Why does my roof have black streaks?

Start with the color, because it explains the rest. The Asphalt Roofing Manufacturers Association (ARMA) says in its algae and moss bulletin that the growth most often behind roof stains “protects itself from damaging ultraviolet rays by producing a dark pigmented sheath.” In an algae streak, the black is not soot settling on the roof. It is a pigment the organism produces as its own shade.

Two shingle makers explain the color the same way. GAF's Technical Advisory Bulletin TAB-R-102, dated 9/19/2024, says “A protective sheath creates the dark color the blue-green algae forms to protect itself from UV rays.” CertainTeed's page on algae-resistant shingles says roof streaking is caused by blue-green algae, and that the streaks are black because of “the dark-pigmented sheath” the algae uses against the sun. The sheath shields the organism. It is not a coating for the roof under it. ARMA's June 2024 bulletin adds that the pigments may gradually darken a light-colored roof, and why pale tile in particular goes dark is its own answer, in why white tile goes black.

Naming the organism takes a hedge, and ARMA supplies it. Its technical bulletin, revised June 2024, says Gloeocapsa magma “is probably the most prevalent of several algae species that contribute to discoloration.” GAF's bulletin calls the same organism “tiny cyanobacteria called Gloeocapsa magma”, commonly known as blue-green algae, so the roofing trade's word and the biologist's word are not the same word. The entry for Gloeocapsa magma sorts out which is which. Probably is ARMA's word, and it belongs in any honest sentence about what is on a given roof.

In general biology, a living stain like this is a 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”, and says “Microorganisms attach to surfaces and develop biofilms.” Calling a roof streak a biofilm is a bridge from that general definition to the roofing bulletins, not a sentence either roofing body wrote.

The shape comes from where the growth starts. ARMA's algae and moss bulletin says the algae can form where “moisture tends to dwell on roof surfaces”, and that the discoloration “typically begins as small spots then quickly transforms into streaks on the roof plane.” A streak is a spot that kept going.

GAF adds the moisture source and the outline. Its 2024 bulletin, written for shingle roofs, says “Dew is the primary moisture source” for blue-green algae, and that the stains “usually cover more area on the lower section of the roof and taper as they reach the ridge.” Water on a pitched roof has one direction to travel, downhill, and a growth that needs moisture follows the paths that stay wet longest. That is the outline GAF gives for black streaks on a shingle roof: heavier toward the eaves, thinner toward the peak.

A streak that seems to arrive all at once has likely been growing for a while. ARMA: “By the time the algae is visible, it likely has been present for several months or longer.” Rain changes how the lines read from the driveway as well, and why a wet roof can look more striped than a dry one is taken up in why a roof looks streaky after rain.

Which slope streaks first is less settled than the shape of the streak. GAF and CertainTeed each name a different exposure, and the reasons the makers do not agree are set out in whether roof algae favors the north side.

Not every dark mark on a roof is growth, and ARMA says so. Its June 2024 bulletin writes that algae discoloration “usually has a brown to black appearance and may be mistaken for fungus growth, soot, dirt, moss, or tree droppings.”

The pattern is the clue ARMA offers, in the same bulletin: “The algae discolorations should not be confused with moss or tree droppings, which typically produce only localized discoloration.” ARMA's other bulletin adds that the black streaks seen on many roofs are often mistaken for mold or mildew and are, in its words, actually algae.

For moss on asphalt shingles, the June 2024 bulletin sends the owner to the maker: “Consult the shingle manufacturer for information concerning the effects of moss growing on asphalt shingles and recommendations for its removal.”

Knowing why the streaks are black answers one question and leaves others where they were. None of the bulletins that name the organism says a streak tells you the condition of the shingle or tile beneath it.

They also say the stain can come back. ARMA writes, of its own cleaning procedure, that “The effectiveness of such cleaning is only temporary, and the discoloration may recur.” The organism travels on its own: GAF says blue-green algae spreads from one infected roof to another via airborne spores, and CertainTeed says “wind carries algae spores from roof to roof” and that hot, humid conditions promote algae growth.

How a low-pressure cleaning works on this kind of growth, and why a pressure washer is not part of it, is the subject of how soft washing works. For an asphalt shingle roof, the cleaning instructions that apply are the ones the shingle's own maker publishes.

Anything past the surface belongs to a different trade. A lifted shingle, a cracked tile or a stain that turns out not to be growth at all is a question for a roofing contractor, and a cleaner who has only looked at the streaks is in no position to answer it.