How to Fix a Leaking Sink and Drain Problems

You reach under the kitchen sink for a trash bag and your hand lands on a damp cabinet floor. Maybe there’s a water stain, a musty smell, or a slow drip you never noticed. A leaking sink is one of the most common plumbing problems in any home, and it’s usually easier to fix than it looks.

Most leaks come from a few predictable spots: loose drain connections, worn washers, aging supply lines, or a failed seal around the drain. Below are a five-minute test to find the exact source, step-by-step fixes for supply-line and drain leaks, and the warning signs that mean it’s time to call a plumber.

 

Why Is Your Sink Leaking Underneath?

Quick answer: A sink leaking underneath is usually caused by a loose slip nut on the P-trap, a worn washer, a failing water supply line, or a broken seal around the drain basket. Dry the cabinet, run water in stages to find the source, then tighten or replace the faulty part. Leaks inside the wall need a plumber.

Small leaks add up fast. According to the EPA’s Fix a Leak Week program, household leaks waste more than 9,300 gallons of water a year in the average home. Under a sink, the bigger cost is often the damage: swollen cabinet floors, rotting subfloor, and mold.

Use this table to narrow down the cause based on where you see water.

Where you see water	Likely cause	DIY fix	Call a pro if…
Curved P-trap or its nuts	Loose slip nut or worn washer	Tighten the nut, replace the washer	The trap is cracked metal or keeps leaking
Under the drain basket	Dried-out plumber’s putty or gasket	Remove and reseal the strainer	The sink flange is cracked or corroded
Shutoff valve or supply hose	Loose compression nut or aging supply line	Tighten 1/4 turn or replace the line	The valve won’t close or is badly corroded
Faucet base, only when water runs	Worn O-rings or loose sprayer hose	Replace O-rings or reconnect the hose	The faucet body is cracked
Dishwasher or disposal connections	Loose hose clamp or failed disposal seal	Tighten the clamp, reseat the hose	Water drips from the bottom of the disposal
Back wall where the drain enters	Loose trap arm nut or pipe leak inside the wall	Tighten the trap arm nut	Water keeps coming from inside the wall

How to Find a Leak Under Sink Cabinets in 5 Minutes

Water travels along pipes before it drips, so the wet spot is often inches away from the real source. This quick test pinpoints it.

1. Empty and dry the cabinet. Remove everything and wipe every pipe and the cabinet floor with a dry towel.
2. Add paper towels. Lay a sheet flat on the cabinet floor and wrap a strip around each joint and connection.
3. Check the supply side first. With the faucet off, look at the shutoff valves and supply lines. These are under constant pressure, so a leak here drips even when nobody uses the sink.
4. Run the faucet for one minute. Keep the drain open. Wet towels near the faucet base or sprayer hose point to the faucet. Wet towels lower down point to the drain.
5. Fill the basin, then drain it. Fill the sink 3–4 inches deep and pull the stopper. A full sink puts the drain under load and reveals leaks that a trickle won’t.

Pro tip: Use a flashlight and run a dry finger along the underside of each pipe. Drops collect at the lowest point, so trace the moisture upward until your finger stays dry.

How to Fix a Leak Under Kitchen Sink Supply Lines and Valves

Fix supply leaks first, because those lines stay under pressure around the clock. Tightening a nut or swapping a supply line is usually a short job with two wrenches.

Tools & materials

• Two adjustable wrenches
• Bucket and old towels
• Flashlight or headlamp
• Braided stainless steel supply line (if replacing)
• Rubber gloves

Step 1: Shut off the water

Turn the shutoff valve under the sink clockwise until it stops. Then open the faucet to release any pressure left in the line. If the valve won’t turn or keeps dripping, shut off your home’s main water valve instead.

Safety: Don’t force a stuck valve with a long wrench. Old valves can snap and turn a small drip into a flood.

Step 2: Tighten the compression nut

If water beads where the supply line meets the valve, hold the valve body steady with one wrench. Turn the nut clockwise about 1/4 turn with the other. Turn the water back on slowly and check with a dry paper towel.

Step 3: Replace a worn supply line

Replace the line if it’s bulging, kinked, rusty, or you have no idea how old it is. Unscrew both ends and take the old line to the hardware store so you match the length and fittings; many valves use a 3/8-inch compression outlet. Hand-tighten the new line, then add a 1/4 turn with a wrench.

 

Step 4: Check the valve stem

If water seeps around the valve handle, gently tighten the packing nut behind the handle about 1/8 turn. If it still weeps, or the valve is crusted with green or white deposits, it needs replacement. That’s often a plumber’s job, especially on soldered copper pipes.

Sink Drain Pipe Leaking? How to Repair the P-Trap and Drain Seals

Drain leaks only appear when water flows, and the pipes aren’t under pressure. That makes them a good first plumbing repair for a homeowner with basic tools.

Tools & materials

• Bucket and rag
• Slip-joint pliers (for metal nuts only)
• New slip-joint washers or a P-trap kit: 1-1/2 inch for kitchen sinks, 1-1/4 inch for most bathroom sinks
• Plumber’s putty
• A small cleaning brush or an used toothbrush
• Rubber gloves

Step 1: Hand-tighten the slip nuts

Many P-trap leaks come from a nut that has worked loose. Tighten plastic slip nuts by hand, then add about a quarter turn. Fill and drain the sink to test.

Step 2: Remove and clean the trap

Place the bucket under the trap, unscrew both slip nuts, and pull the trap free. Scrub out grease and debris with the toothbrush.

A partly clogged trap holds water longer and puts extra strain on the joints. If the drain is still slow after cleaning, our guide to common clogged drain problems covers the next steps.

Step 3: Replace the washers or the whole trap

Flattened or cracked washers won’t seal, no matter how hard you tighten. Install new ones with the tapered side facing the pipe joint, not the nut. Line the pipes up straight before tightening, because a trap forced into place at an angle will leak again.

Pro tip: Snap a photo of the trap before you take it apart. It saves guesswork when you put the pieces back together.

Step 4: Reseal the drain basket

If water appears right under the sink opening, the seal around the drain basket has failed. Loosen the large locknut under the sink, lift out the strainer, and scrape off the old putty. Press a fresh rope of plumber’s putty under the strainer’s rim, reinstall it, tighten the locknut, and wipe away the excess putty that squeezes out.

Safety: Plumber’s putty can stain natural stone and some composite sinks. Check your sink manufacturer’s instructions and use a silicone sealant made for plumbing if they recommend it.

 

What If the Leak Comes From the Faucet or Disposal?

If water only shows up while the faucet runs, and it appears near the base or the sprayer hose, the faucet itself is the likely culprit. Worn O-rings and cartridges are common. Our step-by-step guide on how to fix a leaking faucet walks you through the repair.

For kitchen sinks, check the dishwasher drain hose and the disposal connections too. A loose hose clamp is a quick fix. Water dripping from the bottom of the disposal usually means its internal seals have failed, and the unit needs replacement.

Safety: Turn off the disposal at the breaker, not just the wall switch, before you touch it.

When a Leaking Sink Is an Emergency

Most sink leaks can wait until the weekend. These signs can’t:

• Water is spraying or flowing, not dripping
• The shutoff valve won’t stop the flow
• Water appears on the wall behind the sink or the ceiling below
• The cabinet floor feels soft, spongy, or is sagging
• You smell mildew or see dark spots on the cabinet
• Water is near an outlet or the disposal’s electrical connection

Shut off the water at the valve or main, and turn off power at the breaker if water is near anything electrical. Then dry the area fast. The EPA recommends drying water-damaged areas within 24–48 hours to prevent mold growth.

DIY or Hire a Pro?

Good DIY project if…

• The leak comes from a slip nut, washer, or P-trap
• You’re replacing a supply line with an accessible shutoff valve
• The drain basket needs new putty
• The pipes are plastic and in good condition

Call a pro if…

• The leak is inside the wall or at the drain pipe where it enters the wall
• A shutoff valve is corroded, stuck, or soldered in place
• Your home has old galvanized steel or cast-iron drain lines
• The leak comes back after you’ve replaced the parts
• The cabinet floor or subfloor already shows water damage

Knowing how to fix leaking sink connections yourself saves a service call on the simple jobs. Here’s a simple rule: if you can see the leaking part and remove it by hand or with a wrench, try it yourself.

If fixing it means opening a wall or cutting pipe, hire a licensed plumber. Before you call, check local prices in the HireRush plumbing cost guide.

Common Mistakes to Avoid

• Overtightening plastic nuts. Plastic threads crack under a wrench. Hand-tight plus a quarter turn is enough.
• Using tape or pipe dope on slip joints. Slip joints seal with the washer. Tape can keep the washer from seating and cause new leaks.
• Reusing old washers. A flattened washer can keep a “fixed” trap dripping. New washers are inexpensive, so replace them whenever you open a joint.
• Testing with a trickle. A light stream won’t reveal a drain leak. Always fill the basin and drain it all at once.
• Covering a leak with caulk or silicone. Sealant on the outside of a joint only hides the problem while water keeps damaging the cabinet.
• Ignoring a damp cabinet floor. Even a slow drip can rot particleboard and grow mold within days.

How to Prevent Future Leaks Under the Sink

• Check under your sinks every few months with a flashlight.
• Keep grease, coffee grounds, and food scraps out of the drain.
• Don’t store heavy items that can bump or strain the pipes.
• Replace rubber or plastic supply lines with braided stainless steel lines.
• Place a leak detector or drip tray on the cabinet floor for early warning.

Hire a Plumber

Final Thoughts

A leaking sink usually comes down to a loose slip nut, a worn washer, a failing supply line, or a dried-out drain seal. Drying the cabinet and testing in stages shows you exactly where the water comes from.

Most of these repairs take basic tools and under an hour. Leaks inside the wall, corroded valves, older pipe systems, or signs of water damage are a job for a licensed plumber.