Ice dams. Ice dams and snow end up weakening the structure of your roof as well as its integrity. If you find that this is the case, the most appropriate thing for you to do would be to call in a roofing professional as soon as possible so that the matter can be fixed before it gets worse.
Hailstorm. When you suspect that your roof may have been exposed to a hailstorm, the best thing to do would be to look at the pits that are usually on the roof surface.
Warranty. If you find that your roof warranty has expired, this should be a sign that you are about to start experiencing a whole load of other problems.