Staining and Bubbling
The most obvious danger involving drywall water damage is staining. When drywall is exposed to water, it can be irreparably stained with minerals contained in the water. If the drywall is painted, water can become trapped between the paper of the drywall and the layer of paint, causing bubbling and pitting.
Structural Weakening
When drywall is exposed to water, it becomes mushy and easily crumbles. In a bathroom, tile installed over drywall will be too heavy to be held by the material and will fall away. Items hung on wet drywall, such as frames and shelves, may tear out of the wall and fall. If it becomes sufficiently waterlogged, the drywall itself may fall away from its beams or studs.
Mold Growth
The most health-hazardous outcome of wet drywall can be mold growth. Wet drywall makes an excellent breeding ground for mold and mildew and can harbor potentially hazardous strains. If you see any black mold on your water-damaged drywall, avoid contact with the material and call an expert immediately to assist you in removing the contaminated drywall.
Wire Exposure
If the piece of drywall that has been exposed to water contains electrical wires, you may have a larger problem on your hands. Once exposed to water, electrical wiring and junction boxes can be unpredictable and present a fire hazard. If you suspect the area of drywall that has been water damaged contains electrical wiring, shut off your main circuit breaker before probing behind the wall. If you confirm that there is, in fact, wiring behind the drywall, contact an electrician to assist you.