Water Leaking Under the Kitchen Sink? How to Find the Source

Water under a kitchen sink does not always mean the P-trap is leaking. The water may be coming from a supply connection, shut-off valve, drain connection, or loose slip nut.

Before replacing anything, dry the cabinet and identify exactly when and where the water appears. This simple inspection can help you avoid repairing the wrong component.

Watch the video below for a step-by-step inspection:

Three Common Sources of Water Under a Kitchen Sink

Start by checking these three areas:

1. Water supply connections and shut-off valves
2. Drain connection directly below the sink
3. P-trap and slip-nut connections

The timing of the leak can provide an important clue.

• If water appears while the faucet is off, check the pressurized water-supply connections.
• If water appears only while the faucet is running or the sink is draining, inspect the drain system.
• If water appears around a P-trap slip nut, the connection may be loose, misaligned, damaged, or improperly seated.

Before You Begin

Remove stored items from the cabinet and completely dry the cabinet floor, pipes, valves, and fittings.

Place a bucket or towel under the drain assembly before loosening any connection. If water is actively leaking from a supply line or valve, turn off the appropriate shut-off valve if it can be operated safely.

Do not touch an electrical outlet, plug, or cord with wet hands. If water has reached an electrical component, avoid the area and contact a qualified professional.

Step 1: Check the Water Supply Connections

Begin with the faucet turned off.

Use a clean, dry paper towel to check around:

• Hot- and cold-water shut-off valves
• Braided supply-line connections
• Faucet supply connections
• Dishwasher water-supply connection, if present

A paper towel can reveal a small amount of moisture that may be difficult to see.

If the paper towel becomes wet while the faucet is off, the leak may be on the pressurized water-supply side rather than the drain system.

Do not continue tightening a damaged valve, corroded fitting, or leaking supply line. These components may require repair or replacement by a qualified plumber.

Step 2: Check the Drain Connection

If the supply connections remain dry, run water into the sink while watching the drain assembly.

Inspect the connection directly below the sink and follow the drainpipe toward the P-trap and wall connection. Use a flashlight if necessary, but keep electrical devices away from standing water.

A leak that appears only while water is draining usually indicates a problem somewhere in the drain system. Possible causes include:

• A loose slip nut
• A misaligned drainpipe
• An incorrectly seated or damaged washer
• A cracked fitting
• A failed seal at the sink drain connection

Do not assume that the lowest wet point is the source. Water can travel along the outside of a pipe before dripping onto the cabinet floor.

Step 3: Inspect the P-Trap

The P-trap is the curved section of drainpipe beneath the sink. Its purpose is to retain a small amount of water that helps prevent sewer gases from entering the home.

If moisture appears around a P-trap slip nut, turn off the faucet and place a bucket underneath the connection.

For a plastic slip-joint connection:

1. Loosen the slip nut carefully by hand.
2. Check that the pipes are properly aligned.
3. Inspect the washer for damage, distortion, or incorrect seating.
4. Reassemble the connection without forcing the pipes together.
5. Hand-tighten the plastic slip nut.

Avoid overtightening. Excessive force can distort the washer, damage plastic threads, or crack the fitting.

This procedure applies only to an accessible plastic slip-joint connection in serviceable condition. Glued, corroded, cracked, or concealed drain components should not be handled in the same way.

Step 4: Test Every Connection

After reassembling the connection, dry the entire area again.

Run water into the sink and use a fresh, dry paper towel to check each connection. Test one location at a time so you can identify the exact source if moisture returns.

The paper towel and cabinet floor should remain completely dry.

Also check the drain while the sink contains and releases a larger volume of water. A connection may remain dry under a small flow but leak when the drain is carrying more water.

When Should You Call a Plumber?

Contact a qualified plumber if:

• A shut-off valve or supply line continues leaking
• A pipe or fitting is cracked, corroded, or damaged
• Water has reached an electrical outlet or cord
• The leak is inside a wall or cabinet structure
• The P-trap will not align correctly
• The leak returns after the connection is reassembled
• You cannot identify the source safely

For condo residents, report a continuing leak promptly to the building manager or emergency contact, especially if water may affect another unit.

Final Tip

Always identify the source before attempting a repair. Completely drying the area and checking each connection with a clean paper towel is often the simplest way to determine whether the problem is on the water-supply side or the drain side.

This information is based on practical plumbing experience and is intended for general homeowner and condo-maintenance education. Plumbing systems and repair requirements may vary. When in doubt, contact a qualified plumber.

For more practical plumbing information, follow CJ BC Plumbing Tips.