How to Repair a Leaking Water Supply Line

Water is the ultimate home intruder, and a leaking supply line—whether it is under a bathroom sink or behind a toilet—is a problem that demands immediate attention. Left unchecked, a steady drip rots vanity cabinets, warps flooring, and invites mold to take hold in the dark, damp spaces behind your fixtures. Most of these leaks occur at the connection points where the shut-off valve meets the flexible supply hose, or where the hose connects to the fixture itself. Successfully stopping a leak is less about brute force and more about understanding the geometry of the connection. You are either dealing with a worn-out rubber gasket that has lost its pliability or a cross-threaded fitting that never stood a chance. Once you master the technique of tightening to the point of a firm stop—without overtightening—you will find that most plumbing repairs are well within your capabilities.

  1. Stop the water flow first. Locate the dedicated shut-off valve for the fixture and turn the handle clockwise until it stops. If there is no dedicated valve, you must turn off the main water supply to the house.
  2. Release trapped water pressure. Open the faucet or fixture that the supply line feeds. This allows the remaining water in the line to escape so you do not get sprayed when you loosen the nut.
  3. Unscrew the old hose. Use an adjustable wrench to hold the valve steady while using a second wrench to loosen the nut on the supply line. Carefully unscrew the fitting and set the hose aside.
  4. Check for damage below. Check the rubber washer inside the supply line nut for cracks or flattening. Examine the threads on the shut-off valve to ensure they are not stripped or covered in calcification.
  5. Seal the threads securely. If you are replacing the connection point, wrap plumber's tape clockwise around the male threads of the valve. Smooth the tape firmly into the grooves.
  6. Install the new supply line. Hand-thread the nut onto the valve to ensure it is aligned correctly. Once tight by hand, use your wrenches to give it an additional quarter to half-turn.
  7. Verify the seal holds. Slowly turn the shut-off valve back on. Watch the connection point closely for several minutes for any sign of moisture.