The most common cause of a leaky toilet is a failed wax seal, which connects the toilet to the drain line. Such failures allow water to leak from under the toilet onto the floor, often resulting in serious structural damage, according to This Old House.
Before replacing the wax seal, This Old House recommends taking time to diagnose the source of the leak by drying up any puddles with old towels. When a new puddle starts to form, its location indicates where the toilet is leaking.
If the leak forms around the base of the toilet, the seal is the most likely cause. Sometimes, tightening the bolts that hold the toilet to the floor stops it from leaking. Tightening in an alternating pattern, while avoiding over-tightening, prevents the base of the toilet from cracking.
Replacing the wax ring requires removal of the toilet. The Family Handyman recommends inspecting the evidence to determine if replacing the wax ring alone is enough to stop the leak. A broken or improperly positioned flange prevents the ring from creating a proper seal and require repair before reinstalling the toilet.
If the floor is starting to show signs of rot due to the leak, installing a flange support transfers the load of the toilet to a more solid portion of the floor. If the rot extends more than 2 inches beyond the flange, the required repairs are much more extensive.