Like all plumbing fixtures that require some type of seal to prevent leaking, your toilet depends on a thick wax gasket between its base and the connecting flange in the floor. A wax ring is a simple but effective concept; the pressure of the toilet base expands the sticky, pliable wax until it forms a watertight seal. Because the wax is soft and malleable, however, the seal can occasionally fail, necessitating prompt replacement of the wax ring.
Signs of Wax Ring Failure
Often, the first clue that a wax ring is failing is the presence of water on the floor around the base of the toilet. If the toilet installer caulked around the toilet, though, water might not be visible on the floor -- but it could still be leaking and damaging the subfloor or the ceiling of any rooms beneath. Smelling an unpleasant odor is another sign the wax ring seal is broken, which allows sewer gasses to seep into the bathroom.
Water on the floor doesn't always mean the wax ring is bad. It could be seeping out between the tank and toilet base, the mounting bolts could be loose, or the toilet could have a cracked base. If the toilet rocks back and forth, the toilet flange could be damaged. Whether or not the wax ring is at fault, once you remove the toilet you'll still need to install a new wax ring.
Wax Ring Replacement
If you've never replaced a wax ring -- don't be intimidated; it's a straightforward do-it-yourself task. After removing the old wax ring, check the flange for damage, and repair it with a flange repair kit, if necessary.
Once you're ready to install the new wax ring, you can do it in one of two ways, by either positioning it on the toilet flange, or by fitting it on the boot base of the toilet and then setting the toilet carefully back in place. To get a good seal with the new wax ring, gently rock the toilet back and forth while pressing down until it's snug and flat on the floor.
Choosing a New Ring
Wax rings are one-size-fits-all, and are available with or without an attached rubber or polyethylene "boot" that extends into the toilet flange opening. The booted rings are nice, but if the toilet flange sits a little high, the boot can keep the toilet from resting flat on the floor. Plain wax rings don't have the added boot protection, but they fit all toilets.