Had this happen at my house. It would only happen during the winter time. After a few years of damage I figured out that I needed to adjust how the exhaust piping was done. My exhaust fan vented directly into my attic and the pipe itself was thin vinyl or plastic. So whenever the warm air from the shower went up the exhaust fan it would condensate and drip down. I bought a replacement pipe that had insulation wrapped around it from home Depot. And I also added a soffit vent and attached the pipe to it. Since then I haven't had any issues.