How to Fix a Leaky Pipe Under the Sink

Difficulty: Medium • Time: 30 min active, 45 min total • Estimated cost: $5-40 • Safety: DIY-friendly

Overview

A leak under the sink is easy to ignore at first — maybe you only notice when you open the cabinet to get something. But a slow drip is quietly rotting the cabinet floor, growing mold inside the enclosed space, and wasting water every day. Catching it early and fixing it yourself saves you from a much bigger headache later.

The pipes under a sink form what's called the drain assembly: the tailpiece (vertical pipe from the sink strainer), the P-trap (the curved section that holds water to block sewer gases), and the horizontal drain arm that connects to the wall pipe. Most leaks occur at the slip joint connections between these sections — the threaded plastic or metal slip nuts that hold them together.

The good news: under-sink drain plumbing is designed to be user-serviceable. It doesn't require soldering or special skills. The pipes are connected with hand-tightenable slip nuts, and replacement parts cost $5-15 at any hardware store. For supply line leaks (the smaller braided hoses that bring hot and cold water to the faucet), tightening the connection usually stops the leak — and if not, replacing a supply line takes 10 minutes and costs $8.

This guide covers all the common leak points: slip joint connections, the P-trap, and supply lines. Read it through, identify where your leak is coming from, and go straight to the relevant steps.

Tools Needed

Slip-joint pliers or channel-lock pliers
Adjustable wrench
Bucket (to catch water when disconnecting pipes)
Old towels
Pipe thread tape (Teflon tape)
Flashlight or headlamp

Materials Needed

P-trap replacement kit (1-1/4" or 1-1/2" depending on drain size) — $8-15
Slip joint washers and nuts (repair pack) — $5-8
Braided stainless supply lines (hot and cold) — $8-15
Teflon tape — $3-5
Plumber's putty (if strainer leaks) — $4-7

Step-by-Step Instructions

Identify the exact source of the leak: Clear out everything from under the sink so you have room to work. Dry all the pipes and the cabinet floor thoroughly with a towel. Place a dry piece of cardboard or paper towel under the pipes. Run water in the sink (hot and cold) for 30 seconds and watch carefully. Check: the slip nut connections where pipes join together, the curved P-trap section, the connection where the drain arm meets the wall, and the supply lines (braided hoses running from the wall valves up to the faucet). Mark or note exactly where you see dripping — this tells you precisely what to fix.
Tighten loose slip joint connections (most common fix): Slip joints are the plastic or metal threaded nuts at each pipe connection. Many leaks are caused by nothing more than a slightly loose nut — the connection was installed hand-tight but vibration or thermal expansion has loosened it slightly. Try tightening each nut at the leak location by hand first, then 1/4 turn with slip-joint pliers. Don't over-tighten plastic fittings — they crack. Run water and recheck. If the leak stops, you're done. If it continues, the washer inside the fitting is worn and needs replacement.
Replace worn slip joint washers: Turn off the water supply valves under the sink. Place a bucket under the P-trap. Unscrew the slip nuts by hand at both ends of the P-trap — the curved section should come free and water will drain into your bucket. Look inside each slip nut: there's a tapered rubber or plastic washer inside. If it's flattened, cracked, or has grooves worn into it, it's not sealing properly. Take the washer to the hardware store to match the size, or buy an assortment pack. Install the new washer into the slip nut, reassemble the P-trap, and tighten hand-tight plus 1/4 turn with pliers.
Replace the entire P-trap assembly: If the P-trap itself is cracked, corroded (on metal traps), or if the slip nuts are so old the threads are damaged, replace the whole P-trap assembly. These are inexpensive ($8-15) and come as a kit with all the pieces. With the water off and a bucket ready, unscrew all the slip nuts on the drain assembly — the tailpiece, P-trap, and drain arm. Take the old P-trap to the store to match the diameter (1-1/4" for bathroom sinks, 1-1/2" for kitchen sinks). Assemble the new trap, insert each slip nut washer with the tapered side facing the fitting it will press against, and tighten hand-tight plus 1/4 turn with pliers.
Fix a supply line leak: If the leak is coming from one of the braided hoses connecting the shut-off valve to the faucet, first try tightening the nut at the leaking connection with an adjustable wrench — a quarter turn often stops the leak. If tightening doesn't help, or if the hose itself shows corrosion or bulging, replace the supply line. Turn off the shut-off valve (clockwise), unscrew both ends of the supply line, and take it to the store to match the length and end type. Wrap the faucet tailpiece threads with 2-3 wraps of Teflon tape before installing the new line. Hand-tighten both ends then add a quarter turn with a wrench.
Test all repairs and check for leaks: Once everything is reassembled, slowly turn the water supply valves back on. Run water for 2-3 minutes — both hot and cold. Watch every connection point under the sink with a flashlight. Check the slip joints, both ends of the P-trap, and the supply line fittings at both ends. Dry any drips and watch again. Any leak that persists after tightening means the washer isn't seating properly — disassemble, check the washer orientation (tapered side faces the fitting), and reassemble. Put a dry paper towel or piece of cardboard in the cabinet and check it 24 hours later to catch any slow drips.

When to Call a Professional

Call a plumber if the shut-off valves under the sink leak when you turn them or won't fully close (they need replacement before any other work can proceed), if the wall drain pipe is leaking where it exits the wall (requires opening the wall), if you see mold or significant water damage to the cabinet floor or surrounding cabinets (water may have been leaking for a long time — assess the damage first), or if the kitchen sink drain is connected to a garbage disposal and the disposal itself is leaking from the bottom (internal seal failure requires disposal replacement).