Install Shutoff Valves Under a Bathroom Sink

Shutoff valves are the difference between a five-minute faucet repair and a whole-house water shutdown that involves your main valve and a trip to the basement in wet socks. Most older homes have bathroom sinks plumbed directly into the wall supply without individual shutoffs, which means every small repair becomes a production. Installing quarter-turn angle stops takes about an hour and gives you control over the hot and cold water feeding just that sink. The work happens in tight quarters under the vanity, but the valve bodies themselves are forgiving—compression fittings require no soldering, no flux, no torch. The project breaks into three phases: cutting the existing copper stub-outs cleanly, sliding on the compression hardware in the correct order, and tightening everything just enough to seal without cracking. Overtightening is the most common mistake. The brass ferrule inside the compression nut does the actual sealing work as it bites into the copper pipe, and that happens with surprisingly little force. Most leaks come from undertightening out of caution, then overtightening in response. Get it right the first time and these valves will outlast the faucet above them.

1. Shut off the main water supply and drain the lines. 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.
2. Cut the copper supply stubs cleanly. 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.
3. Deburr and dry the pipe ends. 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.
4. Slide on the compression nut and 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.
5. Thread the valve body onto the pipe. 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.
6. Tighten the compression fitting. 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.
7. Turn on the water and check for leaks. 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.
8. Connect the supply lines to the faucet. 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.