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Photograph of mossy growth on asphalt roof shingles Identify, Remove, Prevent Algae, Fungus, Lichens, Moss on Roofs

  • MOSS on ROOFS - CONTENTS: 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
  • POST a QUESTION or READ FAQs about cleaning off moss from building roofs
  • REFERENCES
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Moss on roofs or roof shingles:

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.

Roof treatments to prevent or remove lichens & moss include use of metal flashing strips, algae-resistant shingles, or chemical treatments. Other roof cleanig procedures such as power washing or sweeping are discussed (and are not recommended).



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

Mossy roof shingles (C) Daniel FriedmanCauses 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.

What are the effects of moss growth on roofs?

Mossy roof (C) Daniel FriedmanBecause 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.

[Click to enlarge any image]

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.

Photograph of lichens growth on asphalt shinglesTherefore 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.

Photograph of fungal growth on shingles(C) Daniel FriedmanWhen 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 ROOFS
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

Roof Treatments to Remove or Prevent Algae, Liches, Moss Growth: Moss B Ware®

Corry's Moss B Ware roof treatment to rmove moss from roofs, patios, walkways, Retta manufacturing, North Bend WAChemicals that "kill" the moss or lichens on roofs can be effective as we illustrate below for a roof located in Seattle where the homeowner applies Corry's Moss B Ware® treatment as a granulated powder sprinkled on the roof along the ridge line. The powder is sprinkled on to a dry roof surface.

Watch out: don't try walking on wet or slippery roof surfaces. It's not the fall that hurts, it's the impact when you hit the ground that is likely to cause severe personal injury.

Moss B Ware® is sprinkled along the roof ridge where subsequent rainfall distributes the chemical (zinc sulfate monohydrate) over the roof surface. The company recommends that the powder should be applied only when the wind speed is less than 10 mph to avoid spreading corrosive dust.

The moss killing powder can be mixed with water to reduce the risk of wind-blown dust hazards to people, animals, or nearby plants or parklands.

The manufacturer of Moss B Ware, Retta Manufaacturing in North Bend WA (USA) recommends applying their roof de-mossing product during the rainy season for best results. In 3-4 weeks you should see the moss begin to turn more brown or gray in color and as the moss dies it is "washed away".

Depending on the volume of moss on the roof you may need to clean gutters of the mossy debris.

Watch out: Moss B Ware® uses zinc sulfate monohydrate as its active ingredient to kill moss or lichens on roofs. The product can cause irreverisble eye damage and is harmful if absorbed through skin or inhaled.

The company warns not to get the product in your eyes nor in your clothing, to wear protective goggles or face shield, to wash with soap and water after handling and to wash your clothes as well. The MSDS for Moss B Ware® as well as the product label details warn that the product must be kept out of reach of children.

Here are photographs of a Seattle WA roof that has been treated with Moss B Ware®. You can see that small dried fragments of "dead" moss have not washed off of the roof surface (photo at below right) on this rather low-slope roof. But generally the roof surface is nearly free of moss and the remaining fragments are harmless enough to be left alone.

Seattle WA roof treated with Moss B Ware from Corey (C) Daniel Friedman Seattle WA roof treated with Moss B Ware from Corey (C) Daniel Friedman

Watch out: as we explain in this article we do not recommend using more aggresive roof cleaning methods such as power washing or rough broom sweeping as forcibly removing live lichens or moss from a roof are likely to damage the roof surface by pulling away the shingle granules. Below is a close-up photo of the dried moss remains on this roof.

Dead moss on a roof treated with Corey's Moss B Ware product (C) Daniel Friedman

Watch out: roof treatment chemicals for algae, lichens or moss may risk also damaging the shingles, may corrode or discolor copper gutters or downspouts, and if not rinsed off of flowers or shrubs or lawns it may damage these plants.

Runoff of some roof treatments for moss or lichens may risk contaminating the environment, and some products may 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, Lichens & Algae Growth on Roofs

Moss on a wood shingle roof (C) Daniel FriedmanOn 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.

Wood shingle roof in Key West FL

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 or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Treat the roof surface with a lichens or moss killing chemical such as the Moss B Ware product discussed above
at ROOF TREATMENTS for LICHENS / MOSS

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 ROOFS 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.

Question: What kind of metal strips at the ridgeline are recommended to slow or remove moss & algae growth on roofs

Galvanized steel flashing at ridge helps keep moss and algae off of this wood shingle roof (C) Daniel Friedman2016/09/02 Anonymous said:
What kind of metal strips at the ridgeline are recommended and most effective, zinc or copper?

[Click to enlarge any image] The galvanized steel ridge flashing helps keep algae and moss from growing on this wood shingle roof, as does the absence of nearby trees and shade. The same effects will be found from metal flashings on asphalt shingle roofs.

Reply: copper or zinc or aluminum

Interesting question Anon. Moss, mold, and algae resistant roof shingles have been around for a while now, so a look at what roofing manufacturers choose to treat their mineral granules would be instructive, except that of course cost will be a critical factor.

Copper and zinc are most-often combined in the forms of cuprous oxide and zinc sulfide to form a copper oxide for moss and algae resistance, is added to most asphalt granule-coated roof materials while some roofing manufacturers use zinc, magnesium, or even titanium dioxide as the mineral granule treatment for moss or algae-resistant asphalt roof coverings.

Some roofing companies also use a ScotchGuard product to improve algae resistance of shingles, though I'd add that my own experience with ScotchGuard on building exteriors found that it needed frequent re-application (such as on masonry surfaces) to maintain its effectiveness.

Here's an excerpt from a GAF roofing warranty statement:

Algae Discoloration. All StainGuard®-labeled Shingles and Ridge Cap Shingles are warranted against algae discoloration for 10 years. There is no coverage for algae
discoloration for any other GAF Accessory Products.
- 2016/09/02 retrieved from www.gaf.com/Warranties_Technical_Documents/Steep_Slope_Warranties/GAF_Shingle_Accessory_Warranty.pdf

To choose between copper or zinc add-on strips for use on an existing roof to fight off algae and moss, where I've seen good results from copper, zinc, and aluminum flashing in those locations, my guess is that zinc oxide might form more rapidly and wash down the roof surface below more rapidly than copper strips affixed along a ridge or other roof location to try to kill off or prevent moss, algae, or fungus growth on a roof surface, but I also speculate that copper, while more costly, may have a longer effective life in that application.

Absence of algae stains below metal flashing at dormer (C) Daniel FriedmanSo do we want speed or life. Speed might be a reasonable choice since asphalt roof shingles will probably not last as long as either sort of metal strip along the ridge.

There is some research on the interaction of copper, zinc, and other metals with moss; I'll cite it below. Some of those studies were interested in the opposite reaction: removal of copper, zinc or other metals from solutions by moss, peat, or other organic materials, but the interaction and its effectiveness remain.

Our photo illustrates the remarkable absence of algae staining below the dormer on this New York roof. Take a closer look and you'll see metal flashing running horizontally just below that dormer window.

[Click to enlarge any image.]

Research on Roof Shingle Treatments to Retard Moss, Algae, Lichens, Fungus or Mold Growth

Ho, Y. S., DA John Wase, and C. F. Forster. "Kinetic studies of competitive heavy metal adsorption by sphagnum moss peat." Environmental Technology 17, no. 1 (1996): 71-77.

Patent research on stain and algae resistant shingles gives both history of those solutions and the common approaches.

Roof Algae Lichens Moss or Stain Articles

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Continue reading at MOSS on ROOFS, FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BLACK or GREEN ALGAE STAINS on ROOFS

Or see STAIN DIAGNOSIS on ROOFS

Suggested citation for this web page

MOSS on ROOFS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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