Does roof algae spread from house to house?

ARMA lists two carriers in one sentence, wind first and animals second, without ranking one above the other.

ARMA's bulletin on prevention states that “algae spores are carried by the wind or by animals and can quickly spread from one rooftop to another. That is why it is common to see algae growth on rooftops throughout an affected neighborhood.” The same document adds “the same is true for apartment or condominium complexes, townhouses or row houses.”

ARMA's separate technical bulletin on discoloration says every species it discusses, not only the most common one, travels through the air: “all species are transported through the air and tend to collect and grow on roofing structures and other outdoor surfaces.”

GAF's technical bulletin describes the same process from the roof's side, what happens once a spore arrives.

GAF writes that “blue-green algae spreads from one infected roof to another via airborne spores. These spores land randomly on a roof and take root.” GAF's bulletin is written for shingled roof surfaces, as are ARMA's algae and moss page and the CertainTeed shingle page below.

CertainTeed's page on algae-resistant shingles names the same carrier in one short line.

CertainTeed writes that “wind carries algae spores from roof to roof,” on the same page where it explains why the black streaks are black. An association and two shingle makers, in three documents, describe the same airborne route.

Algae is not the only rooftop organism that travels by air.

ARMA says “moss spores may also be carried by wind or animals and can easily spread throughout neighborhoods,” the same two carriers it names for algae. Spores are how algae and moss travel. Lichen mainly travels a different way: UF/IFAS says it releases microscopic bundles of all its components into the air, and where a bundle lands somewhere suitable it begins a new lichen. The U.S. Forest Service adds that the lichen's fungal partner also produces spores.

Airborne spread explains why growth can show up across a block. It does not explain why one roof on that block stays cleaner than the rest.

Why a neighbor's roof stays clean is its own question, and so is whether growth on a roof reaches the pavers below it. In a condominium, whether the roof is a common element the condominium association maintains, and who orders work on it, is the declaration's to say, and the roof cleaning page quotes the Florida rules on condominium maintenance.