Step-by-step fix
Step 1: Make sure the supply line is the problem
- Look for water at the braided or smooth supply tube, at either compression nut, or along a visibly kinked or corroded section.
- Dry the line and both connections with a towel, then watch for fresh moisture to show where the leak starts.
- Confirm the shutoff valve body and the fixture itself are not the actual source of the drip.
- Check that the line runs from the shutoff valve to the faucet, toilet, or other fixture and is not part of a different plumbing connection.
If it works: You have confirmed the fixture compression supply line is leaking, damaged, or worn and is the right part to replace.
If it doesn’t: If the valve body, wall pipe, or fixture shank is leaking instead, fix that problem first before replacing the line.
Stop if:- The shutoff valve will not close fully.
- The valve, wall pipe, or fixture connection is cracked, badly corroded, or loose in the wall or fixture body.
- You cannot clearly identify which connection is leaking.
Step 2: Shut off water and prep the area
- Turn the fixture shutoff valve clockwise until it stops.
- Open the fixture to relieve pressure and let the remaining water drain out.
- Place a bucket or bowl under the supply line and lay down a towel to catch drips.
- Use a flashlight to check that you have enough room to turn the wrench safely without stressing the valve or fixture.
If it works: The water is off, pressure is relieved, and the area is ready for removal.
If it doesn’t: If water keeps flowing strongly after the valve is closed, shut off the home's main water supply before continuing.
Stop if:- The shutoff valve leaks heavily from the stem or body when you operate it.
- The valve or supply pipe moves in the wall when you touch it.
Step 3: Remove the old supply line
- Hold the shutoff valve steady if needed and loosen the lower compression nut with an adjustable wrench.
- Loosen the upper nut at the fixture connection and lower the old line into the bucket.
- Remove the line completely and wipe both connection points clean.
- Compare the old line to the new one for overall length, end size, and connection style before installing anything.
If it works: The old line is out and both connection points are clean and ready for the replacement.
If it doesn’t: If a nut is stuck, apply steady pressure and reposition the wrench for a better grip rather than forcing the valve to twist.
Stop if:- The fixture shank or shutoff valve starts turning with the nut.
- The threads are damaged, flattened, or cross-threaded on either connection.
Step 4: Install the new fixture compression supply line
- Start the top connection by hand first, then start the bottom connection by hand so you know both nuts are threading correctly.
- Route the line in a smooth curve without a sharp bend, twist, or kink.
- Tighten each compression nut snugly with the wrench after hand-threading. Use firm pressure, but do not crank down hard.
- Make sure the line is not rubbing a sharp cabinet edge or pulling sideways on either connection.
If it works: The new line is installed straight, untwisted, and snug at both ends.
If it doesn’t: If a nut will not thread easily by hand, back it off and realign it before tightening again.
Stop if:- The new line is clearly the wrong length or has the wrong end fittings.
- A connection will only start if forced with a wrench.
Step 5: Turn the water back on slowly and check for leaks
- Close the fixture if you opened it earlier.
- Turn the shutoff valve on slowly while watching both ends of the new line.
- Wipe each connection dry, then hold a dry finger or paper towel under each nut to check for fresh moisture.
- If you see a slight seep, tighten that connection a little more and check again.
If it works: Water is back on and both connections stay dry under pressure.
If it doesn’t: If a connection still seeps after a small retightening, shut the water back off, loosen the nut, and reseat the line before trying again.
Stop if:- A steady drip continues after careful reseating and slight retightening.
- The shutoff valve itself now leaks or will not control the water properly.
Step 6: Confirm the repair holds in normal use
- Run the fixture for a minute or two, then shut it off and inspect the line and both fittings again.
- Check the cabinet floor or area below for any delayed drips.
- Look once more after 15 to 30 minutes to make sure no slow seep has started.
- Leave the area dry so any future leak is easy to spot.
If it works: The supply line stays dry during use and after the fixture sits under pressure.
If it doesn’t: If moisture returns later, recheck which connection is wet first so you can reseat or tighten only the leaking side.
Stop if:- Water is appearing from behind the wall, under the fixture body, or from another hidden location.
- The line keeps leaking even though the fittings are matched and properly seated.
Replacement Parts
Repair Riot may earn a commission from qualifying purchases, at no extra cost to you.
FAQ
Do I need thread tape on a fixture compression supply line?
Usually no. Compression-style supply line connections seal at the fitting, not on the threads. Adding tape can make tightening feel misleading and does not fix a poor fit.
How do I know what replacement line to buy?
Match the line length, both end sizes, and the connection type at the shutoff valve and fixture. Bring the old line with you or compare it carefully before ordering.
Can I reuse the old supply line if it only leaked once?
It is better to replace it. Once a supply line is kinked, corroded, or has started leaking at the fitting, replacement is usually the more reliable fix.
How tight should the compression nuts be?
Hand-tight first, then snug with a wrench. They should be tight enough to seal without forcing them hard. If a connection seeps, a small additional turn is better than overtightening from the start.
What if the shutoff valve leaks after I replace the line?
That points to a valve problem, not a supply line problem. Shut the water back off and plan to repair or replace the shutoff valve before relying on that connection.