Most Likely Causes (Ranked)
Braided stainless steel supply lines contain a rubber hose inside the braiding. After 5-8 years, this rubber degrades from constant water pressure and chlorine in municipal water. The failure mode is sudden — the rubber develops a weak spot, bulges, and bursts. Water releases at full municipal pressure continuously until the valve is closed. Insurance claims from supply line failures average $10,000+.
The compression fitting or threaded connection at the shut-off valve loosens from vibration (especially if the toilet rocks), thermal expansion, or water hammer. Even a tiny gap allows water to seep — and any seep eventually becomes a drip, then a leak.
The top connection at the toilet fill valve tailpiece uses a rubber washer and coupling nut. If overtightened, the rubber washer deforms and leaks. If undertightened, it drips. The plastic tailpiece on cheaper fill valves can also crack if the supply line puts lateral stress on it.
The shut-off valve itself may be the leak source, not the supply line. Gate valves and globe valves in older homes corrode internally and externally. The packing nut leaks when the valve is operated after years of sitting unused. Multi-turn shut-off valves are particularly failure-prone.
If the supply line was installed cross-threaded (started at an angle), the threads are damaged and can't seal properly. This also happens with over-tightened connections that strip the threads. The only fix is replacement of the damaged component.
How to Fix It
Replace the Supply Line
$8-15 (5 minutes)- 1Turn off the shut-off valve. Flush the toilet to reduce water in the tank.
- 2Place a towel under the connections. Unscrew the coupling nut at the fill valve and the compression nut at the shut-off valve.
- 3Install a new braided stainless steel line. Measure the length — too short creates stress, too long creates a kink.
- 4Hand-tighten both connections, then add a quarter turn with pliers. No Teflon tape needed — these use rubber washers.
- 5Turn on the valve slowly and check both connections with a dry paper towel.
Tighten the Connection
Free- 1Try tightening the leaking fitting a quarter turn. Hold the valve body steady with one wrench and tighten the nut with another.
- 2If it still leaks after a quarter turn, the rubber washer or line needs replacement — don't keep tightening.
- 3Check for cross-threading by disconnecting, inspecting the threads, and reconnecting carefully.
Replace the Shut-Off Valve
$15-30 DIY / $100-200 Pro- 1If the valve itself leaks, replace it with a quarter-turn ball valve ($10-15) — they're far more reliable than multi-turn valves.
- 2Turn off the main water supply. Compression-type valves can be replaced without soldering.
- 3If it's a soldered valve, this requires a torch — consider calling a plumber ($100-200) or converting to a compression valve.
When to Call a Professional
A burst supply line releases water 24/7 at full pressure — know where your shut-off valve is and ensure it works.
Replace all rubber or vinyl supply lines immediately — these have the highest failure rate. Braided stainless is the minimum standard.
Never over-tighten plastic fittings — they crack and strip easily.
If you leave for vacation, turn off the toilet shut-off valve (and the washing machine valves) as a precaution.
How to Prevent It
Replace supply lines every 5-7 years proactively ($8-15 each, 5 minutes to install).
Use braided stainless steel lines only — never rubber or vinyl.
Exercise (turn on and off) shut-off valves annually so they don't seize.
Install a water leak detector ($20-30) behind each toilet for early warning.
Frequently Asked Questions
Every 5-7 years. They're $8-15 and take 5 minutes. Insurance claims from supply line failures average $10,000+. The math is clear.
Don't force it — you'll break the valve. Turn off the main water supply instead. Then replace the valve with a quarter-turn ball valve ($15 DIY or $100-200 pro).
Braided stainless is the standard. Solid chrome supply lines are rigid and harder to install but virtually burst-proof. Either is acceptable — avoid rubber or vinyl.
Yes. A burst line releases 3-5 gallons per minute. Over 8 hours (while you're at work), that's 1,400-2,400 gallons. Average insurance claim: $10,000+.