Cracks in roll roofing are usually the result of shrinkage. This is most often caused by aging. Ultraviolet light from the sun boils the volatiles out of the asphalt and the asphalt becomes brittle and unable to withstand the shrinkage and expansion forces induced by temperature changes. Cracking is occasionally the result of stresses caused by failure to allow the material to relax prior to application, failure to cut the material into 12- to 18- foot lengths before installing it, or unusual expansion and contraction of the substrate. Cracks may occasionally be caused by manufacturing defects.
Cracks are generally easy to identify. The trick is to look at the entire roof surface.