Metal would seem like a good material to cover a roof with, it is strong, dependable, and solid and indeed it is very good, if properly maintained.
When metal roof sheets are manufactured, the corrugated sheets are coated in plastic such as Plastisol. This prevents the metal from being corroded over time and also improves its appearance. After the cladding is cut to size in the factory, the cut edge is left exposed. A steel edge exposed to oxygen will corrode, and then this is exacerbated by water and pollutants. Consequently, the coating will peel off from the edges.
A corroding edge will rapidly peel the coating away and spread rust across the sheet if left untreated, leading to leaks cracks and general deterioration. With the correct attention this can be caught, treated and even prevented altogether.The earlier cut edge corrosion is treated, the better.
If your metal roof is upwards of 5 years old there is a good chance that there will be signs of cut edge corrosion setting in, if it is over 10 years old it is almost certain that the roof will need treating to prevent further damage.
Repairing cut edge corrosion will protect cladding from the elements and pollution as well as reducing the peel back of the coating. It will provide a tight seal on the horizontal laps, lap joints and the edges of cladding. If done by a professional company, repairs will improve how a building looks as well as protecting it structurally.