An overflowing toilet can be caused by one of three things: a clogged drain that prevents it from flushing properly, an improperly adjusted float that causes the cistern to overflow, or a clogged vent pipe that replaces the air in the pipe after each flush.

Clogged Drains: Spotting a clogged drain is a fairly easy task. Water would overflow from the toilet instead of the cistern, creating a huge mess. In order to stop the spill, you have to deal with the clog. For simple blockages, a plunger and some manual labor will usually do the trick.

Clogged Vent: If you notice that your toilet is clogging more frequently, a clogged vent may be to blame. The vent moves outside air into the ductwork to replace the air pumped with each flush. When it becomes clogged, your toilet won't flush properly, causing an overflow.

High Fill Float: The top two causes of toilet overflow cause water to overflow from the toilet itself. In this case, a higher fill float would cause the overflow to flow from the toilet's cistern. If the fill float is set too high, the tank will overfill with water, causing side leaks. You can fix this by adjusting the mechanism inside the tank to control the level of the float.