What You Need to Know Before Starting

A shut off valve (also called a stop valve or angle stop) is the knob under your sink that lets you turn off water to that fixture without shutting off water to the whole house. When it leaks, won't turn, or won't shut off completely, you need to replace it. The job takes 30 to 60 minutes and requires only basic tools — a wrench, bucket, and possibly a valve removal tool if the old one is stuck.

The valve sits where the water line comes up from the wall or floor and connects to the faucet supply line. Replacing it means turning off the main water, draining the line, unscrewing the old valve, and screwing in a new one. You do not need to call a plumber for this, though if the valve is corroded solid or you are not comfortable working with water lines, that is a reasonable choice.

Key Takeaways

  • Turn off the main water supply before you start, then open the faucet to release pressure in the line.
  • Place a bucket under the valve connection to catch water that drains out when you disconnect the supply line.
  • Use two wrenches — one to hold the fitting steady and one to turn the nut — so you do not twist the water line itself.
  • New valves come in different types (angle stop, straight stop) and sizes (usually 1/2 inch); match your old valve or bring a photo to the hardware store.
  • Wrap the threads of the new valve with plumber's tape before screwing it in to prevent leaks at the connection.

Gather Your Tools and Materials

Before you touch anything under the sink, collect what you need. You will need two adjustable wrenches (or one adjustable and one fixed wrench in the right size), a bucket, and plumber's tape (also called PTFE tape or Teflon tape). A flashlight or headlamp is useful because the space under a sink is usually dim. Have a towel nearby to wipe up drips.

For the new valve itself, you need to know two things: the size (almost always 1/2 inch for a kitchen or bathroom sink) and the type. An angle stop has the inlet coming from below and the outlet going up toward the faucet — this is the most common type. A straight stop has both inlet and outlet in line. Look at your old valve or take a photo to the hardware store. Buy a valve rated for hot and cold water (it will say so on the package). A basic brass valve costs $8 to $15.

Turn Off Water and Drain the Line

Locate your main water shut off valve. In most homes this is in the basement near where the water line enters, or outside near the street. Turn it clockwise until it stops. Do not force it — if it will not budge, leave it alone and call a plumber, because a stuck main valve can break and cause a bigger problem.

Once the main is off, go to the sink and turn on the faucet (both hot and cold) to release pressure in the line and let any remaining water drain out. Leave the faucet open. Place your bucket under the shut off valve to catch the water that will spill when you disconnect the supply line. You will lose maybe a cup or two, but the bucket prevents a mess.

Remove the Old Valve

Look at the connection where the shut off valve meets the water line coming from the wall or floor. You will see a nut (usually brass or chrome) holding them together. This nut is what you unscrew. Use one wrench to hold the fitting (the part attached to the wall) steady, and use the other wrench to turn the nut counterclockwise. This two-wrench method keeps you from twisting the water line itself, which can crack it inside the wall.

Once the nut is loose, unscrew it by hand and set it aside. Water will drip into your bucket. Now look at the other end of the valve — where it connects to the supply line going up to the faucet. Use the same two-wrench method to disconnect this nut. Once both nuts are off, the old valve will come free. Empty any remaining water from it into your bucket.

If the old valve will not unscrew — if the nut spins but the valve does not come loose — the fitting inside may be corroded. A valve removal tool (a specialized wrench that costs $10 to $20) can grip the fitting and help you break it free. If you do not have one and the valve is truly stuck, this is the point to call a plumber rather than risk damaging the water line.

Install the New Valve

Before you screw in the new valve, wrap the threads with plumber's tape. Wrap it clockwise (the same direction you will turn the wrench) three to four times around. This tape seals the connection and prevents leaks. It is cheap insurance and takes 30 seconds.

Screw the new valve onto the fitting that comes from the wall. Hand-tighten it first, then use your wrench to snug it another quarter turn. Do not over-tighten — you are looking for snug, not crushing force. The valve should sit at the angle it needs to be (usually pointing up and slightly forward so the supply line can connect easily).

Now connect the supply line that goes to the faucet. Again, wrap the threads of the valve outlet with plumber's tape, then screw the supply line nut onto it by hand first. Use your wrench to snug it another quarter turn. Both connections should be tight enough that they do not leak, but not so tight that you strain the fitting.

Test for Leaks and Restore Water

Before you turn the main water back on, double-check that both nuts are tight and that the valve is pointing the right direction. Then go to the main shut off and turn it counterclockwise slowly. Water will flow back into the line. Listen and watch for drips at both connections under the sink.

If you see a drip, turn the main back off, wait a minute for pressure to drop, and use your wrench to tighten the leaking nut another quarter turn. Then turn the main back on and check again. Most leaks stop after one or two quarter-turn tightens. If a connection keeps leaking after you have tightened it twice, turn the main off, disconnect that nut, add more plumber's tape to the threads, and reconnect it.

Once both connections are dry, turn on the faucet to make sure water flows normally. Then test the shut off valve by turning its handle clockwise. Water should stop flowing. Turn it back counterclockwise and water should flow again. If the valve does not shut off completely or is hard to turn, you may have installed it backwards or cross-threaded it. Turn the main off, disconnect the valve, and try again.

Common Mistakes to Avoid

The most common mistake is using only one wrench, which twists the water line fitting and can crack it. Always use two wrenches — one to hold the fitting, one to turn the nut. The second mistake is forgetting plumber's tape on the threads, which leads to slow leaks that show up hours or days later. Wrap the threads every time, even if the connection looks tight.

A third mistake is over-tightening the nuts. Brass fittings are soft and can crack if you crank too hard. Snug is enough. If a connection leaks after you have tightened it reasonably, the problem is usually not enough tape on the threads, not a loose nut. The fourth mistake is installing the valve backwards — the inlet (where water comes in from the wall) must face the wall, and the outlet (where the supply line goes to the faucet) must face up. Check the direction before you tighten.

When to Call a Plumber Instead

If the old valve is corroded solid and a valve removal tool does not free it, a plumber has specialized tools and experience to remove it without damaging the water line. If you turn off the main and no water comes out of the faucet, the line may be blocked or the main valve may not be working — a plumber can diagnose this. If you finish the job and water leaks from inside the wall (not from the connections you just made), the water line itself may be damaged, and you need a professional.

If you are not comfortable working with water lines or you do not have the tools, hiring a plumber is reasonable. The job usually costs $150 to $300 in labor, which is worth it for peace of mind if you are unsure.

Frequently Asked Questions

Do I have to turn off the main water, or can I just turn off the shut off valve?

You must turn off the main water. The shut off valve is the part you are replacing, so it may not seal completely — water will keep flowing if you only turn it off. Always shut off the main first.

What size shut off valve do I need?

Most kitchen and bathroom sink shut off valves are 1/2 inch. Look at the size marked on your old valve, or measure the diameter of the water line with a ruler. If you are unsure, take a photo of the old valve to the hardware store and staff can match it.

Can I use a straight stop instead of an angle stop?

Yes, if the water line and supply line are already in a straight line. An angle stop is more common because it lets you route the supply line neatly. Check your current setup — if the water line comes straight up and the supply line goes straight up, a straight stop works. If the water line comes from the side, an angle stop is easier.

How tight should I make the connections?

Snug enough that the nut does not turn by hand, then one more quarter turn with the wrench. If you can turn the nut by hand after that, it is too loose. If you need to strain hard to turn the wrench, it is too tight. Brass fittings are soft and can crack.

What if water is still leaking after I tightened the nut?

Turn off the main, wait for pressure to drop, disconnect the leaking nut, and check the threads. If they look dry, add more plumber's tape (wrap it three to four times), then reconnect and tighten. If the threads are wet or corroded, the fitting may be damaged and need replacement.