To understand how to deal with the recurring problem of algae and moss growth on rooftops, it is important to learn what they are and how they grow and spread. Although some similarities exist, algae and moss are very different and require slightly different procedures to prevent or to remove.
Unlike most plants that gather water through its root system, moss is a non-vascular plant that obtains water through its leaves. Moss must have a moist environment to survive. In Bend, moss tends to grow on north facing roof planes that receive less direct sunlight and stay damp longer than south facing planes. Overhanging tree branches provide additional shade and drop debris on the roof that further hold in moisture and acts as a food source for moss. Like algae, moss spores may also be carried by wind or animals and can easily spread throughout neighborhoods. Unfortunately, unlike algae, moss can be detrimental to asphalt shingle performance. Moss can cause the leading edges of the shingles to lift or curl, which increases the risk of shingle blow-off during wind storms. In the worst cases, moss build-up can cause water to move down into the roof resulting in moisture damage to the roof deck or may even cause leaks in the home.
Often mistaken as mold or mildew, the black discoloration and streaks visible on many roofs in Bend, Oregon are actually algae. The algae can form w here moisture tends to dwell on roof surfaces. The same weather that makes Deschutes County into a premier farming community for grapes and hops, also encourages the growth of algae and moss. The most common type, gloeocapsa magma or blue green algae, protects itself from damaging ultraviolet rays by producing a dark pigmented sheath. The algae discoloration typically begins as small spots then quickly transforms into streaks on the roof plane. By the time the algae can be seen, it likely has been present for several months or longer.
Because algae spores are carried by the wind or by animals, it can quickly spread from property to another. That is why it is common to see algae growth on rooftops throughout an affected neighborhood. The same is true for apartment or condominium complexes, townhouses or row houses. There is no scientific evidence that algae is damaging to asphalt shingles, although it certainly affects the aesthetics of a roof. However, it can affect the look of a roof and make the home look less attractive. That is why it is important to address an algae problem as soon as possible.