Damage to the flashing can be caused by improper installation or by drying and cracking. Flashing can generally be repaired without replacing the roof. If you decide to re-roof, you might consider specially designed shingles, like Owens Corning's new WeatherGuard High Wind 110 Roofing System, which stands up to strong wind conditions. Curling causes the corners of the shingle to turn up, and is usually caused by a lack of ventilation, nails positioned too high up, an incorrect number of fasteners or under-saturation of the shingle. They are usually caused by moisture in the shingle.
Shingles rot when the mat at the core of the shingle absorbs moisture.
In winter, snow accumulates on roofs and as it melts down to the overhang and refreezes, then an ice dam is formed. An ice dam can be detected by long icicles on overhangs. Ice dams cause water backups and this allows water to seep through the roof causing even more damage. The best way to prevent ice dams is making sure that the attic is properly insulated with at least 12 inches of insulation. In addition to preventing ice dams, insulation can also help reduce energy bills and keep homes cooler during spring and summer.