Long-term, uniform granule loss is not considered functional damage by insurance companies. It’s considered part of the natural aging process. The rate at which shingles lose granules depends on the quality of the shingles and the climate zone of the home on which they’re installed.
Granule loss that is uniform across the roof is usually a result of normal weathering. Over time, the bond between the granules and asphalt deteriorates, and granules will be loosened and carried away by runoff.
Prolonged exposure to hail can also loosen granules. With older roofs, even marble-size hail may loosen granules.
Premature failure of the bond between the granules and asphalt can be caused by poor-quality asphalt. It can also be caused by other conditions, like foot traffic (not uniform), blistering, batch problems or roof slope orientation.
Granule loss limited to slopes facing a particular direction may be caused by increased exposure to the sun or to storms. South- and west-facing slopes are more likely to lose granules due to increased thermal cycling and loss of volatiles that help keep asphalt from becoming brittle; a condition in which it is more likely to lose granules.
Newly installed roofs can sometimes shed significant amounts of granules. These granules- called “hitchikers”- are interlocked with granules embedded in the asphalt surface, but are not themselves embedded. Runoff from precipitation will eventually wash them off the roof.