Ceiling leaks occurring beneath a bathroom are often a sign that there is a break in the water supply drain or one of the bathroom fixture drains. Finding the leak is usually the most challenging part of repairing it.
According to Reuben Saltzman, a Minnesota home inspector, the level of moisture in a ceiling stain is often the first clue as to what is causing it. If the spot is alternately damp and dry, it is likely caused by a leak in a drain line. If it is constantly wet or even dripping, the leak is most likely due to a breach in the pressurized water supply. In some cases, the leak is not coming from the upstairs bathroom, but rather from a leak in the seal around the vent stack that protrudes through the roof.
According to the owners of The Twin Plumbers, a Van Nuys, Calif.-based plumbing business, it is important to run several tests to determine the cause of a ceiling leak below a bathroom. For example, if the leak is sporadic, it is possible it is a worn washer behind the bathtub overflow plate. Once this washer is excessively worn, it is no longer capable of sealing the water between the plate and the tub, which causes leaking. They suggest looking into the access panel behind the tub while running water to see if the washer is working.