How to Fix a Leaking Bathroom Supply Valve: Step-by-Step Guide

A dripping angle stop valve under the bathroom sink or behind the toilet is one of those leaks that starts small and eventually causes serious damage — warped cabinet floors, mold growth, and even subfloor rot. The good news is that angle stop valves are simple, inexpensive, and entirely within DIY range to repair or replace.

A dripping angle stop valve under the bathroom sink or behind the toilet is one of those leaks that starts small and eventually causes serious damage — warped cabinet floors, mold growth, and even subfloor rot. The good news is that angle stop valves are simple, inexpensive, and entirely within DIY range to repair or replace. This guide covers both a quick fix for a packing-nut leak and a full valve replacement.

Tools and Materials You Will Need

• Adjustable wrench or channel-lock pliers (two pair)
• Teflon tape (PTFE thread seal tape)
• Replacement angle stop valve (1/2-inch FIP inlet, 3/8-inch compression outlet is standard)
• Replacement braided stainless supply line
• Bucket and old towels
• Pipe cutter or hacksaw (if cutting old compression fitting)
• Emery cloth or fine sandpaper

Diagnosing the Leak

Before shutting anything off, identify exactly where the water is coming from:

• Leak at the packing nut (around the stem): The packing material inside has worn and water seeps around the valve stem. Try tightening first.
• Leak at the supply line connection: The connection at the valve outlet has loosened or the supply line is corroded. Tighten or replace the line.
• Leak at the inlet (where valve meets the wall): The threads or compression fitting at the water supply pipe are failing. Full valve replacement is needed.

Quick Fix: Tighten the Packing Nut

If the leak is at the stem, turn the valve to the open position first. Use an adjustable wrench to tighten the hexagonal packing nut — the nut just below the handle — one-quarter turn clockwise. Turn the valve a few times and watch for drips. If the leak stops, you are done. If the nut will not tighten or the leak persists, replacement is the next step.

Full Valve Replacement

Step 1: Shut Off the Main Water Supply

Locate your main house shutoff valve and turn it fully clockwise to close it. Open a faucet on the lowest level of the house to drain pressure from the lines. Place a bucket and towels under the work area.

Step 2: Remove the Supply Line

Use one wrench to hold the valve body and a second to loosen the supply line nut at the valve outlet. Turn counterclockwise. Once loose, unscrew by hand and pull the line free.

Step 3: Remove the Old Valve

If the valve connects to the wall supply pipe via a compression fitting, hold the supply pipe steady and loosen the large compression nut on the valve inlet. If the old compression ring is stuck to the pipe, slide it off with an adjustable wrench and clean the pipe end with emery cloth until it is smooth and bright.

If the valve threads into an iron or copper fitting with NPT threads, use two wrenches — one on the fitting, one on the valve — and turn the valve counterclockwise to unthread it.

Step 4: Install the New Valve

For a threaded inlet connection, wrap the valve’s male threads with three layers of Teflon tape clockwise before threading it into the fitting. Hand-tighten, then snug it an additional one to two turns with a wrench. Do not over-tighten threaded fittings — brass threads strip easily.

For a compression inlet connection, slide the new compression nut and new compression ring (ferrule) onto the supply pipe in that order. Insert the valve inlet over the pipe end, thread the nut onto the valve body, and tighten it firmly — about one full turn past hand-tight. No tape or sealant is used on compression fittings.

A SharkBite quarter-turn angle stop valve is an excellent option if you want to avoid compression fittings entirely — it pushes onto copper, CPVC, or PEX pipe and seals without any tools.

Step 5: Install a New Supply Line

Thread the new braided stainless supply line onto the valve outlet and the fixture inlet. Hand-tighten, then snug each connection a quarter turn with a wrench. These fittings have rubber sealing washers — over-tightening cracks the washer and causes leaks.

Step 6: Restore Water and Test

Turn the main water supply back on slowly. Watch every connection as pressure returns. Open the angle stop valve fully by turning the handle or stem counterclockwise. Check all joints — valve inlet, valve outlet, and supply line at the fixture — for any sign of moisture. Dry the connections with a paper towel and recheck after a few minutes.

If you find the angle stop valve has totally seized and will not close, that is a sign the whole fixture shutoff system needs updating. Seizing is common in homes with hard water and valves older than 15 years.

The total cost for a valve and supply line replacement is typically under $30, and the repair takes less than an hour. Catching this problem early saves hundreds in water damage repairs.