Install Shutoff Valves Under a Bathroom Sink

Materials

• 2 Quarter-turn angle stop valves, ½-inch compression inlet (Match the inlet size to your copper supply stubs—½-inch is standard for bathroom sinks, but measure to confirm) — $8 - $15 each
• 2 Braided stainless supply lines, ⅜-inch compression x ½-inch FIP (Measure from valve outlet to faucet tailpiece and add six inches—typical lengths are 12, 16, or 20 inches) — $6 - $10 each
• 1 roll Teflon tape or pipe thread sealant (Not needed for compression fittings themselves, but useful for the threaded supply line connections) — $2 - $4

Step by step

1. Prep · 10 min

   Kill the Water Flow First

   Turn off the water at your home's main shutoff valve. Open the bathroom faucet and let it run until water stops flowing completely. Flush the toilet once to drain the tank. Place a bucket and towels under the work area to catch residual water in the pipes.

   Tip: Open a faucet on a floor below the bathroom to help drain stubborn standing water from the vertical supply lines.
2. Prep · 15 min

   Square Cuts, Not Hacksaw Hacks

   Measure up from the wall escutcheon about half an inch and mark both hot and cold copper pipes. Use a tubing cutter to make clean, square cuts at both marks. Rotate the cutter around the pipe, tightening a quarter-turn after each full rotation. Clean cuts prevent leaks—hacksaw cuts leave burrs that interfere with compression seals.

   Tip: Keep a rag handy to catch the small amount of water that will dribble out when you complete each cut.
3. Prep · 5 min

   Smooth Edges Save Seals

   Use the reamer blade on the tubing cutter or a deburring tool to smooth the inside edge of each cut pipe. Wipe both pipe ends completely dry with a clean rag. Any moisture, oxidation, or debris will prevent proper compression seal.

   Tip: Run your finger inside the pipe end—if you feel a sharp edge, deburr again until it's smooth.
4. Assembly · 5 min

   Order Matters—Nut Then Ferrule

   On each pipe, slide the compression nut over the pipe first, threads facing out, then slide the brass ferrule (the small ring) onto the pipe behind it. The tapered end of the ferrule should face the nut, away from the wall. This order matters—you cannot add the nut after the valve is on.

   Tip: Set the hardware for both valves at once so you don't forget which pipe still needs parts.
5. Assembly · 10 min

   Position Outlets for Easy Connection

   Slide the angle stop valve body onto each pipe until it seats against the wall escutcheon. The outlet should point downward toward where the supply line will connect. Hold the valve body steady and hand-tighten the compression nut until it stops. The ferrule should now be captured between the nut and the valve body.

   Tip: Orient both valve handles in the same direction for a cleaner look and easier operation.
6. Installation · 10 min

   Snug—Not Vise-Grip Tight

   Hold the valve body with one wrench to keep it from rotating. Use a second wrench to tighten the compression nut one full turn past hand-tight—about a quarter turn with the wrench. Stop there. The ferrule is now biting into the copper and forming the seal. Do not overtighten.

   Tip: If the nut spins freely without resistance, you've stripped it or the ferrule wasn't seated—back off and start that valve over.
7. Testing · 10 min

   Verify Every Joint Holds

   Leave both new valves in the open position. Go turn on the main water supply slowly. Return to the bathroom and inspect both compression fittings for leaks. If you see water beading at the nut, tighten it another eighth-turn. If it still leaks after a quarter-turn total, you've likely overtightened and damaged the ferrule.

   Tip: Dry the fittings completely with a rag before you turn the water on so you can see new moisture immediately.
8. Final · 10 min

   Link Valves to Faucet Safely

   Attach flexible braided supply lines from each valve outlet to the faucet tailpieces. Hand-tighten, then snug with a wrench. Turn each angle stop to the on position. Check all connections—valve bodies, supply line ends at the valves, and supply line connections at the faucet—for leaks. Wipe dry and monitor for ten minutes.

   Tip: Buy supply lines six inches longer than you think you need—the extra length makes installation easier and prevents kinking.

Field Notes

• Marcus T. — The stub-outs on houses built before 1985 are almost always oxidized enough that you'll see green or white crust when you cut into them. Don't panic—it's surface corrosion, not a through-failure. Just make sure you get past it with your cut and deburr thoroughly. That crusty layer will prevent the ferrule from sealing if it's anywhere near your compression joint. I keep a wire brush in my kit specifically for cleaning up old copper before valve installs. — Nov 14 — Cleveland, OH
• Angela K. — The biggest mistake I see is people installing both valves, connecting the supply lines, turning on the main water, and then realizing they never opened the new shutoff valves. They think the valves are defective when actually they just didn't turn the handles. The valves ship in the closed position. Open them before you test. Also, if you have old galvanized pipe instead of copper, stop here and call someone—compression fittings don't work reliably on galvanized, and you're looking at threading or transitioning to PEX. — Feb 3 — Portland, OR
• David R. — Work in a well-lit space or bring a headlamp. The back of a vanity cabinet is dark, and you will not see whether the ferrule is properly seated or if you've cross-threaded something until it's too late. I've watched people tighten a compression nut thinking it felt right, turn on the water, and flood a bathroom because they couldn't see that the ferrule had slipped off to the side. Five seconds with a flashlight saves a hundred dollars in drywall repair. — Aug 21 — Phoenix, AZ