Why are some tiles darker than others after cleaning?

The simplest explanation has nothing to do with the tile or the cleaning solution. A wet tile reads darker than a dry one, and a roof does not dry all at once.

Sun, shade and the way water runs down a slope all change how fast one part of a roof dries compared with another. Look at a roof in that in-between state and it can show some tiles darker than others, in patches that follow the drying rather than the growth.

The documents cited here give no drying time for tile and no measurement of the effect. It is a mechanism, and time is the only way to separate it from the other explanations on this page: a patch that is just water evens out as the roof dries.

Once a roof is dry, some variation can remain that a cleaning did not create and cannot remove, because it was made into the tile.

Eagle Roofing Products says so in the owner information attached to its Florida limited warranty, as archived: on single-color, mono-tone roofs there will be a variation in hue from one tile to another, because concrete tiles are made from natural raw materials and those materials vary in color. Eagle adds a second line that matters to anyone comparing a roof with a memory of it: “As the roof ages, the variation tends to be less noticeable.”

That variation comes from the raw materials, not from the cleaning, and the same document states that “Eagle does not warrant color due to the tile exposure to the elements.” How each tile and shingle maker frames cleaning its own product is set out on the roof cleaning page's maker-by-maker roof types.

The Tile Roofing Industry Alliance explains that concrete roof tile gets its color in one of two ways, and the two age differently.

Integral-colored tile has pigment added to the body of the tile during mixing. A slurry-coated tile has a concentrated coat of pigment applied to its surface after it is formed. TRI Alliance describes what the years do to each: slurry-coated tile can oxidize and turn chalky before gradually wearing down to the concrete base, while integral-colored tile may show some surface lightening but keeps its base color indefinitely.

That gives a roof two ways to look uneven once the growth is gone. Tiles whose slurry coat has worn in places can read lighter or duller than tiles where it has held. TRI's word for that slurry coat is chalky, and the term as the paint makers define it is set out under chalking. Nothing read for this page shows a cleaning reversing the oxidation or the wear, and which kind of tile is on a given roof is a question for the roofing contractor who installed it.

Clay tile sits apart from concrete in TRI Alliance's guide, and its two sentences on clay color belong together.

TRI Alliance writes, “In general, clay tile doesn't fade in the sun.” In the same passage it adds, “Color softening will occur, but the results are subtle.” Read as a pair, those lines say a clay roof keeps its color in general while that color still softens a little, and a cleaning shows whatever softening has already happened.

A reader hoping a cleaning would make every tile match is asking a different question, whether a cleaning brightens the tile at all, and that is answered on whether roof cleaning brightens tile color.

The last documented cause runs the other way: lighter marks on dark tile that make the untouched tiles beside them look darker by contrast.

Eagle's owner information says “Temporary light patterning may appear on darker tiles,” names the cause as efflorescence, and says that if it occurs “it will disappear completely after additional exposure to rain.” TRI Alliance calls efflorescence a temporary surface condition common in many concrete building products, one that can appear as an overall chalky white bloom or in concentrated patches, and says that in most cases carbon dioxide and rainwater gradually remove the deposit and leave the original color intact.

Efflorescence is a cement deposit, not growth. A white bloom on a particular roof is not diagnosed from those two documents, because TRI also describes slurry coats turning chalky. The white powder on roof tiles has an answer of its own.

Some unevenness is not about individual tiles at all. It follows the slope, the rain or the neighborhood, and each of those has its own answer.

GAF describes blue-green algae on shingles as dark stains that usually cover more area on the lower section of the roof and taper toward the ridge, and says the spores land randomly on a roof and take root, so growth was uneven before any cleaning began. A whole slope that reads darker than the other is answered on why one side of a roof is darker, and a roof that looks striped once the rain comes is answered on why a roof looks streaky after rain.