Granule loss

CertainTeed's own anatomy of a shingle starts with the granule: "Granules are fine grain, opaque, crushed rock particles that are pressed into the surface of the asphalt. They block ultraviolet (UV) light from reaching the asphalt in the shingle."

CertainTeed also describes ceramic coated mineral granules as a topcoat used "to protect them from the sun's ultraviolet rays, and to add an attractive finish," so the same layer carries the shingle's color. Granule loss, in those terms, is that layer coming away a piece at a time, and the asphalt under each missing granule loses its cover from the sun. What the sun does to asphalt that loses that cover is the subject of the neighboring entry, UV degradation.

Two roofing documents name what takes granules off a shingle. ARMA's technical bulletin says scrubbing friction "may loosen and remove granules," and GAF's bulletin on algae staining says power washing "may dislodge granules, which can lead to premature shingle failure."

ARMA's same bulletin says high-pressure washing systems are likely to damage asphalt roofing. Both documents are quoted in full, beside the metal roofing guide, on how soft washing works, where three roofing documents give one instruction on pressure.

None of the documents cited here says a cleaning restores a granule that has already come off. Why the trade uses a soft wash instead of pressure on a roof is set out in the soft wash section of the roof cleaning page.

ARMA's moss sentence sits beside this term without being the same thing: unlike algae, ARMA says, moss can be detrimental to asphalt shingle performance and can cause the leading edges of shingles to lift or curl. Whether a shingle roof has lost granules, or whether a given mark is one of the lichen marks left after cleaning instead, is a licensed roofing contractor's call, not a cleaner's.