Toilet tank sweating or dripping

Bob Vila says the main reason a toilet tank sweats is too much humidity in the bathroom meeting the cold tank. This Old House says what looks like sweating can sometimes be a leak. Let's figure out which one you have by drying the tank and watching where the water comes back.

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Before you start. If water is spraying or will not stop, turn off the toilet's own supply stop by hand and never force it. If the water is coming from that stop valve itself, do not turn it. Shut off the main water supply by hand instead. If it will not turn, shut off the main water supply, also without forcing it, and call a plumber. If water has reached an outlet, plug, cord, switch or light, including a light in the ceiling below, do not touch it and call an electrician. Ready.gov says not to touch electrical equipment if it is wet or if you are standing in water, and to turn off the electricity only if it is safe to do so. Go to the breaker panel only if the floor is dry and nothing electrical near you is wet, and open only its hinged door.

Every question and answer on this page, in order

Question 1: Is water pouring, spraying or running steadily, or is the tank cracked? If so, pick the first answer right away. If not, dry the outside of the tank with a towel and watch it for a few minutes. Where does the water come back?

Condensation looks like beads or a film all over the outside, like a cold glass on a warm day. This Old House says what looks like a sweating tank can sometimes be a leak, so the next questions check for one.

It is pouring, spraying or running steadily, or I can see a crack in the tank: A leak that will not stop, or a cracked tank. Call a plumber now
Fine beads or a film all over the outside of the tank: go to question 2
From one spot: a supply line connection, the bolts under the tank, the seam between tank and bowl, or the floor around the base: go to question 3

Question 2: Do a dye test. Put a few drops of food coloring in the tank and do not flush. After at least 10 minutes, does color show in the bowl?

EPA says color in the bowl after 10 minutes means you have a leak. This Old House's sweating guide waits 30 minutes before it checks the floor and the outside of the tank, and Bob Vila waits about an hour, so give it longer if you can before you flush. Look at the outside of the tank and the floor first, then flush right after the test so the dye does not stain the tank, as EPA says. This Old House says colored water outside the tank can come from tank cracks or faulty plumbing. If you see it, choose the last answer.

Yes, the bowl water turned color: Likely cause: water leaking into the bowl
No, the bowl stayed clear: go to question 4
Colored water shows on the outside of the tank or on the floor: go to question 3

Question 3: Where does the water seem to start? Dry it, then look with a flashlight.

This Old House says to dry everything off, grab a flashlight and look closely, and do it after a half-dozen flushes, to see where the water is coming from.

It is pouring or spraying, or it comes from the shutoff valve at the wall, from the tank itself, or I cannot tell: A leak that will not stop, or a cracked tank. Call a plumber now
At the supply line where it meets the underside of the tank: Likely cause: a loose supply line connection
Under the tank at the bolts or the tank to bowl seam, or pooled on the floor around the base: Likely cause: a leaking tank joint or base seal. Call a plumber

Question 4: Lift the lid while no one is using the toilet. Do you hear hissing, or does the tank keep adding water?

This Old House's Richard Trethewey says to check this first, because new cold water coming in all the time would make any tank condense. The overflow tube is the open pipe standing in the tank.

Yes, it hisses, refills on its own, or water runs into the overflow tube: Likely cause: the fill valve keeps adding cold water
No, it is quiet and the water level holds: go to question 5

Question 5: How often does the tank sweat?

Only on humid days or after hot showers, and it dries up: Probably normal: condensation
Most days, even with the bathroom fan on, or it has stained or soaked the wall or floor: Sweating that will not stop: call a plumber or an HVAC technician

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