Oxidation

The Cambridge Dictionary defines oxidation as "the process of a substance or chemical element oxidizing," and defines the verb itself: "If a substance oxidizes, it combines with oxygen and loses hydrogen to form another substance." Its own example is the familiar one: "Iron oxidizes to form rust."

In the dictionary's terms, a substance meets oxygen, loses hydrogen and becomes a different substance. What the new substance looks like depends on the material, and roofing documents use the word for more than one material.

The Tile Roofing Industry Alliance uses the word for a concrete tile's color coat: "Slurry-coated tile can oxidize and turn chalky before gradually wearing down to the concrete base." That is a change in the tile's own coating, not growth sitting on top of it. The paint-film condition with a similar look is a separate entry, chalking.

Eagle Roofing Products' Florida warranty text, in an archived copy headed NOT AN ACTIVE WARRANTY, lists the word among its specific exclusions: "color, surface cracking, chipping, topical slurry coating delamination, efflorescence, carbonation, oxidation." What a given tile warranty covers is answered by that tile maker's own warranty text.

On painted metal, the Metal Roofing Alliance says paint "adds a protective layer to the metal roof which can help prohibit rust, corrosion, UV damage, and weather damage". Rust is the dictionary's own example of oxidation, iron combining with oxygen, which is where the chemical word and the metal roof meet.

None of that is the black on a streaked roof. ARMA and GAF describe the dark color of algae streaks on shingle roofs as a pigmented sheath the algae produces, set out under what causes black streaks on a roof, and dark marks along a tile course have their own answer under what causes the dark drip lines under roof tiles. A white deposit on concrete tile can have more than one sourced cause, laid out under the white powder on roof tiles, and a chalky look across materials under why a roof looks chalky. Whether a given tile or panel has oxidized, and what if anything it calls for, is a licensed roofing contractor's call.