Plumbing how-to

How to Replace a Shutoff Valve Compression Supply Line

To replace a shutoff valve compression supply line, shut off the water, relieve pressure, remove the old line with two wrenches, install a matching new line, then reopen the water slowly and check for leaks.

This is a good repair when the supply line is kinked, corroded, dripping at the fittings, or damaged during another repair. The key is using the same connection style and not overtightening the compression end.

Before you start: Match the line length, end sizes, and connection type before ordering.

Prepared by: Repair Riot Last updated: 2026-06-05 How we build and check guides

Safety first

  • Use two wrenches whenever possible so you do not twist the shutoff valve or the pipe behind it.
  • Open the water back on slowly to avoid shocking a new connection.
  • Do not use thread tape on a standard compression connection unless the fitting instructions specifically call for it.
  • If the valve or pipe is heavily corroded, treat this as more than a simple supply line swap.

Tools you may need

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Step-by-step fix

Step 1: Make sure the supply line is really the problem

  1. Look at the flexible line that runs from the shutoff valve to the faucet, toilet, or appliance connection.
  2. Check for drips at the compression nut, rust or green corrosion on the fittings, a split outer covering, or a sharp kink in the line.
  3. Dry the line and both ends with a towel, then watch closely for a minute to see where fresh water appears.
  4. Confirm the shutoff valve body itself is not cracked and the leak is not coming from the valve stem or the pipe in the wall or floor.

If it works: You have confirmed the supply line or its end connection is the likely source of the leak or damage.

If it doesn’t: If the valve body, stem, or supply pipe is leaking instead, replace or repair that part instead of the supply line.

Stop if:
  • The pipe coming out of the wall or floor moves when touched.
  • The shutoff valve is badly corroded, cracked, or loose on the pipe.
  • You cannot tell where the leak starts because water is coming from inside the wall, cabinet, floor, or escutcheon area.

Step 2: Shut off water and relieve pressure

  1. Close the shutoff valve feeding that fixture by turning it clockwise until it stops.
  2. Open the faucet or flush the toilet served by that line to relieve pressure and drain most of the water.
  3. Place a bucket or shallow pan under the shutoff valve and supply line connection.
  4. Wipe the area dry so the fittings are easier to grip and inspect.

If it works: Water flow from the fixture has slowed to a stop or nearly stopped, and the line is depressurized.

If it doesn’t: If the shutoff valve does not fully stop the water, shut off the home's main water supply before removing the line.

Stop if:
  • The shutoff valve will not close and you cannot shut off water at the main.
  • The valve handle breaks or the valve starts leaking heavily when you turn it.

Step 3: Remove the old compression supply line

  1. Use one wrench to hold the shutoff valve body steady.
  2. Use the second wrench to loosen the compression nut on the supply line at the shutoff valve.
  3. Disconnect the other end of the line from the fixture connection.
  4. Lower the line into the bucket and let the remaining water drain out.
  5. Compare the old line's length, end sizes, and connection style to the replacement before installing the new one.

If it works: The old supply line is off, and you know the new line matches the old one closely enough to install.

If it doesn’t: If the new line does not match both ends or is too short to route without strain, get the correct replacement before continuing.

Stop if:
  • The shutoff valve or supply pipe twists when you try to loosen the nut.
  • The compression outlet on the valve is cracked, deeply scored, or too corroded to seal reliably.

Step 4: Install the new supply line

  1. Start the new line by hand at the shutoff valve first so you do not cross-thread the nut.
  2. Thread the other end onto the fixture connection by hand.
  3. Make sure the line hangs in a smooth curve without a sharp bend, twist, or side pull.
  4. Hold the valve body with one wrench and tighten the compression nut with the other until snug.
  5. Tighten the fixture end snug as well, using hand-started threads and steady pressure rather than forcing it.

If it works: The new line is connected at both ends, routed cleanly, and the fittings are snug without obvious strain.

If it doesn’t: If a nut will not thread on smoothly by hand, back it off and realign it before tightening again.

Stop if:
  • You feel the fitting bind after only a partial turn, which usually means cross-threading.
  • The new line must be bent sharply or stretched to reach the connection.

Step 5: Turn the water back on slowly and check for leaks

  1. Close the faucet or make sure the toilet fill valve is ready to refill.
  2. Open the shutoff valve slowly while watching the compression connection and the fixture end.
  3. Wipe both fittings dry, then watch for several minutes for beads of water, drips, or a damp ring forming around the nuts.
  4. If you see a slight seep, tighten the leaking connection a little more while holding the valve body steady, then dry and recheck.

If it works: Both ends stay dry with the water back on and the fixture operates normally.

If it doesn’t: If a connection still seeps after a small retightening, shut the water back off, disconnect it, and inspect for the wrong size, damaged threads, or a poor seating surface.

Stop if:
  • Water sprays from the fitting instead of a minor seep.
  • The shutoff valve body starts leaking after the line is reconnected.
  • The leak appears to be coming from inside the wall, floor, or cabinet rather than the exposed fittings.

Step 6: Verify the repair holds in normal use

  1. Run the faucet for a minute or flush the toilet several times so the line sees normal pressure changes.
  2. Check the line again after 10 to 15 minutes and once more later the same day.
  3. Feel under each nut with a dry finger or tissue to catch a slow seep that is hard to see.
  4. Leave the bucket out until you are confident the area stays dry.

If it works: The supply line stays dry during normal use, and the repair is holding.

If it doesn’t: If moisture returns later, shut the valve off and inspect the leaking end again or replace the line with the exact correct size and connection type.

Stop if:
  • The fitting keeps leaking after careful retightening and rechecking.
  • You find hidden water damage, swelling cabinet material, or staining around the pipe opening.

Replacement Parts

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Verify the repair

  • No water appears at the shutoff valve connection after the line is pressurized.
  • No water appears at the fixture end during use.
  • The line is not kinked, twisted, or rubbing hard against the cabinet or wall.
  • The fixture works normally and the area stays dry after repeated use.

FAQ

Do I need thread tape on a compression supply line?

Usually no. Compression connections seal at the fitting surfaces, not on the threads. Adding tape can interfere with proper tightening unless the fitting instructions specifically say to use it.

How tight should the compression nut be?

Tight enough to seal without forcing it. Start the nut by hand, then snug it with a wrench while holding the valve body steady. If it seeps, tighten a little more and recheck.

Can I reuse the old supply line if it looks okay?

It is better to replace it if you already removed it for a leak, corrosion, or damage. Old lines can seal poorly when reinstalled, especially if they were kinked or stressed.

What if the shutoff valve will not fully stop the water?

Shut off the home's main water supply before removing the line. If the valve itself leaks or will not close reliably, the valve may need repair or replacement too.

How do I know I bought the right replacement line?

Match the overall length, both end sizes, and the connection style from the old line. The new line should reach comfortably without stretching, twisting, or making a sharp bend.

Related guides

Related problem pages
Outdoor faucet inside shutoff stuck
Shutoff valve not shutting off water
Shutoff valve wont close
Related helper guides
How to Replace a Fixture Supply Line For Shutoff Valve Connection
How to Replace a Hose Bib Shutoff Valve Compression Supply Line