The outside surfaces of toilet tanks sometimes display condensation that runs down the tank and drips to the floor. Water on the floor can easily be mistaken for a toilet leak, instead of a sweating tank. A leaking toilet and sweating tank will cause the same water damage to your floor, but toilet tank condensation has different causes than a leak.
Cold Tank Water
The toilet is only hooked up to a supply of cold water. Each time the toilet is flushed, a fresh supply of cold water enters the tank. This cold water cools the surface of the tank. This is the reason that if you touch the toilet tank, you notice the surface is considerably colder than room temperature. This meeting of warm air and the cold tank causes condensation to form on the tank surface.
Toilets can even sweat during the winter. Lower outside temperatures cause the water entering the toilet to be even colder. Additionally, the bathroom is often heated when it's cold outside. When the warm air meets the tank surface that is holding even colder water, condensation results.
Condensation can form on the tank when a toilet is flushed frequently. If a faulty flapper or fill valve causes the toilet to continually run or "phantom-flush," the tank surface is never given a chance to warm up to a temperature closer to room temperature. The tank water is constantly flushed out, with fresh, cold water refilling it. The same is true of toilets that receive heavy use.
The warmer summer months are often accompanied by higher levels of humidity. Humidity is the amount of water vapour that is present in the air. Heavier levels of condensation can occur when the humid air meets the tank surface; that air moisture forms water beads on the tank.
Warmed Bathroom Air
With cold water already inside a toilet tank, condensation on its surface is also more likely to occur when the bathroom is suddenly warmed, as, for instance, during a hot shower.