Why does a clean roof photograph differently?

Start with the picture rather than the roof. A photograph, from a phone or from a listing photographer, flattens a surface into lighter and darker regions, and what the eye reads as a stain is a region darker than what surrounds it.

Two things follow. A mark shows most where it differs most from its background, and a picture cannot say what made a region dark. Growth, soot, shadow and a wet patch can all land in a photograph as the same darker tone. No photography standard is quoted on this page; the point is the mechanism the roofing sources' own descriptions of discoloration lead to.

What an insurer or underwriter does with roof pictures is a separate matter, answered in whether a clean roof changes insurance photos. Glass is its own case as well, where the picture shows whatever dried on the pane, covered in whether windows should be cleaned before listing photos. Glass below a roof also carries a chemistry requirement. ARMA's bulletin names a solution of liquid household bleach, which is sodium hypochlorite, and water for lightening roof algae, and the National Glass Association's cleaning paper names bleach, with other solutions containing sodium hypochlorite, as corrosive materials to keep off laminated glass. Impact windows are built with laminated glass, so where a roof sits above them, keeping that solution off the glass is part of the cleaning itself.

On a roof, the darker tone often has a living source. GAF's Technical Advisory Bulletin TAB-R-102 says the dark stains on a roof are “often caused by tiny cyanobacteria called Gloeocapsa magma,” commonly known as blue-green algae, and ARMA's page on algae and moss says the most common type, “gloeocapsa magma or blue green algae, protects itself from damaging ultraviolet rays by producing a dark pigmented sheath.” ARMA's technical bulletin puts a hedge on the name, calling Gloeocapsa magma “probably the most prevalent of several algae species that contribute to discoloration.”

GAF's bulletin describes the color the same way: “A protective sheath creates the dark color the blue-green algae forms to protect itself from UV rays.” Where algae is the source, then, the tone a camera picks up is pigment the organism makes. ARMA's bulletin describes what that pigment does to a roof's color over time: “Natural pigments produced by these algae may cause white or light-colored roofs to gradually turn dark brown or black. Discoloration may appear uniform or as streaks.”

Those two patterns, uniform and streaked, are the two ways the stain reaches a photograph: an overall darkening that makes a pale roof read as a darker one, or lines that read as marks. What produces the streaks in the first place is covered in what causes black streaks on a roof.

Contrast needs a light background, and ARMA says as much about the stain: it is “most readily visible upon light-colored roofs, while it is not so visible upon darker shades of roofing.” The same growth on a charcoal shingle and on a white tile photographs very differently, because on the dark roof the pigment has little to stand out against.

GAF describes the shape the stain usually takes on shingles. Blue-green algae stains “usually cover more area on the lower section of the roof and taper as they reach the ridge,” and the algae generally grows “on the northern exposures of the roof since this exposure generally receives less sunlight.” In a wide photograph that pattern reads as a gradient: darker low on a slope, lighter toward the ridge, and heavier on one face of the house than another. The full account of the black streaks on a Naples roof, in GAF's and TRI Alliance's words, sits on the roof cleaning page's section on black streaks.

Take the growth off and its dark tone goes with it, so the contrast the camera was recording goes too. Where growth was the source of the dark tone, that is why a clean roof photographs differently. The method that does it, and why the trade describes it by what it does rather than by a pressure figure, is laid out in what a soft wash is and why no number defines it.

Not every change in a roof's tone is something growing on it, and a photograph cannot tell the difference.

The Tile Roofing Industry Alliance describes aging in the tile's own color layer: “Slurry-coated tile can oxidize and turn chalky before gradually wearing down to the concrete base. Integral-colored tile may show some surface lightening, but will retain its base color indefinitely.” Those are changes in the tile, not a film on it, and in a picture they read as a paler, flatter roof rather than a darker one. That look is taken up in why a roof looks chalky.

Efflorescence is a third look. TRI Alliance says it “can appear as an overall chalky white bloom (softening of color) or in concentrated patches,” and that in most cases “the action of carbon dioxide and rainwater will gradually remove the deposit, leaving the original color of the roof intact.” No source read says what a cleaning does to it; what that white film is gets its own answer in what the white powder on roof tiles is.