Quick Answer:
If a toilet shutoff starts leaking after you turn it, shut off the supply, drain the tank, inspect the valve and connections, try to clear any debris, and replace the valve if it’s damaged. If you cannot stop the flow or the valve is badly corroded, turn off the house main and call a plumber.
Why This Happens
Common causes of a shutoff leaking after being turned include worn internal seats, cracked bodies, a failed packing or compression ferrule, and mineral or debris buildup that prevents a proper seal. Valves that haven’t been moved in years can seize or crumble, and soft plastic parts can fail under pressure. Problems range from an easy cleanup to a full replacement — and in other cases the shutoff behaves like an Under-sink valve frozen in place or follows the pattern of a Valve stops working after years of no use.
Step-by-Step What to Do
1. Isolate the supply
- Turn the shutoff at the toilet. If it still flows, immediately shut off the house main valve.
- Flush the toilet to empty the tank and lift the lid; hold the flush lever down to drain the bowl as much as possible.
- Place a bucket and towels under the valve and supply line to catch water.
2. Inspect the valve
- Look for obvious cracks, corrosion, or a loose compression nut where the supply line meets the valve.
- Check for slow weeping from the stem or around the packing nut — that indicates internal wear or a failed seal.
- If the valve body is brittle, discolored, or crumbling, plan to replace it rather than try to repair it.
3. Flush debris
- With the supply isolated and tank drained, carefully disconnect the flexible supply hose from the valve (have the bucket ready).
- If the valve turns a little, open it gently to let any trapped debris or sediment clear into the bucket. Do not force movement — stop if it’s stiff.
- Inspect the hose and valve inlet for grit or fragments; clean or replace the hose if it’s contaminated.
4. Replace if needed
- If the valve leaks after the above checks, replace the shutoff. Turn off the house main, drain the lines, remove the old valve, and fit a new angle stop or quarter-turn valve using the proper fittings.
- Use a compression-style replacement if the existing connection is compression; use appropriate tools and don’t over-tighten nuts.
- If you’re not comfortable with plumbing connections or the existing pipe is damaged, hire a licensed plumber to ensure a safe replacement.
What Not to Do
- Do not force the valve; forcing can break the body or twist the supply line free and make the leak worse.
- Do not ignore leaks; even a small persistent leak can cause damage and lead to mold or rot.
- Do not use open flame or heat near plastic fittings or flooring; use correct tools and shut water off before working.
When to Call a Professional
- You cannot stop the water flow (house main fails or you can’t reach it).
- The valve or pipe is visibly cracked, corroded, or embedded in finished flooring/wall and you’re unsure how to access it.
- Replacing the valve involves soldering or changes to copper that you’re not trained to do; or water damage is already present.
- A quick repair doesn’t stop the leak or water damage is accelerating.
Safety Notes
- Wear gloves and eye protection when disconnecting lines; use towels to protect floors and fixtures.
- Always relieve pressure by draining the tank and bowl before disconnecting the supply hose.
- Keep a bucket and rags handy to manage water. If unsure about shutting off the house main, call a pro before attempting repairs.
Common Homeowner Questions
- How long will a replacement shutoff last? — A good-quality quarter-turn valve can last many years; plastic or low-quality valves may fail sooner.
- Can I tighten a leaking compression nut without replacing the valve? — Sometimes tightening the compression nut stops a leak, but if the valve body or ferrule is damaged, replacement is best.
- Is a slow drip from the valve an emergency? — Not usually immediate, but address it quickly to avoid water damage and mold; if it worsens or you can’t stop it, call a plumber.