Water is the major enemy of your fireplace system. It can seep in many places, cracking and corroding joints, mortar and brick. Excess condensation caused by improper operating flue systems adds water to your chimney. Cracks can let water in, un-capped chimneys invite water damage and defective flashing can allow roof leaks.
At the first sign of water, have an inspection! You will end up saving money and put an end to any potentially unsafe conditions. Many times if the problem is caught early enough, the cost to repair is much less than a future cost to replace.