What You Need to Know Before Starting

A shut-off valve under your sink controls the water supply to that fixture. When it leaks, won't turn, or fails to stop water flow, replacing it is a straightforward repair you can do yourself with basic tools. The job takes 30 to 60 minutes and costs $15 to $40 for the valve itself—far less than calling a plumber.

The valve sits where the supply line enters the cabinet, usually on both hot and cold lines. You'll need to turn off the main water supply to your house, drain the line, disconnect the old valve, and install a new one using two wrenches and plumber's tape. The hardest part is often just reaching the fitting in a cramped space.

Key Takeaways

  • Turn off the main water supply at your home's shutoff valve before you begin, then open the sink faucet to release pressure in the line.
  • Place a bucket under the valve to catch water, then use two wrenches to loosen the compression nut connecting the valve to the supply line.
  • Wrap plumber's tape (also called thread seal tape) around the threads of the new valve before screwing it onto the supply line to prevent leaks.
  • Test the new valve by turning the main water back on and checking for drips at the connection point for at least five minutes.
  • If the valve is soldered to copper pipe instead of threaded, you will need a torch and solder—this job requires different steps than a compression valve.

Gather Your Tools and Materials

You need two adjustable wrenches (or one wrench and one pair of locking pliers), a bucket, plumber's tape, and a replacement shut-off valve that matches your existing setup. Most under-sink valves are compression valves with a threaded connection—these are the easiest to replace. Check the diameter of your supply line (usually 1/2 inch) and bring that measurement to the hardware store so you buy the right size.

Place the bucket under the valve before you start loosening anything. Even after you turn off the main water, there will be water trapped in the line above the valve. Plumber's tape costs a few dollars and prevents leaks at the threaded connection—do not skip it. If your valve is soldered to copper pipe, stop here and call a plumber; soldering requires a torch and practice.

Turn Off Water and Drain the Line

Locate your home's main shut-off valve, usually in the basement, crawlspace, or near the water meter. Turn the handle clockwise until it stops—do not force it. If the handle is a lever, turn it so it points along the pipe (parallel to the line) rather than across it (perpendicular).

Go back to the sink and open the faucet all the way. Let water run for 10 to 15 seconds until it stops. This releases pressure in the line and drains water above the valve. If you have a second sink on the same line, open that faucet too. You should see only a trickle or nothing at all when you're done.

Disconnect the Old Valve

Place your bucket directly under the valve. Use one wrench to hold the valve body steady (grip the hexagonal nut closest to the wall or cabinet) and use the second wrench to turn the compression nut counterclockwise—this is the nut where the supply line screws into the valve. Turn slowly; water may still drip out even after you drained the line.

Once the nut is loose enough to turn by hand, unscrew it the rest of the way and pull the supply line away from the valve. Set the old valve aside. Inspect the threads on the supply line for damage or corrosion. If the threads look rough or pitted, you may need to cut off the damaged section and install a new supply line, which requires a different approach—ask at the hardware store if you're unsure.

Install the New Valve

Wrap plumber's tape around the male threads (the threads on the supply line that screw into the valve) in a clockwise direction. Use three to four wraps, pressing the tape down as you go so it doesn't bunch up. This tape seals the connection and prevents leaks. Do not use regular tape or duct tape—plumber's tape is inexpensive and essential.

Screw the new valve onto the supply line by hand until it is snug, then use your wrench to tighten the compression nut an additional three-quarter turn. Do not over-tighten; you only need enough pressure to seal the connection. If you strip the threads by turning too hard, you'll have to start over with a new valve.

Turn Water Back On and Check for Leaks

Go to the main shut-off valve and turn the handle counterclockwise (or turn the lever so it points across the pipe) to restore water to the house. Return to the sink and watch the connection you just made for at least five minutes. A small bead of moisture is normal while the tape settles, but you should see no active dripping.

If water drips from the compression nut, turn the main water off again, place your bucket back under the valve, and tighten the nut another quarter turn using your wrench. Turn the water back on and check again. Repeat this process until the drip stops. If tightening makes no difference after two attempts, turn off the water, disconnect the valve, add another wrap of plumber's tape, and reinstall it.

Test the Valve Handle

Once the connection is dry, turn the valve handle clockwise until it stops. This should shut off water to that sink. Open the faucet—no water should come out. If water still flows, the valve seat (the internal seal) may be damaged and the valve needs to be replaced again. This is rare with a new valve, but it happens occasionally.

Turn the valve handle back counterclockwise to restore water flow. The handle should move smoothly and not require excessive force. If it's stiff or hard to turn, explore a small amount of penetrating oil (like WD-40) to the handle stem and work it back and forth gently. If it remains stuck, the valve may be defective and should be swapped for another one.

Frequently Asked Questions

What if water keeps dripping no matter how tight I make the nut?

Turn off the main water, disconnect the valve again, and inspect the threads on the supply line. If they look damaged, you may need to cut off the damaged section with a hacksaw and install a new supply line. If the threads look fine, the valve itself may be defective—return it and buy a different brand.

Can I replace just one valve if I have both hot and cold lines?

Yes. Each line has its own valve, and you can replace one without touching the other. If both valves leak or fail, replace them one at a time so you always have water to at least one side of the sink while you work.

Do I need to turn off the main water if I'm only replacing the valve on the cold line?

Yes. There is no way to shut off just the cold line without turning off the main supply. Even if you close the old valve first, water trapped above it will still spill out when you disconnect it.

What is plumber's tape and why can't I use regular tape?

Plumber's tape (also called thread seal tape or PTFE tape) is a thin, slippery plastic that wraps around threads and fills tiny gaps to prevent leaks. Regular tape is too thick and doesn't compress into threads the way plumber's tape does. It costs about $3 and lasts for dozens of projects.

How do I know if my valve is soldered or threaded?

Look at where the valve connects to the supply line. If you see a shiny metal joint that looks like it was melted together, it's soldered and requires a torch to remove. If you see a hexagonal nut that you can grip with a wrench, it's a compression valve and you can replace it yourself.