Why does roof algae grow on the north side?

GAF's technical advisory bulletin says: “Blue-green algae generally grows on the northern exposures of the roof since this exposure generally receives less sunlight.” CertainTeed's page on algae-resistant shingles says: “Algae discoloration most often occurs on east-facing surfaces but can show up anywhere on your roof.” Two roofing makers name two different sides, and each hedges with its own words, generally and most often.

The Asphalt Roofing Manufacturers Association (ARMA) also writes about north-facing planes, but its sentence is about a different organism: “In North America, moss tends to grow on north facing roof planes that receive less direct sunlight and stay damp longer than south facing planes.” That describes moss, a plant, and not the algae behind a black streak.

The roof cleaning page quotes GAF's account of how the black streaks form. Set CertainTeed's sentence beside GAF's and the two makers, read together, do not support a rule that algae starts on any one side.

Under the disagreement about direction, the documents describe the same conditions. GAF writes that blue-green algae “requires moisture and proper temperatures to grow” and names dew as the primary moisture source. CertainTeed writes that “hot, humid conditions promote algae growth” and that “shaded areas serve as excellent breeding grounds.”

TRI Alliance, writing about tile, lists variables rather than a side: “location of the structure, the direction the roof is oriented to the sun and the quantity of trees, canals and lakes in the area.” U.S. Steel, writing about painted metal buildings, says biological growth “commonly appears on surfaces which remain moist for extended periods such as north walls, under eaves or sheltered areas,” and that there is “a sufficient supply of organisms in dust to initiate growth anywhere.” CertainTeed adds that “wind carries algae spores from roof to roof.”

Read together, those sentences point at whichever slope stays shaded and damp the longest, and which slope that is depends on the house: the way it faces, what shades it, and how long dew sits on each plane after sunrise. The dew point has its own glossary entry. Branches are one source of that shade, taken up under why a roof gets dirty under the trees, and why a warm, humid coast supplies the rest is answered under why algae grows faster in Naples.

A slope that streaks first shows where conditions favored visible growth on that roof. It does not show that the other slopes are clear. CertainTeed writes that “an algae infection typically takes hold well before the black streaking is visible,” and ARMA's algae and moss bulletin says that by the time the algae is visible, it has likely been present for several months or longer.

A darker slope is not always growth, either, and why one side of a roof is darker sorts through the other explanations. Metal strips along a ridge are a separate question again, covered under zinc strips and roof algae.

Which slope of a given roof carries what, and whether anything beyond appearance is going on up there, belongs to the roof's own inspection by a licensed roofing contractor. The makers' sentences describe a pattern across many roofs. None of them reads a particular house.