Q: We have a toilet with an intermittent floor leak that a double wax ring and a new toilet could not correct. The toilet sits on a slightly uneven slab floor. There hasn’t been a leak the 50 years we have lived here. Any ideas?
A: The most likely cause is that the wax ring around the closet flange is leaking, allowing a small amount of water to seep from under the toilet with each flush. The wax rings are probably misaligned.
The fix is to reset the toilet using a flange extension and one wax ring. The slight unevenness of the slab might be a contributing factor, but you can fix that by shimming the bottom of the toilet so that it sits level.
Toilet hydraulics are pretty simple. Waste and wastewater are discharged from the toilet bowl through an S curve in the bowl structure into a waste pipe. The toilet bowl is bolted to the floor with closet bolts attached to a flange connected to the waste pipe.
A wax ring, when correctly installed, produces a watertight seal between discharge hole and the waste pipe. If the closet flange sits below the level of the finished floor or the wax ring is installed catawampus, the seal can be incomplete, and the toilet will leak.
Removing and resetting the toilet properly will stop the leak.