How to Install or Replace Water Shutoff Valves Under the Sink

Water shutoff valves under the sink are small steel or brass devices that sit between your main water line and your faucet. They let you stop the flow of water to just that sink without shutting off water to the entire house. Over time these valves corrode, jam, or start dripping. When they fail, you're forced to choose between a puddle under the sink or dealing with the main shutoff valve in your basement or yard. The good news is replacing one is straightforward work that requires only basic tools and takes less than an hour. A new valve costs between 8 and 20 dollars and will outlast the sink itself.

1. Kill the Water First. Locate your main water shutoff valve. This is usually in the basement near the water meter, in a crawlspace, or outside on the property. Turn the valve handle clockwise until it stops. It should feel snug but not forced. Open the nearest faucet to release any remaining pressure in the lines. Water should stop flowing within a few seconds.
2. Catch the Drips. Set a bucket or shallow pan directly beneath the shutoff valve under the sink. Even with the main water off, some water will remain in the supply line. This bucket catches that water and keeps your cabinet dry.
3. Free the Supply Line. Locate the chrome or plastic nut that connects the flexible supply line to the shutoff valve. Use an adjustable wrench to hold the valve body steady with one hand while you turn the nut counterclockwise with the other. Unscrew it by hand once it's loose enough. Water will drip into your bucket. Slide the supply line away from the valve opening.
4. Unbolt the Old Valve. Look at where the valve connects to the copper or steel shutoff coming out of the wall. This is a compression fitting or sweat connection. If it's hand-tight only, turn it counterclockwise by hand. If it's soldered on, you'll need a wrench. Use a pipe wrench or adjustable wrench to hold the shutoff steady while you unscrew the valve body. It should come free after several full rotations. If it's stuck, apply penetrating oil, wait 15 minutes, and try again.
5. Polish the Threads. Wipe the exposed threads on the shutoff with a clean dry cloth. Remove any mineral deposits, corrosion, or old tape. If deposits are heavy, use a soft brush or fine steel wool to lightly scrub the threads. The goal is a clean, smooth surface for the new valve to seat on. Wipe again with a dry cloth.
6. Seal With Tape. Take a roll of PTFE plumber's tape (also called thread seal tape). Wrap it around the male threads of the shutoff in a clockwise direction, overlapping each wrap by half. Use 3 to 4 full wraps. Stretch the tape slightly as you wrap so it adheres and doesn't bunch. The tape should cover the threads completely and smoothly.
7. Thread by Hand First. Take the new shutoff valve and align the threaded inlet with the wrapped shutoff. Begin turning the valve clockwise by hand. Feel for resistance as the threads engage. Keep turning smoothly until the valve is hand-tight. You should feel firm resistance but you're not using strength yet. Stop when it becomes snug to hand pressure.
8. Wrench It Snug. Once the valve is hand-tight, use a pipe wrench or adjustable wrench to tighten it another quarter turn. Hold the shutoff steady with one wrench while you turn the valve body with the other. The valve should be snug but not over-tightened—you're looking for a tight seal, not maximum force. Stop when you feel solid resistance.
9. Reattach the Supply Line. Take the flexible supply line and guide its nut onto the threaded outlet of the new valve. Hand-tighten the nut first by spinning it clockwise. Once hand-tight, use an adjustable wrench to snug it another quarter turn. Hold the valve body steady with your other hand so the entire valve doesn't spin.
10. Restore the Water. Go back to your main shutoff valve and turn it counterclockwise until it stops. Water will flow back into the lines. Return to the sink and open the faucet. Water should flow normally within a few seconds. Leave the faucet open for 10 seconds to flush air from the lines.
11. Spot-Check for Leaks. Look carefully at both the connection where the valve screws onto the shutoff and where the supply line connects to the valve. Watch for drips or moisture for at least 30 seconds. If you see water dripping from either connection, move to the troubleshooting section. If everything is dry, close the faucet and test the shutoff valve itself—turn its handle to the off position and confirm the water stops. Turn it back on.