Leaking Toilet Supply Line or Shut-Off Valve: Causes & Fixes

If you're seeing water dripping behind your toilet — from the flexible supply hose, the connection nuts, or the little oval valve on the wall — you're dealing with one of the most common bathroom leaks, and most versions are a quick, inexpensive fix.

The tricky part is that several different problems all look the same at first glance, and one of them (a shut-off valve that won't fully close) actually blocks the easy DIY repairs. Below we help you pinpoint the exact spot and match it to the right fix.

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Prefer to fix it yourself? The full DIY guide is below. Stuck or short on time? We offer same-day help across Denver.

Is this an emergency?

Slow drip or damp spot

A bead of water on a nut or a slightly damp floor can usually wait a day or two while you get parts.
Put a towel or shallow pan under it and check it isn't getting worse.

Steady running, pooling, or spraying — act now

Close the angle stop (turn clockwise) to cut water to the toilet, then assess.

Valve won't shut off — urgent

If you turn the shut-off valve fully clockwise and water keeps flowing, or it's seized, you have no local control.
Go to your home's main water shutoff (near the meter or where the line enters the house) and shut the whole house off. Treat this as a priority and a reason to call a pro.

How to diagnose it

The whole job depends on finding the exact drip point, so do this first:

1Dry everything completely — the hose, both connection nuts, the whole valve body, and the wall/floor. Water pools at the lowest point, not always where it starts.
2Wrap a dry paper towel around each suspect spot, or tuck one under the valve.
3Watch and feel for several minutes (flush once so the line re-pressurizes), then check which towel got wet first.
4Identify the zone: top nut (at the tank), bottom nut (at the valve), under the handle (packing nut), where the valve meets the wall pipe, or the hose itself.

Common causes

Loose compression/coupling nut

The nut at the tank or valve has simply worked loose over time.

Telltale signs: A slow drip right at a connection nut; the nut turns slightly by hand; the drip slows when you gently snug it.

Worn rubber washer/gasket inside the supply hose

The rubber cone seal at each end of the supply line hardens and shrinks with age.

Telltale signs: Leak at a nut that keeps dripping even after you tighten it; an older hose (5+ years); drip returns shortly after retightening.

Cracked plastic supply-line nut (overtightened)

The plastic nut on a cheap supply line hairline-cracks — almost always from being cranked too tight.

Telltale signs: A persistent weep at the nut that no tightening fixes; a visible crack/split in the plastic nut; often appeared right after an install.

Split or burst flexible/braided supply hose

The hose itself fails along its length — plastic lines crack with age; braided lines can corrode or rupture.

Telltale signs: Water from the middle of the hose; bulging; rust/fraying on a braided line; or a sudden spraying leak.

Packing nut leak on the valve stem

The small nut under the handle seals around the moving stem; its internal packing dries out.

Telltale signs: Water seeping from under the handle / around the stem, often only after you turn the valve; the body and connections stay dry.

Corroded or old angle-stop valve body

The valve itself leaks through pitted/corroded metal or a failed internal seal.

Telltale signs: Drip or mineral crust on the valve body (not a nut); green/white corrosion; drip persists no matter what you tighten.

Valve that won't fully close (seized or worn)

The handle turns but water keeps flowing, or it's frozen — the internal seat/washer is worn or scaled.

Telltale signs: You close it and water still reaches the toilet; handle spins freely with no resistance, or is stuck solid. This removes your local shutoff and blocks most DIY repairs.

How to fix it yourself

Before any fix, know where your main water shutoff is — many repairs assume the angle stop works; if it doesn't, you'll need the main.

1. Tighten a loose connection nut

Difficulty:: Easy
Time:: 5–10 min

Tools & parts: Adjustable wrench, slip-joint pliers to hold the valve, towel

1Dry the connection so you can tell if tightening actually stops it.
2Brace the valve body with pliers so you don't twist the whole valve and damage the pipe in the wall.
3Snug the leaking nut about 1/8 to 1/4 turn — no more.
4Dry, flush, and recheck. If it persists, move to replacing the supply line.

Don't: Don't crank it hard — overtightening crushes the washer or cracks plastic/porcelain and makes it worse.

2. Replace the supply line

Difficulty:: Easy
Time:: 15–30 min

Tools & parts: New braided stainless supply line (match length and fittings — bring the old one), adjustable wrench, towel/bucket

1Turn off the angle stop (clockwise). If it won't fully close, shut off the main instead.
2Flush and hold the handle to drain the tank; sponge out the rest.
3Unscrew the bottom nut at the valve and the top nut at the tank (some water will spill).
4Connect the new line — valve end and tank end. Rubber-cone seals need no tape; hand-tighten plus about 1/4 turn with the wrench.
5Keep the plastic nut at the tank hand-tight only (overtightening cracks the porcelain).
6Turn water back on and check both ends with a dry paper towel; snug slightly if needed.

Don't: Reuse old hardened washers, overtighten the tank nut, or use Teflon tape on rubber-sealed (compression-cone) ends.

3. Stop a packing nut (valve stem) leak

Difficulty:: Easy
Time:: 5–15 min

Tools & parts: Adjustable wrench; optional replacement packing/O-ring or Teflon packing string

1Find the packing nut directly under the handle on the valve stem.
2Turn it clockwise just 1/8 turn to compress the packing.
3Wipe dry, operate the valve, and recheck.
4Still weeping? Shut off the main, remove the handle, back off the packing nut, replace the worn washer/O-ring or wrap fresh packing string, and reassemble.

Don't: Over-tighten the packing nut — it can crack the valve or crush the packing and worsen the leak.

4. Replace the angle stop valve (corroded valve, or valve won't close)

Difficulty:: Moderate–Advanced
Time:: 30–60+ min

Tools & parts: New shut-off valve sized to your pipe (compression or push-to-connect), wrench(es), bucket/towels, emery cloth, tubing cutter/deburrer (for copper), Teflon tape, flashlight

1Shut off the home's main water supply — you can't rely on the broken valve. Drain the tank and open a faucet to relieve pressure.
2Disconnect the supply line. Remove the old valve (compression: unscrew the nut and slide the valve and ferrule off; threaded: unscrew from the pipe).
3If it's a soldered/"sweated" valve, the pipe must be cut — this is where many DIYers stop and call a pro.
4Prep the pipe: cut copper square, then deburr and shine the end with emery cloth.
5Install the new valve. Compression: slide on the nut and a NEW ferrule, push on, hand-tighten, then ~3/4 turn while bracing the pipe. Push-to-connect: push on until it clicks fully home.
6Reconnect the supply line (hand-tight + ~1/4 turn). Turn the main back on slowly and check every joint with a dry paper towel.

Don't: Attempt soldered valves or fragile/corroded pipe unless experienced — old pipes can snap inside the wall and flood the cavity.

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When to call a professional

A new angle stop is a cheap part, but a snapped pipe inside the wall is a much larger repair. Call a pro when:

The shut-off valve won't fully close or won't turn and you're not comfortable replacing it.
It's a soldered/sweated valve needing torch work, or any valve on galvanized/old/corroded pipe that may break.
There's no working shutoff at all (and/or the main shutoff is also stuck).
Leaks keep coming back after you've tightened and replaced the obvious parts.
There's water inside the wall, behind drywall, or you can't find the source.

Frequently asked questions

The supply line is dripping at the nut — tighten it or replace the line?

Try gently snugging the nut first (1/8–1/4 turn). If it keeps dripping, the rubber washer is worn or the nut is cracked — replace the supply line. A new braided line is cheap and a 15-minute job.

Braided steel or plastic supply line?

A stainless-steel braided line with metal/brass nuts is the most durable, kink-resistant choice. Avoid all-plastic lines and braided lines with plastic nuts — the plastic is weak and can crack or burst. Many plumbers replace supply lines roughly every 5–7 years.

Why is water dripping from under the handle when I turn the valve?

That's the packing nut around the valve stem. Snug it clockwise about 1/8 turn. If it still leaks, the internal packing needs replacing (shut off the main first).

I turned the shut-off valve all the way and water still flows. What now?

The valve's internal seal is worn — it can no longer stop the water. Shut off your home's main water supply, then plan to replace the valve or call a plumber.

Can I fix the leak without shutting off the main water?

For tightening a nut, replacing the supply line, or snugging a packing nut, the angle stop alone is usually enough — if it closes properly. To replace the valve itself, or if the valve won't close, you must shut off the main.

Is it a leak or just condensation (sweating)?

In humid weather, cold pipes and tanks sweat and drip. Dry everything, then watch where water reappears. A leak comes back at a specific joint; condensation forms as a fine film over a cold surface and is worse in summer.

How tight is "tight enough" so I don't break anything?

Hand-tighten, then add only a small turn with a wrench (about 1/4 turn for supply-line nuts; ~3/4 turn for a compression valve nut). The plastic nut at the toilet tank should usually be hand-tight only.

My old valve is corroded and I'm afraid the pipe will snap. Should I DIY?

If the pipe or valve is badly corroded, or it's a soldered valve, the risk of snapping the pipe inside the wall is real. Unless you're experienced and the main shutoff works, this is a good job to hand to a plumber.