How to Install Shutoff Valves Under Your Sink

Shutoff valves under your sink are one of those small installations that save you from catastrophe. When a faucet starts leaking or you need to replace it, you don't want to shut off water to your entire house—you want to stop it at the source. If your sink doesn't have shutoff valves, or if the existing ones are corroded or stuck, replacing them is straightforward work that keeps you in control of your own plumbing. The key is working methodically: stop the water, drain the lines, disconnect cleanly, and seal your new connections properly. Done right, these valves will give you 10–15 years of reliable service.

1. Kill Water at the Source. Locate your main shutoff valve, usually near the water meter in the basement, crawlspace, or exterior wall. Turn it clockwise until it stops. Open a faucet upstairs to confirm the water is off. Do not force the valve—it should turn smoothly.
2. Bleed the System. Open the faucet at your sink, and also open a low faucet elsewhere in the house (like a basement spigot or basement sink). Leave them open for 30 seconds to drain residual water from the lines and relieve pressure. You'll hear the water trickle out and stop.
3. Catch the Residual Drip. Position a small bucket or tray under the shutoff valve area. Even though you've shut off and drained the main line, there will be water in the supply lines between the shutoff and your sink—typically 1–2 cups. You'll also have water trapped in the flex lines themselves.
4. Free the Old Lines. Locate where the hot and cold supply lines connect to your faucet (or to the shutoff valves if they exist). Using a basin wrench or adjustable wrench, hold the hex nut on the inlet side with one wrench and turn the compression nut counterclockwise with the other. Work slowly—corrosion can make these stubborn. Turn the nut until the line releases. Do this for both hot and cold.
5. Unseat the Old Valve. If you're replacing an existing valve, use a wrench to grip the valve body and turn the compression nut counterclockwise to release it from the supply line stub. If the valve is seized, apply penetrating oil, wait 15 minutes, and try again gently. Do not force it. If it won't budge, leave it in place and call a plumber—the stub may break and require wall access.
6. Polish the Threads. If you've removed an old valve or are working with an exposed supply stub, clean the threads with a dry cloth or soft brass brush to remove mineral deposits and old tape. Look for visible corrosion or debris. If the stub itself is corroded or damaged, consider calling a plumber—you may need to access the supply line from inside the wall.
7. Tape the Threads. Take a new ball valve and wrap the male threads (the threaded end that screws into the supply stub) clockwise with plumber's thread seal tape, going around 3–4 times. Keep the tape smooth and tight—wrinkles trap water. The tape should cover all visible threads and create a slight resistance when you turn it by hand.
8. Screw Home the Hot Valve. Hand-thread the ball valve onto the hot water supply stub (typically the stub on the right side, but confirm by touching the existing hot line or checking your water heater connection). Turn it clockwise until snug, then use an adjustable wrench to tighten it another quarter turn. Do not over-tighten—you should feel resistance, not strain. The valve handle should point down or to the side for easy access.
9. Seat the Cold Valve. Repeat the tape-and-install process for the cold water supply stub (typically the left stub). Hand-thread, then use the wrench to snug it with a quarter turn. Position the handle for easy reach. Both handles should now be accessible and roughly parallel.
10. Reattach the First Line. Take the supply line that was originally connected to your faucet and thread it onto the outlet side of the new shutoff valve (the side opposite where you screwed it onto the stub). Hand-thread first, then use a wrench on the valve body and a second wrench on the compression nut to tighten it. The line should be snug but not over-tightened.
11. Connect the Second Line. Repeat for the second supply line (the one you removed from the other faucet inlet). Connect it to the outlet side of the second new shutoff valve. Ensure both lines are secure and not twisted or pinched under the sink.
12. Check Every Connection. Go to your main shutoff and turn it counterclockwise slowly to restore water pressure. Listen for water flow. Return to your sink and tighten the loose faucet inlet nut once water flows steadily. Observe the new valve connections for 2–3 minutes. Check under the sink with the bucket in place to catch any drips. If you see water beading up but not dripping, wait another minute—residual seepage stops quickly.
13. Verify the Shutoffs. Once you confirm no leaks around the valve connections, turn the handle of the hot water shutoff valve a quarter turn clockwise (perpendicular to the supply line). The flow from your faucet should stop. Turn it back counterclockwise to restore flow. Repeat with the cold water valve. Both should operate smoothly and stop flow completely.