How to Install Shutoff Valves Under the Sink

Shutoff valves under the sink are one of those small upgrades that pay dividends the first time you need to replace a faucet or fix a leak. Without them, you're forced to shut off water to your entire house for a simple repair. Most older homes never got these installed, and plenty of newer ones skipped them as a cost-cutting move. The work is straightforward plumbing that doesn't require special skills—just attention to detail and the willingness to spend an afternoon in a cramped under-sink space. Once installed, you'll have independent control of hot and cold water at the point of use, which is how every sink should be set up.

1. Kill the Water First. Locate your main shutoff valve, usually near the water meter or where the main line enters the house. Turn it clockwise until it stops. Then open a faucet at the highest point in your house (usually an upstairs sink or tub) to relieve pressure from the lines. Leave it open until water stops flowing.
2. Make Room to Work. Remove everything stored under the sink—cleaning supplies, trash can, drain organizers, all of it. You need clear floor space and access to the back wall. Place a towel or small bucket under the supply line connections to catch any residual water that will drip when you disconnect.
3. Separate Old From New. Locate the two supply lines coming from the wall (hot on the left, cold on the right, typically). Use an adjustable wrench to hold the nut closest to the wall steady while turning the connection nut with a second wrench. Turn counterclockwise. You may see one or two supply lines coming from shutoff valves already installed, or they may connect directly to the wall stubs. Disconnect whichever is there. Have your towel ready—water will spill.
4. Check Before You Commit. Look at the male threads on the copper or PEX stubs sticking out from the wall. They should be clean and undamaged. If they're corroded, pitted, or stripped, they won't seal properly. In that case, you'll need a professional plumber to replace the stubs—this isn't a DIY fix. If the threads look sound, you're good to proceed.
5. Seal the Threads First. Take your roll of PTFE (plumber's) tape and wrap it clockwise around the male threads of the water stub—three to four full wraps. Start at the base of the threads and work upward. The tape should overlap itself slightly on each pass. This tape seals the joint and prevents leaks. Do this for both the hot and cold stubs.
6. Hand-Tight Plus a Quarter. Take your first shutoff valve and thread it by hand onto the hot water stub. Turn it clockwise until you feel resistance—don't force it. Once hand-tight, use a wrench to tighten it another quarter turn. You should feel firm resistance but don't overtighten, or you'll crack the valve. Repeat for the cold water valve on the cold stub. Both valves should be oriented so their ball handles point downward or to the side for easy access.
7. Reconnect the Supply Lines. Take your first flexible supply line and connect it to the outlet port on the hot water shutoff valve. If it's a compression connection, place the ferrule and nut over the line, insert the line into the port, and tighten the nut with a wrench while holding the valve body steady with your other hand. If it's a push-fit connection, simply push the line fully into the port until you feel a click. Repeat for the cold water line on the cold valve.
8. Restore Water and Watch. Go back to your main shutoff and turn it counterclockwise to fully open. Open it slowly—don't rush. Return to under the sink and watch both valve connections and the supply line connections for leaks. Let water run through for 30 seconds, then close the ball handles on both shutoff valves. Check again for any drips.
9. Confirm Full Control. Open both ball handles fully and turn on the sink faucet. Water should flow normally. Now close the hot water valve and verify that only cold water comes out. Close the cold valve and open the hot valve—only hot should flow. Turn off the faucet, then close both valves and open the faucet again. No water should come out. If all three tests pass, your valves are working correctly.
10. Stop Condensation Before It Starts. Wrap the valve bodies and the first few inches of supply line with self-adhesive foam pipe insulation. This prevents condensation from forming on the metal during humid weather and keeps the water inside slightly warmer longer. It's optional but worthwhile in humid climates. Cut the insulation to length with a utility knife and overlap the seam slightly.
11. Restore and Label Everything. Once everything is tested and dry, put back everything that lives under the sink. Keep the area in front of the shutoff valves clear so you can access them easily if needed in the future. Consider labeling the valves hot and cold with waterproof tape so family members know which is which.