Does roof algae damage shingles?

Algae, in ARMA's account, is a growth that changes how an asphalt shingle roof looks. Moss, in the same account, is a growth that can change how the shingles perform.

Everything else on this page follows from that split, so it is worth being exact about the first half. ARMA's technical bulletin on algae discoloration names Gloeocapsa magma as “probably the most prevalent of several algae species that contribute to discoloration,” and says the algae's natural pigments may turn white or light-colored roofs dark brown or black, in a uniform tone or in streaks. That is a description of color. It is not a description of wear.

Moss gets its own paragraphs on ARMA's maintenance page, and the sentence that sets it apart begins “Unlike algae.” The difference is physical: a growth that can lift a shingle's edge rather than a pigment spread across its face. Whether that plant turns up in this climate at all is its own question, answered on the page about moss on a Florida roof.

The association that speaks for asphalt shingle makers puts the ordinary case in one sentence, and it is a no.

ARMA's page on algae and moss prevention says: “There is no scientific evidence that algae is damaging to asphalt shingles, although it certainly affects the aesthetics of a roof.” Note both halves. The first rules out a documented harm to the shingle, and the second keeps the appearance problem honest. So when someone asks whether algae damage shingles, the association's answer is that the evidence is not there, while the streaks are real.

It helps to know what a shingle's working parts are. CertainTeed explains that granules are crushed rock particles pressed into the asphalt that “block ultraviolet (UV) light from reaching the asphalt in the shingle,” and that the asphalt is the shingle's waterproofing agent and holds those granules in place. The growth ARMA describes is a discoloration on that surface, and ARMA's own bulletin says a solution of household bleach and water may lighten it, with an effect that is “only temporary.”

Moss and cool roofs aside, the harm the shingle documents name comes from cleaning done wrong. ARMA warns that scrubbing friction may loosen granules and says “Never use a pressure washer to clean an asphalt shingle roof,” tying it to granule loss and very likely premature failure. What granule loss is, and how a shingle roof gets cleaned without losing granules, each have a page, and what every roof maker says about its own material is on the roof cleaning page's section on cleaning tile, shingle and metal.

ARMA's no has a boundary, and it is drawn around one kind of roof: shingle made to reflect sunlight.

The same ARMA page says that on highly reflective or cool roofs, “algae will undoubtedly have a negative impact on the long-term effectiveness of these roof systems,” and it draws its practical conclusion right after: “That is why it is important to address an algae problem as soon as possible.” The harm ARMA names here is not to the shingle itself. It is to the job a reflective roof was bought to do.

CertainTeed describes the mechanism. On cool roof specialty shingles, it says, algae “can reduce the performance of solar reflectance,” because as it spreads across the face of the shingle it covers the reflecting granules. A dark film over a granule built to bounce sunlight away is a film in the way. Whether a given roof is one of these reflective products is written in the shingle maker's product documents, not in the color of the roof.

The strongest damage language in ARMA's maintenance page is not about algae at all.

ARMA says moss can be detrimental to asphalt shingle performance. It can lift or curl the leading edges of shingles, which raises the risk of blow-off in wind, and in severe cases a moss build-up can move water sideways, leading to moisture damage to the roof deck or even leaks. For the effects of moss and its removal, ARMA's bulletin sends owners to the shingle manufacturer. The state of a particular set of shingles, edges included, is for a roofing contractor to read.

Tile is a different material with its own maker statement, and what Eagle and TRI Alliance say on that is on the page about whether algae damages tile. More roof questions sit among the answers on roofs.