Under Sink Leak at P-Trap: Causes and How to Fix It

The P-trap is the curved pipe section under your sink that holds water to block sewer gases. It’s also one of the most common sources of under-sink leaks — the slip-joint connections that hold the P-trap together loosen over time, and the plastic fittings can crack. Here’s how to identify the leak source and fix it.

What Is the P-Trap?

The P-trap is the curved “P”-shaped pipe section under the sink.

It connects the sink drain basket to the drain pipe in the wall. The bend in the pipe holds a small amount of water — this water plug blocks sewer gases from coming up through the drain into the living space. If the P-trap leaks or dries out, sewer gas enters the home.

P-trap components:
– Trap arm: The horizontal section that goes into the wall drain
– P-trap (curved section): The J-shaped bend
– Slip-joint nuts: The threaded plastic or chrome nuts at each connection
– Washers: Rubber or plastic washers inside each nut that create the seal

Finding the Leak

Dry everything under the sink first.

Wipe the P-trap, drain pipes, and cabinet bottom dry with a towel. Then run water in the sink for 30 seconds and watch carefully.

Where to check:
1. The slip-joint nut where the drain basket connects to the top of the P-trap
2. The slip-joint nut where the P-trap connects to the drain arm going into the wall
3. The curved P-trap body itself (look for cracks or splits)
4. The connection at the wall drain stub-out

Drip location during water flow tells you exactly which connection or section is the problem.

Fix 1: Tighten the Slip-Joint Nuts

Most P-trap leaks are loose slip-joint connections.

The slip-joint nuts are the threaded plastic (or chrome) nuts that hold each section together. They’re designed to be tightened by hand — but they loosen over time from vibration, minor movement, and thermal cycling.

Steps:
1. Hand-tighten the nut at the leaking connection
2. Turn an additional 1/4 turn — use channel-lock pliers if needed, but don’t overtighten (cracks the nut)
3. Run water and check for drips

If the nut is already fully tight and still leaks: The washer inside is worn or the nut is cracked — proceed to replacement.

Fix 2: Replace the Washer

Each slip-joint connection has a rubber or plastic washer inside.

If tightening doesn’t stop the leak, the washer has hardened, deformed, or is missing. Remove the nut, pull out the old washer, take it to the hardware store to match the size, and install a new one. This costs $1–$2 per washer.

Fix 3: Replace the P-Trap Assembly

If the P-trap body is cracked, or the fittings are deteriorated, replace the whole assembly.

P-trap assemblies for standard 1-1/2 inch kitchen and bathroom sink drains cost $5–$15. They come as a kit with the curved trap section, trap arm, and slip-joint nuts.

Steps:
1. Put a bucket under the P-trap
2. Unscrew both slip-joint nuts by hand (counterclockwise)
3. Remove the P-trap — water will drain from the trap into the bucket
4. Install new trap: slide nut over pipe end, insert new washer, connect sections, hand-tighten nuts
5. Run water and confirm no leaks

Matching size: Standard bathroom sink drains use 1-1/4 inch pipe; kitchen sinks use 1-1/2 inch. Verify before buying the replacement kit.

When to Call a Plumber

• The drain stub-out in the wall is corroded or damaged
• The leak is at a metal compression fitting rather than a slip joint
• You find water damage inside the cabinet (swollen particleboard, mold)
• The drain arm angle or height doesn’t match your replacement parts

FAQ

Q: Why is my P-trap leaking under the sink?
A: Most P-trap leaks are at the slip-joint connections — the threaded nuts that hold the sections together loosen over time. Tightening them by hand plus 1/4 turn usually stops the leak. If the P-trap body is cracked, replace the assembly.

Q: How do I fix a leaking P-trap?
A: First tighten the slip-joint nuts at each connection. If that doesn’t work, replace the rubber washer inside the leaking nut. If the P-trap itself is cracked, replace the entire assembly — about $10 in parts and 15 minutes of work.

Q: Can I replace a P-trap myself?
A: Yes. P-trap replacement is one of the simplest plumbing repairs. No tools required for most plastic slip-joint assemblies (just hand-tightening). The assembly costs $5–$15 at any hardware store.

Q: What size P-trap do I need?
A: Standard bathroom sinks use 1-1/4 inch; kitchen sinks use 1-1/2 inch. Measure the existing drain pipe or bring the old trap to the hardware store to match it.

Q: How much does it cost to fix an under-sink leak?
A: $5–$15 for DIY parts (washer or complete P-trap kit). If a plumber handles it, expect $75–$150 for a simple P-trap replacement — it’s typically under 1 hour of labor.