Most Likely Causes (Ranked)
The packing nut behind the handle contains a rubber washer or graphite string packing that seals the stem. Every time the valve is turned, the stem rotates against this packing, wearing it down. Valves that sit untouched for years have packing that dries and shrinks. The irony: valves that are never used are more likely to leak when finally operated because the packing has hardened.
Brass valve bodies last 20-40 years. Cheap zinc alloy valves corrode in 5-10 years. Galvanic corrosion occurs when brass contacts galvanized steel pipe without a dielectric union. Internal corrosion creates rough surfaces that can't seal even with a new seat washer.
Multi-turn gate valves and globe valves use a brass wedge or disc that presses against a seat to stop flow. Mineral deposits, corrosion, and debris prevent full closure. Gate valves are especially unreliable — the gate can break off the stem entirely, leaving the valve stuck partially open.
Most under-sink shut-off valves use compression fittings on the supply pipe. The brass ferrule (compression ring) deforms once during installation to create a seal. If the nut loosens, or if the valve was removed and reinstalled (reusing the ferrule), the seal fails.
Shut-off valves near water heaters experience thermal expansion stress as hot water in the line expands and contracts. The repeated cycling loosens connections and accelerates packing wear. Homes without expansion tanks are most affected.
How to Fix It
Tighten the Packing Nut
Free- 1Try tightening the packing nut (the hexagonal nut just behind the handle) a quarter turn clockwise with a wrench.
- 2Hold the valve body steady with a second wrench to avoid stressing the pipe connection.
- 3If this stops the drip, you're done. Check again in a month.
- 4If it still drips after a half turn, the packing needs replacement.
Replace the Packing
$2-5 DIY- 1Turn off the water upstream (main shut-off or another valve).
- 2Remove the packing nut. Pull the old packing washer or unwrap the old string packing.
- 3Install a new rubber packing washer (take the old one to match at the store), or wrap 3-4 turns of graphite packing string around the stem.
- 4Reassemble, hand-tighten, then a quarter turn with a wrench. Test.
Replace with a Quarter-Turn Ball Valve
$10-25 DIY / $100-200 Pro- 1Quarter-turn ball valves are vastly more reliable than multi-turn gate/globe valves.
- 2Turn off the main water supply. Place a bucket under the valve. Open a faucet downstream to drain.
- 3For compression-type: unscrew the old valve. If the ferrule is stuck on the pipe, use the new valve with a new ferrule, or cut the pipe below the ferrule and use a new one.
- 4For solder-type: cut the pipe below the valve and use a compression-to-push-fit adapter.
- 5Install the new ball valve. Hand-tighten compression nuts plus a full turn with a wrench.
When to Call a Professional
A shut-off valve that doesn't shut off is useless in an emergency. If yours doesn't fully stop flow, replace it now — before a supply line bursts.
Never force a stuck shut-off valve — the stem can snap, leaving you with no way to stop the water. Use penetrating oil and gentle pressure.
Always know where your main shut-off valve is. If an under-fixture valve fails, the main is your backup.
When replacing a valve, have the main shut-off closed and a bucket ready — residual water in the lines will drain.
How to Prevent It
Exercise every shut-off valve in your home every 6 months — turn off and back on. This keeps the packing supple and the mechanism free.
Replace multi-turn gate valves with quarter-turn ball valves whenever you have a plumber on-site for any reason.
Install stainless steel braided supply lines on the new valve while you're at it.
Frequently Asked Questions
Ball valve, by far. Quarter-turn operation, full-bore flow (no restriction), virtually leak-free for decades. Gate valves are obsolete for residential use — they stick, corrode, and fail.
Apply penetrating oil (PB Blaster or WD-40) to the stem. Wait 30 minutes. Gently work back and forth. Never use a pipe wrench on the handle — use an adjustable wrench. If it won't budge, replace it.
$100-200 for a standard under-sink compression valve. $150-300 if soldering is required. $200-400 for a main shut-off valve replacement.
If you can close the valve upstream, yes — cap the outlet side. But this means you have no water to that fixture. It's a temporary measure only.