What is the difference between algae and mold on a roof?

Algae make their own food. Mold lives on food that is already dead. That single difference is where the rest of the comparison starts.

The EPA describes algae as natural and important primary producers in aquatic ecosystems, and cyanobacteria, also known as blue-green algae, as microscopic organisms that use direct sunlight to produce their own food.

Mold works the other direction. The EPA's guide to mold and moisture says molds are part of the natural environment, and outdoors they break down dead organic matter such as fallen leaves and dead trees. Molds reproduce by spores, and none grows without water or moisture.

Biology does not put algae and mold in one group under two names; it puts them far apart.

UF/IFAS EDIS describes fungi as organisms that are either single-celled or form bodies of tubular strands called hyphae, that reproduce by spores, and that secrete digestive enzymes and re-absorb nutrients. That digestive step fits the EPA's description of mold outdoors breaking down fallen leaves and dead trees: it is how a fungus feeds.

The same EDIS publication puts cyanobacteria somewhere else entirely: they are true bacteria, neither plant nor fungus. An alga, in its glossary, is one of a large group of photosynthetic organisms that store food as starch. Neither description includes a digestive step like the one that feeds a fungus.

ARMA's two bulletins and GAF's current bulletin each put a name on the black streaks, and the name is algae.

ARMA's algae and moss bulletin says the black discoloration and streaks on many roofs are often mistaken as mold or mildew and are actually algae. Its bulletin on algae discoloration says the discoloration is usually brown to black and may be mistaken for fungus growth, soot, dirt, moss or tree droppings. In both, mold and fungus sit on the list of what the streaks get mistaken for, not what they are.

GAF's bulletin dated 9/19/2024 says the dark stains on a roof are often caused by tiny cyanobacteria called Gloeocapsa magma, commonly known as blue-green algae. The cause and the pigment are answered on why a roof has black streaks, and the makers' statements are set out where the black streaks on a Naples roof are explained.

Mold is not absent from roofing documents. It shows up in a different sentence than algae does, and for a different material.

The Tile Roofing Industry Alliance, writing about tile, says mildew, algae and moss do not grow directly on the tile but on the dust and other organic material that settles on it. That sentence names mildew, algae and moss; the word mold is not in it.

Eagle Roofing Products, in a 2019 article about its concrete tile, is where the word mold does appear: homeowners may notice a buildup of algae, mold and mildew, which Eagle calls typical of all natural materials and not harmful to the integrity of the tile, though some may find the appearance undesirable. That is a tile maker listing mold among things that can build up on its product, a different sentence from TRI's dust-film mechanism.

A third word shows up in these documents beside algae and mold: mildew.

UF/IFAS Extension puts it plainly: mildew is mold, growing from spores that exist almost everywhere and requiring moist, warm air. What the word covers on its own is set out on what mildew is.

ARMA's bulletin says the black discoloration on roofs is often mistaken as mold or mildew, which is a reminder that the words get swapped by people looking at the same stain.

This page describes the organisms and the documents. It does not say which is on a given roof, and a description of a stain's color does not settle it. Whether a lighter roof shows algae more readily is set out on whether white roofs need cleaning more often. A question about mold and health belongs with a physician or a public health office, and this page does not answer it.