Biofilm

CDC's journal Emerging Infectious Diseases defines a biofilm as "an assemblage of surface-associated microbial cells that is enclosed in an extracellular polymeric substance matrix." The same article says the cells generate that matrix themselves, one of the traits it lists as setting them apart from cells of the same kind floating free.

The article is direct about how ordinary the behavior is: "Microorganisms attach to surfaces and develop biofilms," and biofilms "may form on a wide variety of surfaces, including living tissues, indwelling medical devices, industrial or potable water system piping, or natural aquatic systems." A roof is not on that list, and the article does not study one.

Where attachment happens is the line that matters most for this word. The article says that, in general, attachment "will occur most readily on surfaces that are rougher, more hydrophobic, and coated by surface" conditioning films. That is a finding about surfaces in general, and it names no roofing material and no rate.

Neither the Tile Roofing Industry Alliance nor the Asphalt Roofing Manufacturers Association uses the word biofilm in the documents cited here, so connecting the term to a roof is this page's own reading, not a sentence either body wrote.

On concrete and clay tile, TRI Alliance places mildew, algae and moss on the dust and organic material that settles on the tile rather than on the tile itself, a sentence the roof cleaning page quotes in full under the black streaks. Settled dust is not the conditioning film the CDC article describes, but it plays a similar part as the layer growth sits on, and that parallel is where the bridge starts.

On asphalt shingle, ARMA's algae and moss bulletin says its most common type, Gloeocapsa magma, "protects itself from damaging ultraviolet rays by producing a dark pigmented sheath," and that the discoloration "typically begins as small spots then quickly transforms into streaks on the roof plane." Reading that colony as a biofilm is a bridge from CDC's general definition, not a word ARMA uses.

Two roof questions that lean on this idea have their own answers: what causes the dark drip lines under roof tiles and why a white tile roof turns black. Growth that reaches the hardscape below a roof is taken up under roof algae on pavers. How soon a film returns after a wash, and whether an algaecide changes that, is not something the CDC article, TRI Alliance or ARMA measures in the documents cited here.