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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).
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
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. 
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?
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.
[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.
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 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.
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.
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.
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®
Chemicals 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.
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.
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
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.
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 or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Question: What kind of metal strips at the ridgeline are recommended to slow or remove moss & algae growth on roofs
2016/09/02 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.
So 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.
Lee, C. K., and K. S. Low. "Removal of copper from solution using moss." Environmental Technology 10, no. 4 (1989): 395-404.
Panda, Sanjib Kumar, and Shuvasish Choudhury. "Changes in nitrate reductase activity and oxidative stress response in the moss Polytrichum commune subjected to chromium, copper and zinc phytotoxicity." Brazilian Journal of Plant Physiology 17, no. 2 (2005): 191-197.
Shaw, Jonathan. "Genetic variation for tolerance to copper and zinc within and among populations of the moss, Funaria hygrometrica Hedw." New Phytologist 109, no. 2 (1988): 211-222.
Abstract: Ten individuals from each of lour populations of the moss, Funaria hygrometrica Hedw., were grown on two copper and two zinc treatments and a control in order to assess tolerance to these metals. Tolerances of two life-history stages (protonemal growth and stem production) showed significant variation among populations, and among individuals within all populations.
Heritabilities for tolerance were high within three populations, but were close to zero within a population that originated on a copper mine. Phenotypic correlations among growth responses to the five experimental treatments were similar in the four populations, but this reflected a greater similarity among the environmental correlations than among the genotypic correlations. There was evidence of positive genetic correlations between growth on control medium and growth on media enriched with copper or zinc. This is in contrast to data from flowering plants and fungi, and raises the question of why tolerant individuals are restricted to contaminated sites in nature.
Shaw, Jonathan. "Effect of environmental pretreatment on tolerance to copper and zinc in the moss Funaria hygrometrica." American Journal of Botany (1987): 1466-1475.
Patent research on stain and algae resistant shingles gives both history of those solutions and the common approaches.
Teng, Yihsien. "Algae resistant shingle." U.S. Patent Application No. 11/066,644.
Excerpts: [in current products] .. To combat algae and/or fungus growth, it is generally known to include metallic granules on the surface of the roofing material. The metallic granules are generally either composed primarily of or coated with a coating containing copper and/or other metals such as zinc. When wetted by rain or otherwise, the copper and zinc compounds leach out from the roofing material and act as algaecides and/or fungicides to inhibit the growth of the algae and/or fungus.
Joedicke, Ingo B. "Algae-resistant roofing granules." U.S. Patent 6,214,466, issued April 10, 2001.
Excerpts citing other relevant patents are very helpful in this question where we include as does the patent applicant these illustrative examples of prior art compositions:
U.S. Pat. No. 3,494,727 discloses a microbe resistant roof wherein the roofing material contains embedded therein visually observable metallic chips, such as copper, lead, zinc and iron.
U.S. Pat. No. 3,507,676 discloses zinc-containing coating for roofing granules in the form of zinc, ZnO or ZnS.
U.S. Pat. No. 3,528,842 discloses copper compound-containing algicidal surfacing, the copper compound being cuprous oxide or cuprous bromide.
U.S. Pat. No. 3,598,627 discloses fungus-resistant shingles the outside surface of which are asphalt coated and are covered by a hard, granular weathering material and granular zinc.
U.S. Pat. No. 3,884,706 discloses algicidal roofing granules coated with a substantially water insoluble silicate-clay coating into which small amounts of algicides, e.g. 0.05% to 0.5% zinc algicide and 0.05% to 0.5% copper algicide, are incorporated based on the total weight of the roofing granules.
U.S. Pat. No. 4,092,441 discloses roofing granule treatment by coating the granules with a metallic algicide, such as zinc, copper, nickel and mixtures thereof. The metals are sprayed onto the granules in the form of molten droplets.
U.S. Pat. No. 5,382,475 discloses algicidal granules having a ceramic coating thereon comprising three layers, the first two of which have a copper compound, and the third layer has a pigment, a borate compound and zinc oxide.
 "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
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Green Roof Plants: A Resource and Planting Guide, Edmund C. Snodgrass, Lucie L. Snodgrass, Timber Press, Incorporated, 2006, ISBN-10: 0881927872, ISBN-13: 978-0881927870. The text covers moisture needs, heat tolerance, hardiness, bloom color, foliage characteristics, and height of 350 species and cultivars.
Green Roof Construction and Maintenance, Kelley Luckett, McGraw-Hill Professional, 2009, ISBN-10: 007160880X, ISBN-13: 978-0071608800, quoting: Key questions to ask at each stage of the green building process Tested tips and techniques for successful structural design
Construction methods for new and existing buildings
Information on insulation, drainage, detailing, irrigation, and plant selection
Details on optimal soil formulation
Illustrations featuring various stages of construction
Best practices for green roof maintenance
A survey of environmental benefits, including evapo-transpiration, storm-water management, habitat restoration, and improvement of air quality
Tips on the LEED design and certification process
Considerations for assessing return on investment
Color photographs of successfully installed green roofs
Useful checklists, tables, and charts
Roofing The Right Way, Steven Bolt, McGraw-Hill Professional; 3rd Ed (1996), ISBN-10: 0070066507, ISBN-13: 978-0070066502
Slate Roofs, National Slate Association, 1926, reprinted 1977
by Vermont Structural Slate Co., Inc., Fair Haven, VT 05743, 802-265-4933/34. (We recommend this book if you can find it. It
has gone in and out of print on occasion.)
Roof Tiling & Slating, a Practical Guide, Kevin Taylor, Crowood Press (2008), ISBN 978-1847970237, If you have never fixed a roof tile or slate before but have wondered how to go about repairing or replacing them, then this is the book for you. Many of the technical books about roof tiling and slating are rather vague and conveniently ignore some of the trickier problems and how they can be resolved. In Roof Tiling and Slating, the author rejects this cautious approach. Kevin Taylor uses both his extensive knowledge of the trade and his ability to explain the subject in easily understandable terms, to demonstrate how to carry out the work safely to a high standard, using tried and tested methods.
This clay roof tile guide considers the various types of tiles, slates, and roofing materials on the market as well as their uses, how to estimate the required quantities, and where to buy them. It also discusses how to check and assess a roof and how to identify and rectify problems; describes how to efficiently "set out" roofs from small, simple jobs to larger and more complicated projects, thus making the work quicker, simpler, and neater; examines the correct and the incorrect ways of installing background materials such as underlay, battens, and valley liners; explains how to install interlocking tiles, plain tiles, and artificial and natural slates; covers both modern and traditional methods and skills, including cutting materials by hand without the assistance of power tools; and provides invaluable guidance on repairs and maintenance issues, and highlights common mistakes and how they can be avoided.
The author, Kevin Taylor, works for the National Federation of Roofing Contractors as a technical manager presenting technical advice and providing education and training for young roofers.
The Slate Roof Bible, Joseph Jenkins, www.jenkinsslate.com,
143 Forest Lane, PO Box 607, Grove City, PA 16127 - 866-641-7141 (We recommend this book).
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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