Have you noticed moisture stains next to your bath fan?
A "leaky" bath fan can occur when warm, humid air is not being fully exhausted outside the house. Moisture created from running a hot shower will build up in the duct and eventually leak through into the ceiling. This can be fixed by installing a fan with a higher CFM as well as by installing a bath fan timer switch. The bath fan should run for at least an hour when you plan on taking a shower. This will ensure the warm moist air is exhausted outside the house and will not cause leaks or mildew buildup!