As the asphalt hardens over time, the granules which were once securely embedded begin to break away. Ocassionally you may have seen the colored granules in gutters. Also, as this hardening advances, the asphalt layers begin to shrink. Of course, all of this is occuring at a microscopic level and is not something which will be noticeable on a shingle reinforcement, which resists shrinking.
Another manifestation of the normal aging process may be the development of surface cracks. For example, as the flexibilizing oils of the asphalt are depleted due to heat, the shingle becomes more brittle, to the point where surface cracking may appear. The stresses created by thermal shock and the movement of the roof deck also increase the likelihood of surface cracking.
During the course of natural weathering, small bubble-like raised areas know as blisters may appear on the surface of the shingles. The blisters may be small and pea sized or as large as a quarter. The blisters may be open, exposing the asphalt, or closed. Blisters frequently result when minimum ventilation requirements are not met.
Over a period of time, shingles may develop dark brown or black streaks that are sometimes mistaken for soot, dirt, moss or tree droppings. In actuality, this discoloration may be caused by algae growth. Although most roofing systems are susceptible to algae discoloration, it is most readily visible on white or light-colored shingles.
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