A leaking toilet can cause some serious water damage to a property, beyond just cascading through the ceiling of the room below the bathroom. If the water damage goes unnoticed or is unaddressed for too long, you may find that you need to replace whole sections of the subfloor, including some floor joists.
There often are warning signs of a leaking toilet that will help you address a leak early, before the wood under the toilet starts to rot. Look for any water gathering around the toilet's base, since this indicates that the seal between the bottom of the toilet and the drain pipe has been compromised. Water stains or drips on the ceiling of the room below the toilet also indicate a leak. The bathroom's finished flooring may have water stains or it may bow and flex around the toilet but not elsewhere.
A toilet will leak due to one of a few potential causes. Improper seating of the toilet will cause the seal between the opening in the bottom of the toilet and the drain pipe to be breached, leaking water every time you flush. If the toilet is seated properly, the seal should hold up for years without incident. When placing the toilet on any surface, you must take care to not crack the porcelain. A crack in the opening in the bottom of the toilet, called the horn, or in the curving drain line, called the trap, will also lead to leaks.
Even with leaks that have resulted in widespread rotting, you can repair the damage once you have determine the cause. With minimal rot around a toilet, you can slide a flange support bracket under the flange, which will give a stable surface for the toilet and keep the flange secured tightly down, preventing future leaks. For leaks that have softened the floor more than 2 or 3 inches from the toilet, you must remove the rotting wood and replace it, relay the floor and then reseat the toilet.
Seating a toilet correctly will prevent future water leaks. When you anchor the flange to the bathroom floor, the screw heads need to be flush with the top of the flange and the flange cannot shift on the floor in any direction. The wax ring needs to rest with the flat side touching the flange, while the rounded side touches the bottom of the toilet around the horn. Once you place the toilet over the wax seal and flange and thread the closet bolts through the openings in the toilet's base, you must compress the wax ring by pushing on the toilet until the bottom of the toilet touches the bathroom's finished floor. The nuts need to be tightened on the closet bolts until you feel resistance, preventing overtightening while not allowing the toilet to rock.