Toilet Fill Valve Leaking at the Bottom: Causes and Fixes

A small puddle on the floor beside the toilet can come from four or five different places, and the fill valve shank is one of the most common. When you have a toilet fill valve leaking under the tank, water usually drips from the threaded shank where it passes through the tank, or from the supply line nut just below it. The causes are almost always simple: a loose locknut, a worn or misaligned shank washer, a cracked plastic valve body, or a tired supply connection.

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Most of these fixes cost little and take under an hour. The trick is confirming the drip really starts at the fill valve and not at the tank bolts, the tank-to-bowl gasket or the wax ring at the base, because each of those needs a different repair. Dry everything, add some tissue, and watch where the water shows up first.

How to Confirm a Toilet Fill Valve Leaking at the Shank

The fill valve sits on the left side of most tanks (as you face the toilet) and pokes through the tank bottom as a threaded shank. Underneath, a large plastic locknut holds it tight, and the supply line connects with a smaller coupling nut below that.

1. Dry the entire underside of the tank, the supply line, the shank and the floor with a towel.
2. Wrap a strip of dry toilet paper around the shank just above the locknut, another around the supply coupling nut, and lay one on each tank bolt nut.
3. Wait 15 minutes without flushing, then check the paper. Then flush and check again.

Paper wet only at the shank or supply nut means a fill valve or connection leak. Wet paper at the tank bolts points to bolt washers. Water appearing only after a flush, coming from between the tank and bowl, suggests the tank-to-bowl gasket. Water seeping from around the base of the bowl with no drips above is likely the wax ring. A toilet leaking from bottom of fill valve is easy to separate from those once you have dry paper in place.

One more possibility: condensation. On humid summer days, a cold tank can sweat enough to drip. If every surface of the tank is beaded with water, not just one spot, condensation is the likely cause rather than a leak.

Common Causes

Loose locknut

The plastic locknut can loosen over time from vibration or from someone bumping the supply line. A loose nut lets water seep past the shank washer.

Worn or shifted shank washer

Inside the tank, a cone-shaped or flat rubber washer seals the shank against the tank bottom. It can harden, crack or shift off-center, especially if the valve was turned while the nut was tight.

Cracked valve body or shank

Overtightening with a wrench is the classic cause. Plastic shanks can develop hairline cracks that leak steadily, and no amount of tightening will fix them.

Supply line or coupling problems

The coupling nut on the supply line has its own washer or cone. Old braided lines, crushed washers and cross-threaded nuts all leak at this point. Supply lines also age, and a braided line with frayed strands, bulges or kinks should be replaced right away.

Tools and Materials

• Towels and a small bucket or bowl
• Large sponge to soak up the last water in the tank
• Adjustable pliers or a basin wrench
• Replacement shank washer, or a complete toilet fill valve kit
• New braided toilet supply line in the correct length
• Flashlight
• Gloves

Buying a full fill valve kit often makes more sense than hunting for a single washer. The kit includes a new valve, shank washer, locknut and refill tube, and fill valves wear out anyway after roughly five to seven years in many homes.

Step-by-Step: Tighten or Reseal the Fill Valve

Start with the easiest fix. Shut off the water at the supply stop valve behind the toilet by turning it clockwise until snug. If that stop is stiff or leaks itself, close the main water shutoff instead.

1. Try snugging the locknut. With the water off, hold the fill valve inside the tank so it cannot spin, then turn the locknut a quarter turn by hand. Plastic nuts should be hand-tight plus a slight nudge with pliers, never cranked hard.
2. Turn the water back on and test. Re-wrap with dry paper and check again in 15 minutes. If the drip is gone, you are done.
3. If it still leaks, drain the tank. Shut off the water, flush and hold the handle down to empty the tank, then sponge out the rest.
4. Disconnect the supply line at the coupling nut, with a bucket underneath to catch the water left in the line.
5. Remove the locknut and lift the fill valve out of the tank.
6. Inspect the shank washer and shank. Look for cracks, flattening, debris or mineral buildup. Clean the hole in the tank bottom so the new washer seats on smooth porcelain.
7. Install the new washer or new valve. Seat the washer with the beveled side facing down into the hole if it is a cone type, insert the valve, and hold it straight while you hand-tighten the locknut from below.
8. Reconnect the supply line, preferably a new one. Hand-tighten the coupling nut, then give it about a quarter turn more.
9. Adjust the fill height so the water stops about an inch below the top of the overflow tube, clip the refill tube to the overflow, and turn the water on slowly.

Check for drips at the shank and coupling nut over the next hour, and again the next morning. Slow leaks sometimes show up only after the washer settles.

Troubleshooting

• Leak gets worse after tightening: you may have cracked the shank or pushed the washer off-center. Remove the valve and inspect.
• Drip at the coupling nut only: replace the supply line. A new line costs little and removes a common failure point.
• Water on the floor but all paper stays dry: check the shutoff valve stem. A packing nut leak at the stop valve can dribble down the pipe and look like a toilet leak.
• Tank keeps running after the repair: that is a fill level or flapper issue, not a shank leak. Lower the float and check that the flapper seals.
• Puddle returns only after flushing: look at the tank-to-bowl gasket and the flush valve, not the fill valve.

Fill Valve Leak vs Base or Bolt Leak

Location tells you most of what you need to know. A fill valve leak drips from the left rear underside of the tank. Tank bolt leaks show up as drops forming at the nuts under the tank, often after a flush when the tank is full and heaviest. A tank-to-bowl gasket leak appears at the center rear where the tank sits on the bowl. Wax ring failure brings water out from under the bowl, may smell like sewage, and sometimes stains the ceiling below.

Floor damage gives another clue. A long-running base leak tends to soften or discolor flooring all around the toilet, while a fill valve drip often leaves a stain on one side, near the supply line.

Costs

A fill valve kit typically runs from about $10 to $30, and a braided supply line usually costs $5 to $15. Doing it yourself, the total is often under $40. A plumber will commonly charge somewhere around $125 to $275 for a fill valve replacement, depending on your area and whether the shutoff valve also needs work. Replacing a seized or leaking shutoff valve adds to that.

When to Call a Plumber

Call a licensed plumber if the shutoff valve will not close or leaks when you touch it, if the supply pipe from the wall is corroded, or if the floor around the toilet feels soft or spongy. Soft flooring suggests a long-term leak has damaged the subfloor, and repair may involve pulling the toilet and replacing wood. Also bring in help if you see water stains on the ceiling below the bathroom, because water may be traveling farther than you think.