Spores

The Cambridge Dictionary defines a spore as "a reproductive cell produced by some plants and simple organisms such as ferns and mushrooms." The definition names what a spore does, which is reproduce, and leaves the organism open, which is why the same word can serve a fern, a mushroom and the growth on a roof.

EPA's guide to mold adds the scale. It describes molds as part of the natural environment that, outdoors, break down dead organic matter such as fallen leaves and dead trees. It says mold spores "are invisible to the naked eye and float through outdoor and indoor air," and that none of the many types of mold will grow without water or moisture, so a spore in the air is not yet growth. Mold as an organism is set out under mold, defined, and the same question on a painted metal surface is answered under whether mildew can grow on a metal roof.

GAF's bulletin on blue-green algae staining uses the word for arrival: the algae spreads from one infected roof to another via airborne spores, and the spores "land randomly on a roof and take root."

• ARMA's algae and moss bulletin says algae spores "are carried by the wind or by animals," and it names the same two carriers for moss spores, which "may also be carried by wind or animals."
• A lichen relies mainly on something else: UF/IFAS Extension says lichens reproduce primarily by releasing microscopic bundles of all their components into the air, bundles that begin a new lichen where they land, while the U.S. Forest Service says the fungal partner also produces spores.

None of these documents gives a distance, a speed or a count for spores. Whether roof algae spreads from house to house is answered on its own page, and so is the question of why a neighbor's roof stays clean.