My toilet tank has a crack. Is there anything that can repair this?

[Guest]

Just recently, I noticed that water is leaking from our toilet tank and knew that it is broken. I don't know who or what made it crack but I really want it to be fixed. Can you give me some tips on how to fix this?

[Nestor]

Believe it or not, toilet tanks DO crack by themselves. It doesn't happen very often, but it does happen. See the PS below.

Before you do anything, take a closer look. If you don't see a crack, there isn't one. The water leaking from the tank might just be condensation forming on the tank and dripping off.

If the tank is cracked, I don't think that repairing the tank is a good option. That's because every time you flush, the warmed up water empties out of the tank and the tank is refilled with cold water. The associated expansion and contraction is likely to cause the crack to propagate further, causing the tank to leak again.

Your best bet is to either replace the tank with a used one from your local Habitat for Humanity Re-store, or replace the whole toilet (both the bowl and the tank). Normally there are two brass bolts that hold the tank to the bowl. As long as the distance between those bolts are the same on the new tank, there's a good chance you can get it to fit onto your bowl. Toilets normally have a 12 inch "rough in" which means that the center line of the toilet discharge on the bottom of the bowl is 12 inches away from the finished wall surface behind the toilet. So, even if the distance between the bolt holes is the same, the new tank might sit further back on the bowl so that it hits the wall.

If replacing the tank with another old tank or replacing the whole toilet isn't an option, then I think about the best way to fix a cracked tank would be to allow it to dry completely, fill the crack with silicone caulk by pressing it into the crack with a putty knife, and then giving the silicone a few days to cure completely. You can flush your toilet bowl in the mean time just by pouring a 5 gallon pail of water into it. (Pour quickly.)

For your new tank to work properly on your existing toilet bowl, it has to hold as much or more water and release it as fast or faster than the old tank did. And, therein lies the problem. Old toilet tanks held 5 gallons of water, but nowadays, all the new toilets have tanks that only hold 6 liters of water, or about 1/3 as much as the old tanks. So, the smaller tanks for new toilets won't work on an old toilet bowl. (Post again if you want me to explain why.)

PS:

One possible reason for toilet tanks to crack by themselves is something called "moisture expansion" which happens to all fired clay ceramic materials, which includes toilet tanks and bowls. Clays will swell as they absorb moisture. During firing, nearly all of the moisture is driven out of the clay. Consequently, the fired clay will absorb moisture if it's in contact with water, or even from the humidity in the air and expand slightly as a result. The expansion is most rapid immediately after firing the clay, but continues at a progressively slower rate for centuries. The rate of moisture expansion in fired clay materials is inversely proportional to the time elapsed after firing raised to the 4th power, so a 20 year old toilet tank is expanding at 1/16 the rate that a 10 year old toilet tank is. It sounds like the plot from a cheap horror movie, but it's true. All the fired clay materials in your house, from the dishes in your kitchen cupboards to the bricks in your fireplace to the toilet in your bathroom are very very slowly getting bigger (and slightly heavier cuz of the moisture absorbed).