, white mineral deposits around fittings, or any soft spots in the cabinet floor. Catching a slow drip early avoids water damage to the cabinet base and the floor beneath.

Sources and editorial notes

Prices and permit requirements vary by location, property, equipment, and scope. Treat price ranges as planning estimates and confirm current requirements with the relevant authority and contractor.

• US Small Business Administration - business guide

Related guides

• How to Fix a Slow Drain in Your Sink or Shower
• How to Unclog a Sink Without Chemicals
• How Much Does It Cost to Fix a Leaking Roof?

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