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• Algae, moss, lichens, or fungal growth on roof shingles: causes, effects on roof life, cure & prevention
• Types & photographs of organic felt asphalt roof shingle defects & failures

Here we discuss the causes of algae, moss, lichens, or fungal growth on roof shingles, the effects of these conditions, and how to cure or prevent them.

Also see STAINS on ROOF SHINGLES

Algae, Fungus, Liches, Moss Effects on Asphalt Shingle Roofs

Causes of moss or lichens growth on roofs

Moss growing on any roof surface will be more severe on roof sections that area shaded and exposed to periodically damp cool weather conditions. Shade over the roof is often the determining factor but shingle chemistry is also involved - some roof shingle materials contain algecides or fungicides which will also retard moss growth on the roof surface. You'll also notice that moss, lichens, and usually algae and fungus will be conspicuously absent from a roof surface down-roof from areas where galvanized metal, copper, or even aluminum flashing are installed. Mineral salts washing off of these metals will retard moss or algae growth on the shingles.

What are the effects of moss growth on roofs?

Because moss or lichens growing on a roof surface will hold moisture on the roof longer than other areas, these growths can reduce the life of the roof covering. Particularly where the roofing materials are asphalt shingles or wood shingles, holding water on the roof surface by any means (leaves, debris, moss, or lichens) speeds up wear on these shingles. In freezing climates there may be faster frost damage, cracking, and wear of the shingles under the moss or lichens. Even in non-freezing climates, the roots or growth structures of moss or lichens eventually penetrate and separate the roof shingle materials, speeding their demise.

Is lichens on a roof as much of a problem as moss?

Because lichens growing on a roof surface does not have as much thickness of body as moss, it will hold less water on the roof surface and is less of a wear factor than moss. Therefore if we have only lichens growing on a roof surface we would be less quick to try to clean it off since lichens not only has a tighter "grip" on the roof surface but the cleaning process for lichens risks doing more harm than good to the roof surface.

Is green algae on roof shingles as much of a problem as moss?

Algae on a roof surface appears as a thin green coating which is mostly a cosmetic concern. The presence of algae on the roof is an indicator that this roof area is in a shaded spot where you may want to be alert for development of moss or other roof problems, but the level of damage from algae is probably low, even less than that caused by lichens. Avoid any aggressive cleaning methods that might damage the roof surface.

Black algae stains on asphalt singles

Some black stains on asphalt roof shingles are caused by a black algae (sometimes mis-named as a black fungus or even a "mildew") such as that shown in the photograph here. Black algae stains may be mistaken for but are not "extractive bleeding" - a product cosmetic defect. When the staining or bleeding appears to run down the roof from individual small points or "spots" I think this is extractive bleeding or "bleed through" on shingles. (See STAINS on ROOF SHINGLES for a more detailed discussion of bleed through or extractive bleeding as well as black algae stains on roof shingles. When black stains on the roof surface are more uniform and cover a wide area that does not originate at one or more single pinpoints in the shingles, this may be a black algae growing on the shingle surface. As we said about green algae on roofs, the presence of these black fungal or algal stains on the roof is an indicator that this roof area is in a shaded spot where you may want to be alert for development of moss or other roof problems, but the level of damage from the black fungus or algae is probably low, even less than that caused by lichens and certainly less than that caused by moss. Avoid any aggressive cleaning methods that might damage the roof surface.

How to clean off mossy or lichens-covered roofs

Do not try to clean a roof like the one shown in this photograph. The shingles are worn out and fragile. It will be impossible to clean the roof without damaging it. A new roof is needed. Power washing or brushing: it is possible to remove moss from a roof surface by gentle cleaning using a soft brush or a power washer. But be careful: power washing or even brushing or sweeping an asphalt shingle roof (or a wood shingle roof in old, worn, fragile condition) is itself a process that can damage the roof by breaking shingles or by loosening the protective mineral granules from the shingle surface. Chemicals that "kill" the moss or lichens risk also damaging the shingles or contaminating the environment, and have the further disadvantage that they still leave the moss or lichens in place where it holds water on the roof surface.

How to prevent moss or lichens growth on roof surfaces

Trim back trees whose branches overshadow the roof surface. Keep the roof clean of organic debris like leaves or pine needles which may collect in valleys or at other roof locations.

Installing copper or other metal strips along the ridge of an existing roof will slowly kill off moss or lichens as rainwater washes over the metal and down the roof surface. This method is suitable for both prevention of future or further moss or lichens growth on the roof and for gently treating an othewise fragile old roof.

Chemically treated roof shingles are available from several asphalt roofing manufacturers who offer these products which are resistant to moss, lichens, or algae growth on roofs. Discuss this option with your roofer when it's time to replace the roof.

Readers should also see STAINS on ROOF SHINGLES

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