UV degradation

CertainTeed's anatomy of a shingle names the threat directly: "UV rays, if allowed to reach unprotected asphalt, will age the asphalt prematurely." The word unprotected carries the sentence, because CertainTeed says the granules pressed into the shingle's surface are what block ultraviolet light from the asphalt.

That is why granule loss sits next to this term: a shingle that has shed granules has less cover between the sun and the asphalt. CertainTeed gives no timeline for how fast that aging runs.

U.S. Steel's guide to cleaning painted metal buildings calls chalk a natural white by-product of ultraviolet degradation of the paint system, a light white film that builds over several years. The word itself is defined under chalking.

The same guide ties the effect to exposure: "Chalking increases with greater exposure to the sun." It says the roof, the south side of a building and buildings in southern climates may chalk more, a statement about painted metal buildings that a painted metal roof in Naples falls within. Why one slope can look different from the other is answered under why one side of a roof is darker.

Sherwin-Williams' Problem Solver pages keep chalking and fading apart. Its fading entry reads: "Premature and/or excessive lightening of the paint color, which can occur on surfaces with sunny exposures."

Sherwin-Williams adds that "colors will fade slightly when exposed to intense sunlight," and that "excessive chalking of the paint film will cause colors to appear lighter." Neither page publishes a rate. How a chalky look sorts across roof materials, tile included, is answered under why a roof looks chalky.

The dark streaks ARMA describes on asphalt roofing come from a different mechanism: ARMA's algae and moss bulletin says its most common algae type shields itself from ultraviolet rays with a dark pigmented sheath. That pigment is a living organism's response to UV light, not UV wear on the roofing material.

On metal, the Metal Roofing Alliance lists UV damage among what paint on a metal roof can help prohibit. Age is its own question, with its own answer on whether a roof can be too old to clean.