I am sure most of you have heard of the term "Alligatoring" as applied to roofing----particularly related to flat roofs. This is a condition of the roof that indicates that the roof is near the end of its life (maybe way past it), and gets its name from the way an alligator's back looks (if you use your imagination). As the roof expands and contracts over time, and as the volatiles of the tar boil off in the heat from the sun, cracks start to develop in the surface. This condition is due to the lack of UV protection, and is why roofs of this type should be covered with aggregate or reflective coatings. Some of the paint-on reflective type coatings, if routinely maintained, can double the life of a roof of this type. Obviously if these cracks get big enough leaks will develop. While minor Alligatoring may be able to be successfully coated-over, a roof as bad as this next picture would likely require replacement.
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WA State, Home Inspector Advisory Licensing Board