Replace a Corroded Water Supply Line

Corrosion in water supply lines is a slow-motion emergency—the line weakens invisibly until it fails, sometimes spectacularly. You'll notice it first as discolored water from that line, or a pinhole leak that stains your cabinet. Catching it early and replacing the line yourself saves you the callout fee and gives you a water system you actually trust. A corroded line isn't a temporary fix situation; it needs replacement, not patching. This repair is straightforward once the water is off, and it's one of the cleanest wins in home plumbing—visible, tangible improvement in half a day.

1. Find the Corrosion. Find where the corroded supply line begins and ends. Trace it from the shutoff valve or main line to the fixture it serves. Look for green or white mineral deposits, pinhole leaks, or discolored water. If the corrosion is visible on the exterior, assume it's worse inside. Take a photo of the line routing and connection points before you start—you'll need to replicate that path exactly.
2. Kill the Water. Locate your main water shutoff valve, typically near the meter or where the supply enters the house. Turn the handle clockwise until it stops. Do not force it. Then open a nearby faucet—kitchen or bathroom—and let any pressure drain out. This confirms the water is truly off. If you can't locate the main shutoff, call your water company; they can show you.
3. Isolate if Possible. Many fixtures or supply lines have an isolating shutoff valve nearby. If the corroded line feeds a single fixture like a toilet or sink, look for a small valve directly below or behind it. Turn it clockwise. This is faster than the main shutoff and safer if the main shutoff is unreliable. If no isolating valve exists, use the main shutoff.
4. Catch the Spillage. Position a bucket or towel directly under where the old line connects. Even after the water is off, there will be water sitting inside the pipe that will spill when you disconnect. Position the bucket to catch drips. Have a second dry towel ready for wiping.
5. Disconnect the Source. At the point where the corroded line connects to the supply (either a shutoff valve, a T-fitting, or the main line), use two wrenches: one to hold the fitting steady, one to turn the nut clockwise. If the line is soldered to copper, you'll need a hacksaw or tubing cutter to cut it cleanly just before the solder joint. If it's compression fittings, unscrew hand-tight first, then apply wrench force. Let any remaining water drip into your bucket.
6. Disconnect the End. At the far end of the corroded line—where it enters the toilet tank, faucet, or fixture—use the same two-wrench technique to disconnect. If it's soldered, cut cleanly with a tubing cutter. Unscrew completely. You should now have a free length of old corroded pipe. Leave it sitting in the bucket to drain any remaining water.
7. Clean the Fittings. Using a pipe brush or fine steel wool, clean both the source fitting and the destination fitting. Remove any old plumber's tape, corrosion, or mineral deposits. Dry both fittings completely with a clean cloth. If you're connecting to old copper with corrosion visible, use a desoldering bulb or wick to remove old solder, or sand the connection point with 120-grit sandpaper until it's bright copper.
8. Size and Cut New Line. Hold the new supply line up against the old routing path. Mark where it needs to connect at both ends. For flexible supply lines (recommended for most repairs), they're forgiving on length—add 2 inches to account for the fitting connection depth. Cut with a tubing cutter or hacksaw. If you're using rigid copper, measure twice and cut once. For PEX or other flexible materials, use a line-specific cutter if you have it.
9. Connect at Source. If using compression fittings (easiest for most people): Slide the compression nut over the new line, then the compression ring (ferrule), then insert the line into the fitting. Hand-tighten the nut, then use a wrench to tighten it one more full turn. Do not over-tighten—it will crack the ferrule. If soldering copper: Apply flux to both the fitting and the pipe end, heat with a propane torch until the fitting is hot enough that solder melts on contact, feed solder into the joint, and let it cool naturally. Test by opening the shutoff valve slowly and checking for leaks.
10. Connect at Fixture. Repeat the same connection method at the fixture end. Ensure the line isn't kinked or stressed. If it's a toilet inlet, the line should come in from below and the connection should be hand-accessible for future toilet replacement. For sink faucets, ensure the line isn't bent sharply—gradual bends only. Tighten compression fittings by hand plus one full turn of the wrench.
11. Test for Leaks. Open the shutoff valve slowly—do not crank it open suddenly. Open the destination fixture (faucet or toilet) and let water run for 30 seconds to purge air from the line. Watch both connection points carefully for any drips. It's normal to see a drop or two appear at the connection immediately after turning on; tighten the fitting another quarter-turn if water is actively running. Turn off the fixture and let it sit for five minutes, watching the connections. If they remain dry, the job is done.
12. Dispose and Document. Remove the old corroded line from the bucket. Many municipalities accept copper and brass for recycling; verify with your local recycling center or water utility. Stainless steel braided lines go in regular trash. Take a photo of your completed connection from the same angle as your initial assessment photo. This documents that the repair is done and helps future owners or contractors understand what you replaced.