Sometimes you'll glance up and notice a truly hideous brownish stain on your ceiling that has appeared seemingly out of nowhere. These stains are a fairly common occurrence in homes, especially homes in which a lot of deferred maintenance has piled up. There are exactly two causes -- both with their own remedies -- for these types of stains.
The single most common cause of discolorations on the ceiling is water damage, either from a leaking pipe somewhere near the stain or water from another source such as a leaky roof or the run-off from a bathtub. Staining occurs when water seeps through an assortment of building material and permeates the gypsum-based drywall ceiling panels. The ceiling will eventually dry out, but the stains will not disappear unless you take steps to make them disappear.
The second cause of ceiling discoloration may be related to moisture, but is not directly caused by it. When a portion of the ceiling stays damp for an extended period of time, a yellowish brown mold stain can form overhead. Left untreated, the mold stain may grow and become darker over time.
Water Damage Remedies
First, you must locate and fix the seepage or leak. If it's caused by a leaky pipe behind the ceiling, you'll have to tear out the ceiling panels and fix the leak, then re-drywall the ceiling. If water is seeping in from the roof or from an overactive child in the bathtub, then you can simply wait for the spot to dry out completely and paint over the stained area. Fix any roof leaks first to prevent further stains.
Mold Damage Remedies
You will have to kill the mold if you want the stain to go away. A solution of household laundry bleach mixed with water is effective on most common forms of mold. When you apply the mix to the affected area it will kill the mold, after which you can simply wipe away the residue as if the stain had never existed.