Guide to Identifying, Removing, Preventing Algae, Fungus, Lichens, Moss on Roof Shingles

Algae, moss, lichens, or fungal growth on roof shingles: causes, effects on roof life, cure & prevention
- How to clean off roof moss and lichens: brushing, raking, washing, power-washing?
- How to prevent moss and lichens growth on roof surfaces: algae-resistant shingles, metal flashings, special products, sunshine
- Caution advised before trying to remove moss or lichens growth from roof
LIFE / WEAR FACTORS in SHINGLES - separate article
Questions & Answers about cleaning off moss from building roofs
References

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Here we explain the causes of algae, moss, lichens, or fungal growth on roof shingles, the effects of these conditions, and how to cure or prevent them. Just brushing or raking off moss won't prevent future moss growth, and if roof cleaning is not done with care you risk damaging the shingles and reducing the life of the roof. We cite and discuss roofing manufacturer shingle warranty exclusions or inclusions for algae and algae-related roof stains at ALGAE STAINS ON ASPHALT ROOF SHINGLES.

Algae, Fungus, Lichens, 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.

The presence or absence of much sunlight on a roof surface is often a determining factor in the amount of moss or lichens growth (more sun, less moss and lichens. When shade and cool moist conditions combine, moss or lichens growth on a roof surface is more likely. [1]

Moss and lichens are more than a cosmetic issue on many kinds of roofing materials - asphalt shingles, roll roofing, wood shingle roofs, wood shake roofs. By holding moisture against the roof surface lichens but more so moss speed the wear of the asphalt shingle surface in freezing climates by increasing frost damage to the mineral granule coating on the shingles.

Sometimes there is so much moss and crud on a roof that we're not sure what roofing material was used. The photo at right is actually of a wood shingle roof, though it may make more sense to call this a moss roof in Rhinebeck, New York.

The differences among algae, lichens, and moss on roofs are described at ALGAE, FUNGUS, LICHENS, MOSS COMPARED where we also show what moss looks like under the microscope. And at ALGAE STAINS ON ASPHALT ROOF SHINGLES we show what roof shingle algae looks like under the microscope.

Also see BLACK STAIN REMOVAL & PREVENTION and see STAINS on ROOF SHINGLES. This website tells readers how to identify & explain the most-common asphalt roof shingle failures and how to obtain asphalt roofing shingle failure claims assistance. We also discuss green algae and moss on building and artifact surfaces at Green Stains on Stone.

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.

At ALGAE, FUNGUS, LICHENS, MOSS COMPARED we show photographs of lichens damage to asphalt shingle roofs.

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

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 misnamed 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" we think this is extractive bleeding or "bleed through" on shingles. (See ALGAE STAINS ON ASPHALT ROOF SHINGLES and also 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

Photograph of mossy growth on a worn out asphalt shingle roof

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.

In our opinion, any roof surface brushing or raking should be done with great care to minimize damage to the shingles themselves. If there is any doubt about the condition of the shingles underneath the moss, work gently by hand on a small area first to see the condition of the shingles below.

If the roof shingles are fragile, brittle, cracking, curled, the risk of serious damage to the roof during moss removal is much more likely. The roof may look cleaner, but its remaining life may be reduced by aggressive cleaning.

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. As stated at Power Washing Roofs we do not recommend power-washing asphalt shingles nor asbestos-cement roof shingles nor any other roofing product that can be damaged by high pressure spraying.

Raking off roof moss: using a flexible leaf rake to remove moss, providing the raking is done gently, may be easier than brushing. Debris on the roof after raking off moss can make it more difficult to see the actual condition of the shingles. Also, raking leaves more loose debris on the roof that may need to be gently washed off with a garden hose. Otherwise the debris will wash down and clog roof gutters

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

How to Prevent Moss, Lichens & Algae Growth on Roofs

On an asphalt or possibly some (treated) wood shingle roofs, shingle chemistry is also involved in moss, lichens and algae growth or resistance to it. Some roof shingle materials contain algaecides 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.

Moss, Lichens, Algae Resistant Roof Shingles

Use algae-resistant shingles when re-roofing. 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.

New asphalt roof shingles are available with an inclusion of chemicals that resist moss, lichens, algae, and even fungal growth. Resist does not mean moss-proof or lichens proof however for shingles subject to difficult conditions such as extensive shade under trees and lots of organic debris left on the roof surface.

At A Brief History of Algae-resistant AR-Shingles and What They Are Made-Of we give details about how shingles are made algae resistant - a property that appears to also retard moss growth by using zinc or copper treated mineral granules or flashings.

Metal Flashings can Retard Moss & Lichens Growth on Roofs

We have observed that the chemical or mineral salt wash-off from some building materials like aluminum flashing and copper flashing and even some paints which appear to kill of moss, lichens, algae, and fungus, as their extracts are washed over the roof surface. It's particularly easy to spot this effect by noticing where there are moss-free areas on an otherwise mossy roof surface.

One of our most obvious photos of rain wash off of copper flashing keeping moss off of a roof happens to be on a wood shingle roof, But we see this effect below copper flashing (and often aluminum flashing) on asphalt shingle roofs as well.

Notice the clear area below the turret on top of this wood-shingled pyramid shaped roof located in Key West Florida.

Chemicals from the metal cap atop the turret and from flashing at the turret base appear to be washing down the wood shingles in a path which prevents or even kills off moss, algae, and lichens on this roof (which we viewed from the Key West lighthouse museum.)

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 otherwise fragile old roof.

Here is another photograph demonstrating the effect of copper flashing on algae, moss or lichens on a roof.

Give the roof surface more sunlight: 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.

Steps to prevent or retard moss growth on roof shingles also work about as well for preventing lichens growth. For advice on diagnosing, cleaning, and preventing algae stains on roofing, see Black Stain Removal & Prevention

Trim back trees too close to the building: see our tree trimming advice at TREES & SHRUBS, TRIM OFF BUILDING

Frequently Asked Questions (FAQs) about about cleaning off moss from building roofs

Question: What about using bleach to clean off roof moss?

I recently bought a home that has green moss on parts of the roof. The roof is only 12 years old and original, but it looks more like 18 years old where the moss is present. I was hoping to get a few more years out of it, before having to replace the shingles.

From everything I've read and/or been told that a mixture of hydrogen peroxide and water will do the job. I understand there are commercial products available like "Spray and Forget" that uses hydrogen peroxide as the main ingredient and is effective, environmentally friendly and won't damage trim, siding or decks. However, they are not readily available in Canada. My next door neighbor uses a bleach/water mix, this may do more harm than good!

Can you suggest a mix ratio for the HP/Water? If you have any other suggestions, they would be appreciated. - G.F.

Reply: Go Easy on Mossy Roofs or Risk Short Remaining Roof Life

Above at How to clean off mossy or lichens-covered roofs we discuss the pros and cons of even trying to remove moss from asphalt shingle roofing. I'm worried that with the heavy roof moss growth that you describe, you will pull off so much of the protective granule shingle coating along with the moss that your roof will be rather short-lived if it does not leak immediately, no matter how you remove the moss. Therefore, gentler is better.

At BLACK STAIN REMOVAL & PREVENTION we discuss using bleach, laundry detergent, and TSP-substitute cleaners to try to remove black algae stains from roof shingles, and some rough formulas are given. You could try those solutions on a mossy roof, but I wouldn't.

Algae stained shingles are not the same as mossy shingles. Most often on an algae-stained roof the shingle granules remain pretty much intact, so the risk of converting from a stained roof to a leaky roof is lower than when converting from a mossy roof to a "cleaned" roof.

Therefore in your case I'd either leave things alone entirely or I'd consider installing a zinc or copper ridge strip to slow down further moss growth on the roof.

And in any case I'd be saving up for a new roof. When you re-roof, see How to Prevent Moss, Lichens & Algae Growth on Roofs so that your roof moss problem is less severe in the future.

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

[1] "Poor Season for Sunshine is a Great One for Spores", William Yardley, The New York Times, 20 April 2011, web search, 20 April 2011, http://www.nytimes.com/2011/04/20/us/20moss.html?_r=1&emc=eta1
The Ecology of Algae, F.E. Round, Cambridge University Press, 1984 ISBN-10: 0521269067 ISBN-13: 978-0521269063 (Available at Amazon.com)
After an introduction outlining the chemical and physical characteristics of the environment, the book goes on to look at the actual habitats in which algae occur. The communities of the individual habitats such as open water, sediments, rocky shores, coral reefs, hot springs, sea ice, soil, etc., are then discussed with special phenomena highlighted, for example rhythmic activity, nitrogen fixation and buoyancy. There are also chapters on seasonal cycles of algal growth, energy flow, geographical dispersion, palaeo-ecology and contribution to sediments. The importance of algae in symbiotic relationships and their considerable significance to animal grazers in aquatic food chains are also discussed. The final chapter deals with the relationships of algae to eutrophication and pollution of water. This is an important aspect, which can only be understood through an appreciation of algal ecology.
Lichens of North America, Irwin M. Brodo, Yale University Press, 2001, ISBN-10: 0300082495, # ISBN-13: 978-0300082494 (Available at Amazon.com)
Quoting from Library Journal:
Lichens are a combination of a fungus and an alga but have a unique structure and appearance quite different from either. Existing worldwide and growing on a variety of surfaces, including rocks, soil, and trees, they may appear leafy, shrubby, mossy, crusty, or jellylike and are seen in a wide range of colors, from brilliant oranges, yellows, and reds to dull grays and browns. This huge new book, written by a world authority on lichens and emeritus research scientist at the Canadian Museum of Nature, Ottawa, provides information on about 1500 of the roughly 3600 recognized North American lichens. Part 1 introduces lichens in 14 clearly written chapters that discuss their biology, ecology, geography, environmental roles, and collection. Part 2, the heart of the book, is a guide that offers identification keys to groups, genera, and species and their descriptions, with accompanying photographs and North American distribution maps. The more than 900 truly beautiful, full-color photos were taken by the Sharnoffs, nature photographers whose work has been widely published in National Geographic, Smithsonian, and elsewhere. Of value to professionals and amateurs alike, this book is certain to be a classic reference for decades to come. Highly recommended for academic and research libraries and for public libraries where interest warrants; libraries needing only a brief yet informative introduction to lichens should consider William Purvis's inexpensive Lichens (Smithsonian Institution, 2000). William H. Wiese, Iowa State Univ. Lib., Ames

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