Some time ago contractors began using metal chimneys for heating equipment, water heaters and even fireplaces. Metal chimneys are far cheaper than brick or masonry chimneys. They are lighter, easier to install and do not slow down large construction projects. On the whole they are usually trouble free for many years.
Notice I said usually not always. One of the most vulnerable parts of a metal chimney is the metal top tray between the metal chimney flue and the wood chimney box if you will. These metal pan are prone to rusting, leaking and having the trims that attach them to the chimney blow off. Corrective actions are necessary in order to cure the existing hazard and to avoid further damages.