Moss

ARMA draws a line between moss and algae. Moss is a non-vascular plant, taking in water through its leaves instead of roots, and it must have a moist environment to survive. Algae, in ARMA's bulletins, is a discoloration, usually brown to black.

The line matters on an asphalt shingle roof. ARMA writes that, unlike algae, moss can be detrimental to asphalt shingle performance: it can cause the leading edges of shingles to lift or curl, raising the risk of blow-off in wind, and in severe cases moss build-up can cause lateral water movement, moisture damage to the deck or leaks. Of algae, ARMA says there is no scientific evidence that it damages asphalt shingles, although it affects the aesthetics of a roof.

ARMA's moss guidance for asphalt shingle is specific: in North America moss tends to grow on north facing roof planes that receive less direct sunlight and stay damp longer, and overhanging tree branches add shade and drop debris that holds in moisture and acts as a food source for moss. Why a roof gets dirty under trees is its own answer, why does my roof get dirty under the trees.

Moss spreads through the air as well: moss spores may be carried by wind or animals and can easily spread throughout neighborhoods, ARMA writes, and the spores glossary entry covers the word on its own. On the effects of moss on asphalt shingles and its removal, ARMA's Algae Discoloration of Roofs bulletin tells owners to consult the shingle manufacturer.

ARMA's moss documentation is written for asphalt shingle. The Tile Roofing Industry Alliance names moss on tile in a different frame: mildew, algae and moss grow on the dust and organic material that settles on tile rather than on the tile itself, once enough cycles of heat, moisture and light let spores start to grow. Whether moss grows on a Florida roof, and what a green fuzz on a roof can be, are two separate answers: can moss grow on a Florida roof and what is the green fuzz on my roof.

Lichen is a different organism again, a fungus and an alga growing together. The glossary entry for lichen covers how it holds to a surface, and why lichen leaves marks after cleaning is its own answer.