How to Fix a Leaky Pipe Under the Sink — Tighten Compression Nuts, Replace Washers, and Stop Drips

Learn how to identify where a leak under your sink is coming from and fix it yourself — tight connections, fresh washers, and a few basic tools are all you need for most jobs.

You open the cabinet under your sink to grab a sponge and spot a small puddle. Or you feel a damp spot on the pipe when you reach in to stash the trash can. That drip-drip-drip is annoying, but here’s the good news: most leaks under a sink trace back to just a handful of connections, and nearly all of them are fixable with a crescent wrench and a few dollars in parts.

The key is figuring out which connection is leaking before you start wrenching on anything. Tightening the wrong nut can turn a drip into a spray.

What You’ll Need

• Adjustable crescent wrench (or channel-lock pliers)
• Flathead screwdriver
• Bucket or shallow pan
• Old towel or rag
• Flashlight or work light
• Plumber’s tape (PTFE tape)
• Replacement washers (assorted sizes from the hardware store — under $5)
• A small wire brush or old toothbrush

Step 1: Find the Leak (It’s Usually Not Where You Think)

Water travels. A leak at a high connection can run down a pipe and drip off the lowest point, making it look like the leak is somewhere else. Here’s how to pinpoint the source:

1. Dry everything with a towel.
2. Place a dry paper towel under each connection point.
3. Run the water (both hot and cold) for 30 seconds with the cabinet door open.
4. Shine a flashlight on each connection and look for the first bead of water to appear.

The most common leak points, in order of frequency:

• The slip-joint nut where the P-trap connects (drain side)
• The compression nut where the supply line connects to the shut-off valve
• The supply line nut where it connects to the faucet tailpiece
• The P-trap itself if it has a small crack or was cross-threaded

Step 2: Fix a P-Trap or Slip-Joint Leak (Drain Side)

Scenarios a homeowner would search:

• “Water dripping from curved pipe under sink”
• “P-trap leaking after I tightened it”
• “Slip joint washer leak fix”

If the leak is coming from one of the large plastic nuts where the drain pipes connect, you likely have a slip-joint leak. These connections rely on a tapered plastic or rubber washer inside the nut to create a seal.

How to fix it:

1. Hand-loosen the nut (do NOT use a wrench on large plastic nuts — you’ll crack them).
2. Slide it back and inspect the washer inside. If it’s brittle, flattened, or missing, the washer needs replacing.
3. Clean the pipe ends with a rag. Any debris or old plumber’s putty will break the seal.
4. Slide a new washer onto the pipe (tapered side facing the nut, wider side toward the pipe end).
5. Hand-tighten the nut firmly, then give it an extra quarter turn with your hand only. If it still drips, use a pair of channel-locks gently — just enough to snug it, not crush it.

Step 3: Fix a Compression-Fitting Leak (Supply Side)

The thin braided supply lines that connect your shut-off valve to the faucet use compression fittings — a small brass ring (ferrule) that gets squeezed against the pipe to form a seal. These are the most common source of supply-side drips.

How to fix it:

1. Turn off the shut-off valve. (If the valve itself is leaking, see Step 4 first.)
2. Use a crescent wrench to tighten the compression nut clockwise about an eighth of a turn. That’s often enough to stop the drip.
3. If it still leaks, turn off the main water supply, disconnect the nut, and inspect the brass ferrule. If it’s cracked or deeply scored, slide it off and replace it (hardware stores sell ferrule replacement kits for about $4).
4. Wrap the pipe threads with three layers of plumber’s tape (PTFE) before reassembling.
5. Reconnect and tighten the nut snugly with a wrench — about a quarter turn past hand-tight.

Pro tip: If the existing ferrule is stuck on the pipe and won’t slide off, use a small hacksaw to carefully cut a notch in it, then pry it loose with a screwdriver. Don’t cut into the pipe itself.

Step 4: Fix a Shut-Off Valve Leak

If the leak is coming from the handle of the shut-off valve itself, or from where the valve body meets the pipe coming out of the wall, you have a different problem.

For a valve handle leak:

• Tighten the small packing nut (the one directly behind the handle) with a wrench — just a quarter turn. This compresses the internal packing and stops the leak around the stem.

For a valve body leak:

• This means the connection between the valve and the wall pipe is failing. Replace the shut-off valve. This is a slightly more advanced job that involves shutting off the main water supply and unscrewing the old valve. If you’re not comfortable sweating copper or using a compression fitting tool, this is the one place in this guide where calling a plumber is reasonable.

What if the shut-off valve is old and I’m afraid to touch it? If your shut-off valve looks corroded, crusted with mineral deposits, or hasn’t been turned in years, turning it can cause it to start leaking or even break. If this describes your valve, shut off the main water supply to the house instead (usually near the water meter), then fix the shut-off valve by replacing it. You can also call a plumber for a valve replacement — it’s a one-hour service call.

Step 5: Fix a Faucet Tailpiece Connection Leak

Supply lines connect to the bottom of the faucet with another compression-style nut. If the drip is at the top of the supply line:

1. Snug the nut with a crescent wrench — again, just an eighth to a quarter turn.
2. Check the cone washer inside the connection. If it’s worn or missing, replace it.
3. Reassemble and test. If it still leaks, the plastic or metal threads on the faucet tailpiece may be stripped — this is rare but happens. In that case, replace the supply line with a new one that has the same fitting type.

When to Call a Pro

Some leaks are beyond the DIY toolkit:

• Pinhole leak in a copper pipe — not a fitting, but the pipe itself. This requires soldering or a push-fit coupling, which is doable but unforgiving for a beginner. If the leak is in a wall or under the house, definitely call a plumber.
• Cracked PVC or ABS pipe (not the trap, but the wall drain line). This needs to be cut out and replaced with a coupling.
• Leaking main shut-off valve — if the main water shut-off for the house is dripping, call a plumber. You cannot safely work on the one valve that controls all water to the house.
• Mold or water damage behind the cabinet. If the leak has been going on for a while, the cabinet floor, drywall, or subfloor may be compromised. Have a plumber or contractor assess the damage before you patch and move on.

Quick Reference: Leak Location → Fix

Most under-sink leaks are compression nut or slip-joint problems — two of the easiest fixes in home plumbing. A smart diagnosis and a $4 bag of washers will solve 80% of them. Use the checklist above, work slowly, and you’ll have that drip stopped before lunch.