How to Fix a Leaking Supply Line: Step-by-Step Guide

A dripping supply line under a sink or behind a toilet may seem minor, but supply line failures are one of the most common causes of serious home water damage. A pinhole drip can become a flood if the line bursts.

A dripping supply line under a sink or behind a toilet may seem minor, but supply line failures are one of the most common causes of serious home water damage. A pinhole drip can become a flood if the line bursts. Replacing a leaking supply line is inexpensive, fast, and requires no special plumbing skills.

What You Will Need

• Braided stainless steel supply line (correct length and fitting size)
• Adjustable wrench or channel-lock pliers
• Bucket or small bowl
• Old towels or rags
• Teflon tape (PTFE tape)
• Flashlight
• Safety glasses

Estimated time: 15 to 30 minutes Difficulty: Beginner Cost: $5 to $20 for a replacement supply line

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Step 1: Turn Off the Water Supply

Never work on a supply line without shutting off the water first.

1. Locate the shut-off valve for the affected fixture. For sinks, it is usually under the sink cabinet on the wall or floor. For toilets, it is on the wall behind and below the tank.
2. Turn the valve clockwise until it stops. If the valve is the older oval-handle style, turn it fully clockwise.
3. Flush the toilet or open the faucet to release pressure remaining in the line.
4. Place a bucket or bowl under the supply line connection to catch residual water.

If the local shut-off is stuck or does not fully stop flow, shut off the main water supply to the house at the meter before proceeding.

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Step 2: Remove the Old Supply Line

1. Use an adjustable wrench to loosen the coupling nut connecting the supply line to the shut-off valve. Turn counterclockwise. Some connections will drip water — have your rag ready.
2. Loosen the upper connection at the faucet tailpiece or toilet fill valve the same way.
3. Remove the old supply line and set it aside. Note its length and the fitting types at each end (typically 3/8 inch compression at the valve end and 7/8 inch or 1/2 inch at the fixture end).

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Step 3: Inspect the Shut-Off Valve

Before installing the new line, examine the valve stem and seat for corrosion or mineral buildup. Wipe the threads clean with a dry cloth.

If the valve shows visible corrosion or does not hold completely shut, now is the time to replace it. A leaking shut-off valve defeats the purpose of the new supply line.

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Step 4: Select the Correct Replacement Line

Supply lines come in various lengths and end configurations. Match your old line carefully:

• Length: Measure the distance between the shut-off valve outlet and the fixture inlet. Add 2 to 3 inches to allow for a gentle curve without kinking. Common lengths are 12, 16, 20, and 24 inches.
• Valve end: Most residential shut-off valves have a 3/8 inch compression fitting. Confirm yours matches.
• Fixture end: Toilet fill valves typically use 7/8 inch ballcock slip-joint nuts. Faucet tailpieces are commonly 1/2 inch MIP or compression.

Braided stainless steel supply lines are strongly recommended over plastic or chrome corrugated lines for their burst resistance and longevity.

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Step 5: Wrap Threads with Teflon Tape

For any threaded connections (not compression fittings), apply two to three wraps of Teflon tape clockwise around the male threads before threading on the coupling nut. This creates a watertight seal and makes future removal easier.

Do not apply Teflon tape to compression fittings — these rely on a ferrule and washer to seal, not tape.

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Step 6: Install the New Supply Line

1. Hand-thread the lower coupling nut onto the shut-off valve. Ensure it starts straight to avoid cross-threading.
2. Hand-thread the upper coupling nut onto the faucet tailpiece or toilet fill valve.
3. With your wrench, tighten the lower connection first: snug plus one-quarter to one-half additional turn. Do not over-tighten — brass fittings strip easily.
4. Tighten the upper connection the same way.
5. Route the supply line in a gentle curve — never kinked or bent sharply, as this weakens the line and restricts flow.

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Step 7: Restore Water and Check for Leaks

1. Slowly open the shut-off valve by turning it counterclockwise. Open it all the way and then back it off a quarter turn to prevent the valve seat from sticking.
2. Watch the connection points carefully as pressure returns. Have your flashlight and a dry cloth ready.
3. If any connection weeps, do not tighten while fully pressurized — shut off the valve, dry the area, and snug the fitting another quarter turn, then re-pressurize and check again.
4. Run the faucet or flush the toilet to test full flow.

Dry the area completely with a cloth, then lay a dry paper towel against each fitting and leave it for 10 minutes. If the towel stays dry, the job is done.

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Common Mistakes to Avoid

• Cross-threading: Always start coupling nuts by hand and confirm they turn smoothly before applying a wrench.
• Over-tightening: This cracks washers and strips brass fittings. Snug plus a quarter turn is enough.
• Using the wrong line length: A supply line that is too short strains the fittings under pressure. A line that is too long bunches and kinks.
• Skipping the Teflon tape: On threaded connections, tape prevents micro-leaks at the thread roots.

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Final Tips

• Replace all supply lines in a bathroom or kitchen at the same time — if one is old, the others likely are too.
• Label the date of installation on the supply line with a marker so you know when the next preventive replacement is due.
• Check under sinks and behind toilets during your regular seasonal home maintenance walk-through to catch drips early.

A $10 braided supply line replaced in 20 minutes of your time is far preferable to a water damage claim. This is one DIY repair that pays for itself many times over.