Chalking

Sherwin-Williams' Problem Solver page defines chalking as "the formation of fine, white powder on the surface of the paint film due to weathering, which may give the appearance of color fading." U.S. Steel's guide to painted metal buildings names the cause: "Chalk is a natural white by-product of ultra-violet degradation of the paint system."

U.S. Steel adds a timeline Sherwin-Williams does not: "Over a period of several years, a light white chalky film is produced on the surface of the paint film, which makes colors appear lighter and reduces the natural gloss of the original paint." The wider wear behind that film is UV degradation.

Concrete roof tile has a white deposit of its own, which the Tile Roofing Industry Alliance calls efflorescence and traces to "the chemical nature of cement." TRI writes that it can appear as "an overall chalky white bloom (softening of color) or in concentrated patches," and the word chalky there describes a look, not the paint-film weathering Sherwin-Williams and U.S. Steel define.

Which white is on a given concrete tile roof is a licensed roofing contractor's call; the documents behind each are laid out under what the white powder on roof tiles is.

TRI Alliance says slurry-coated concrete tile can oxidize and turn chalky before gradually wearing down to the concrete base. That change has its own entry, oxidation, and it is a change in the tile's own coating rather than a paint film's chalk.

None of the documents cited here says a cleaning restores a worn color coat, reverses chalking or brings back gloss a paint film has lost. Repainting or recoating is roofing or painting work, and a chalked roof's condition is a question for a licensed roofing contractor. How a chalky look sorts by roof material is answered under why a roof looks chalky, and whether age alone rules out a wash under whether a roof can be too old to clean.