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Toilets | May 22, 2025

Is It Condensation…Or Is Your Toilet Leaking From Tank Bolts or Gaskets?

Is It Condensation…Or Is Your Toilet Leaking From Tank Bolts or Gaskets?

Often, what appears to be condensation on our toilet tank is actually a leak that needs to be stopped. Two common reasons for the leak are (1) tank bolts that need to be replaced or tightened, and (2) gaskets that are worked out and need to be replaced.

Either way, here’s how to diagnose if your toilet tank is “sweating” from the outside or if you need to fix an actual leak.

Stopping Toilet Leaks at Bolts & Gaskets

1. Make Sure it’s Not Condensation

Leaks (or seepage) from around the two bolts that hold the tank to the toilet bowl are often mistaken for condensation. To make a sure diagnosis, try this:

  • Pour food coloring (or laundry bluing) into the the tank.
  • Hold a piece of white tissue over the tips of the bolts.
  • If the tissue turns to the color you’re using, you have a leak, not condensation.
  • Move on to the next step to fix the tank bolt leak.
Leaky Toilet Flapper - add food coloring to tank
This is a neat little trick. Pour food coloring into the tank to find out where leaks are occurring in your toilet tank or toilet bowl.

2. Fix the Tank Bolt Leak

  • In most cases, simply tightening the tank bolts will stop the leak. These bolts are also known as hold-down bolts or mounting bolts.
  • Drain the tank of water.
  • Reach into the tank with one hand holding a screwdriver (or have a helper do it). Place the screwdriver into the hold the head of the bolt.
  • With your other hand, use an adjustable wrench (a socket wrench with an extra-deep socket might work even better) to tighten the bolt from underneath the tank.
  • If the leak persists, you will have to remove the bolts and replace the washers. Move on to step #3 below.

CAUTION: Do not overtighten the tank bolt or the brittle finish of the tank will crack.

toilet leaking from tank bolts - unscrew from inside tank

Unscrew the tank bolts from insider the toilet. Place a screwdriver into the bolt head to keep it from turning. Then use a wrench to unscrew the bolt from below.

3. Stop the Flush Valve Gasket Leak

If the leak occurs around the flush valve “” the large opening between the tank and the bowl “” you must remove the tank and replace the conical washer and the spud gasket (also called a spud washer).

FYI: A gasket is another name for a washer, usually made of rubber, that creates a watertight seal between two different surfaces. You might have also heard it referred to as an O-ring. Whatever its name, the function is the same. Here’s how to stop a leak in your toilet’s flush valve gasket.

  • As always, make sure the tank is drained of water.
  • Unscrew the two hold-down bolts and disconnect the supply pipe to the float valve (also called a ballcock). It is the device that allows water to fill the tank without any overflow or backflow.
  • Carefully lift the tank upward and off the bowl, then set it back down on a bed of newspapers or towel.
  • Remove the large locking nut on the threaded valve-seat shaft that protrudes from the bottom of the tank.
  • Pull the shaft into the tank and replace the spud washer on the shaft.
  • Also replace the large conical (circular) washer that covers the threaded shaft of the flush valve.
  • Remount (re-install) the tank.
toilet leaking from tank bolts - diagram of tank bolts and gaskets washers

You will want to replace the conical washer that fits around the shaft of the flush valve, as well as the spud gasket. A close-up image of the spud gasket is used as our feature image for this article.

Final Thoughts

Replacing or tightening the two tank-mounting bolts (which attach the tank to the bowl) is super easy. Replacing the gaskets, while they’re just simple rubber parts, involves more work.

But if you follow the 3-step process above, you’ll be able to diagnose whether the problem you’re having is condensation (we have a fix for that below, too!) or a leak that needs to be stopped.

Call 1-Tom-Plumber

Don’t hesitate to call us at 1-Tom-Plumber (1-866-758-6237) if you need help with a toilet leaking from its tank bolts or gaskets.

1-Tom-Plumber’s certified team of plumbers and drain technicians respond immediately to any emergency plumbing, drain cleaning, or water damage problem. We also handle the excavation of underground water lines and sewer main lines. Our immediate-response team is available every day and night of the year, even on holidays.

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