What is the white powder on my roof tiles?

Lime, water and carbon dioxide are the whole recipe for the white powder the tile industry describes. The Tile Roofing Industry Alliance (TRI Alliance) puts it this way: “Manufactured cement contains free lime, and when water is added, a series of chemical reactions takes place. These reactions are accompanied by the release of calcium hydroxide that can form a white, chalky, crystalline salt deposit on the tile surface when reacting with carbon dioxide.”

The name for that deposit is efflorescence. TRI writes that it “is caused by the chemical nature of cement,” which is why its account belongs to concrete tile, and that the reaction “can appear as an overall chalky white bloom (softening of color) or in concentrated patches.” A roof can show it as a general paling of color or as blotches.

Pavers are concrete too, and the same family of reaction turns up on a driveway. The paver cleaning page sets out efflorescence on concrete pavers with ICPI's account, and it names the compound the deposit becomes there, calcium carbonate. This page stays on the roof.

TRI Alliance does not treat efflorescence as a defect. “Efflorescence is a temporary surface condition that is common in many concrete building products,” its maintenance guide says. “It is superficial and in no way affects the quality or functional properties of the tile.”

TRI also says what usually happens next: “In most cases, the action of carbon dioxide and rainwater will gradually remove the deposit, leaving the original color of the roof intact without further efflorescence.” Eagle Roofing Products, a concrete tile maker, puts it without TRI's hedge in its Florida owner information, as archived: temporary light patterning may appear on darker tiles, it is caused by efflorescence, and “it will disappear completely after additional exposure to rain.”

Neither document singles out a tile profile. Their sentences are about concrete, so they read the same for a barrel roof and a flat tile roof. Eagle's sentence also names darker tiles as the place the light patterning may appear, which is where a pale deposit stands out most.

Not every white on a roof is a salt deposit. TRI Alliance writes that “Slurry-coated tile can oxidize and turn chalky before gradually wearing down to the concrete base.” That is a change in the tile's color coat as it weathers, and TRI does not describe rain taking it away.

Painted metal has its own white. U.S. Steel calls chalk a natural white by-product of ultraviolet degradation of a paint system, a film that builds over several years, and it adds that “rainwater only partially washes it off.” That is chalking, a paint film breaking down, and the UV degradation behind it is a different process from cement chemistry.

So the documents describe three different whites: a salt deposit that rain in most cases removes, a slurry coat that oxidizes and wears, and a paint film that chalks. How the three compare across roof materials is set out under why a roof looks chalky.

Eagle's Florida warranty text, in the archived copy headed NOT AN ACTIVE WARRANTY, lists its specific exclusions, and they include “color, surface cracking, chipping, topical slurry coating delamination, efflorescence, carbonation, oxidation.” Efflorescence and oxidation are both named as conditions of concrete tile that document does not warrant.