If the shutoff valve under your sink won’t stop the water, the valve itself has likely failed internally. This is common in older homes, and it means the valve needs to be replaced, not just turned harder. Forcing a worn valve can break the stem and make things worse.
Why Angle Stop Valves Fail
The valve under most sinks is called an angle stop (or angle valve). It turns the water supply 90 degrees from the wall to the supply line going up to the faucet. Most of them use a compression design: a rubber washer presses against a brass seat to block the flow. Over time, that rubber deteriorates, hardens, or gets gouged by mineral deposits. When that happens, you can close the handle all the way and still get a trickle, or a steady stream, through.
Age is the biggest factor. A compression angle stop that has sat in a cabinet for a decade or more without ever being touched is a valve that’s never been exercised. The internal parts corrode or seize. When you finally try to close it during a repair, it won’t seat properly.
Mineral buildup (calcium and lime scale) is the other common culprit, especially in areas with hard water. Scale accumulates on the valve seat and prevents the washer from making a clean seal.
Ball Valves vs. Compression Valves
Newer homes and recent plumbing upgrades often have ball valves for shutoffs instead of the older compression style. A ball valve uses a rotating ball with a hole drilled through it. When the hole lines up with the pipe, water flows. Rotate 90 degrees and it blocks. Ball valves are generally more reliable and have a longer service life than compression valves.
You can usually tell them apart by the handle. A compression valve has a round multi-turn handle (like a small wheel or a cross shape) that you spin several times to close. A ball valve has a lever handle that moves a quarter turn. If you have the round multi-turn style and it won’t stop the water, it’s almost certainly a compression type that’s worn out.
What Diagnosing Actually Looks Like
A plumber checking a failing shutoff will look at a few things. First, how old is the valve and what’s it made of? Older chrome-plated brass compression stops have a finite lifespan. If the home was built in the 1980s or 1990s and the valve has never been replaced, that tells the tech a lot before even touching it.
Second, they’ll check for mineral deposits on the seat. Descaling or reseating with a new washer is sometimes possible, but on a valve that’s already 20 years old, most plumbers will just recommend replacement. A new quarter-turn ball valve at that location is more reliable than a repaired compression valve of the same age.
Third, they’ll check what the valve connects to: the supply line material, the condition of the ferrule (the compression fitting that connects the valve to the supply pipe), and whether the shutoff feeds just the sink or something else on the same branch.
What to Observe Before You Call
You can safely turn the handle slowly and check whether the flow changes at different positions. That tells a plumber something useful. Don’t force it. If the handle takes unusual effort to turn, or starts to feel like it’s spinning freely without resistance, stop. That’s a stripped stem, and forcing it further makes the job harder.
Look at the pipe connection behind the valve too. Green buildup, white mineral crust, or visible corrosion around the fitting or the wall stub-out are things worth mentioning when you call. So is whether the main house shutoff is in good shape (or unknown). That context helps a plumber come prepared.
Replacing a failed valve involves shutting the main water supply, disturbing old fittings, and ensuring the new valve seats correctly on whatever pipe is behind it. That’s work for a licensed plumber, particularly in older homes where the pipe condition is uncertain and a fitting that looks fine can crack when moved.
When to Call a Licensed Plumber
Call a licensed plumber if:
- The valve is completely seized and you can’t turn it at all
- Closing the valve doesn’t slow the water at all, or the handle spins freely
- You see active corrosion, green buildup, or white mineral deposits around the valve body or the pipe connection behind the wall
- The valve feeds a bathroom or kitchen where you need the water back on quickly
- You’re not sure which shutoff to use if the under-sink valve fails mid-repair
- The main house shutoff is also questionable (that’s a code-level safety issue and needs attention)
Replacing a failed angle stop is routine plumbing work for a licensed plumber. In California, plumbing work in a home requires a C-36 license. You can verify a contractor’s license at cslb.ca.gov before hiring anyone.
Don’t wait on this repair. A shutoff valve that won’t close during a calm moment under a sink also won’t close during a supply line burst at 2 a.m. Getting a new ball valve installed before something goes wrong is the whole point of having a functioning shutoff in the first place.