How to Replace a Shutoff Valve Under a Sink

Water shutoff valves under sinks fail slowly and then suddenly. You'll notice a drip forming at the connection, or the handle won't turn all the way, or it leaks even when cranked shut. When that happens, you're looking at a straightforward swap—one of the most confidence-building repairs a homeowner can do. A shutoff valve is just a brass fitting with a handle that lets you stop water flow to that specific sink without shutting down your whole house. Replacing one means you'll have local control again, and you won't wake up to a cabinet full of water because a guest didn't notice the valve was already leaking. The job requires no special skills, just patience and the right wrench.

1. Shut off the main water supply and open the faucet. Locate your main shutoff valve (usually near the meter or where water enters your house) and turn it clockwise until it stops. Go to the sink you're working on and open the faucet to release any remaining pressure in the line. This is your safety step—it prevents surprises when you crack open the connection.
2. Put a bucket under the shutoff valve. Position a bucket or shallow pan directly under the old valve to catch any water trapped in the supply line. There's usually only a cup or so, but it beats soaking your cabinet floor. Leave a towel nearby for quick cleanup.
3. Disconnect the supply line from the shutoff valve. Use an adjustable wrench to hold the nut on the valve body still while you loosen the nut that connects the supply line to the valve outlet. Turn counterclockwise. Once it's loose enough to turn by hand, unscrew it the rest of the way and gently pull the line away. Expect a small amount of water to drain into your bucket.
4. Unscrew the old valve from the water supply stub. Use your adjustable wrench to grip the supply stub (the short pipe coming out of the wall or floor) with one wrench while you unscrew the old valve body with another wrench. Turn the valve counterclockwise. It will take some effort—these fittings are often on tight. Keep the stub steady so you don't crack the pipe behind the wall.
5. Clean the supply stub and wrap it with tape. Use a rag to wipe the threads on the supply stub clean of any old plumber's tape, debris, or mineral buildup. Inspect the stub for damage—if it's cracked or corroded, you may need a professional to replace it. Wrap the threads tightly with plumber's tape, going around clockwise three to four times. This tape creates a watertight seal.
6. Screw on the new shutoff valve. Thread the new valve onto the supply stub by hand first, then tighten it with your adjustable wrench. Hand-tighten until you can't turn it anymore, then give it another half-turn with the wrench. Don't over-tighten—you'll deform the fitting or crack the valve body.
7. Reconnect the supply line and check the fit. Thread the supply line nut onto the valve outlet by hand. Once hand-tight, use your wrench to tighten it another quarter-turn. The connection should feel snug but not crushing. If you see any gaps around the nut, tighten it a bit more, but stop immediately if resistance increases sharply.
8. Turn on the main water supply and test for leaks. Go back to your main shutoff valve and turn it counterclockwise slowly to restore water pressure. Return to your sink and look for drips at both connections—the valve-to-stub connection and the line-to-valve connection. Turn the sink faucet on and off a few times, then operate the new shutoff valve handle to make sure it moves freely and shuts off the flow. Leave the bucket under the valve for an hour and check it; it should be dry.