Guide to Identifying, Removing, Preventing Algae, Fungus, Lichens, Moss on Roof Shingles InspectAPedia® -
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
Questions & Answers about cleaning off moss from building roofs
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.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers nor with topics or services discussed at this website.
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. 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.
These roof staining defects occur on both organic-mat and fiberglass-mat
asphalt roof shingles.
Readers are also invited contribute roof failure information to the web author for research purposes.
web author for research purposes.
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.
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 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
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.
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
Questions & Answers 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.
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Additional technical contributors & reference sources for this article are listed below.
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[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
Online Guide to Types of Stains on Building Roofs, Surfaces, Walls
In the following guide we list types of stains by stain color & appearance, by building location or material, and by stain cause. We distinguish among the following stuff that may stain or be found growing building roofs, walls, or other surfaces, with extra focus on asphalt shingle roofs as well as other roofing materials such as wood shingles, wood shakes, roll roofing, and even slate or tile roofs. Some of these types of roof stains or discoloration are only cosmetic in nature, while others may indicate growths that are likely to reduce the roof covering life. A more detailed, illustrated version of the list below is given at STAIN DIAGNOSIS on BUILDING EXTERIORS.
Black stains on asphalt roofing products such as asphalt shingles or roll roofing, caused by extractive bleeding (product defects) - see Bleed-Through and see Roofing material defect / environmentally-caused roof shingle stains.
Black, brown, or gray stains on roofing products due to debris left on the roof surface, such as piles of leaves and organic waste from nearby trees. See Debris Staining and see Proximate cause roof shingle stains
Black or other colored stains on roof surfaces may be caused by soot from chimneys, both wood-fired and oil-fired devices. Black or other colored stains may also appear on roof surfaces from nearby industrial activities. See Soot Staining.
Brown or red stains on roof surfaces are often caused by rust from wearing steel flashing or other on-roof metal products. See Rust Stains on Shingles.
Green roof stains on roof shingles, clay tiles, cement tiles, and asbestos cement roof shingles or newer fiber cement roof shingles are often caused by algae. For details see ALGAE STAINS ON ASPHALT ROOF SHINGLES. Moss on roofs is also often green and should be unmistakable in appearance. See ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES.
Moss on roofs and other building surfaces: see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES, and Green Stains on Stone. Green or other colored mosses are a bit easier to remove than lichens on roofing surfaces, but both can carry away mineral granules and damage the roof surface. Moss tends to hold more water and moisture on the roof surface. So in our opinion moss is more likely to damage a roof surface than lichens or algae. Also see ALGAE, FUNGUS, LICHENS, MOSS COMPARED.
Rust Stains on buildings - see Rust Staining - Red or Brown Rust Stains (on shingles)
Soot or Creosote black or brown stains on roof surfaces near or below chimneys - see SOOT STAINING - Black Soot Stains on Roof Shingles Around Chimneys and see Proximate cause roof shingle stains
Stains on stone or masonry surfaces - see STONE CLEANING METHODS for cause, cure, and prevention
White stains on roof surfaces may be from wash-down of pigments in paint on surfaces above the roof such as a gable end or dormer wall; white or less-black roof surfaces may also be found below copper or aluminum flashing whose salts run down the roof surface in wet weather, preventing algae or moss growth and leaving a white area on an otherwise stained roof surface. Our article at ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES includes a photograph of this condition on a wood shingle roof.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book 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. InspectAPedia.com ® editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
Architectural elements: the technological revolution: Galvanized iron roof plates and corrugated sheets; cast iron facades, columns, door and window caps, ... (American historical catalog collection), Diana S Waite, available used out of Amazon.
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
Problems in Roofing Design, B. Harrison McCampbell, Butterworth Heineman, 1991 ISBN 0-7506-9162-X (available used)
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).
Solar heating, radiative cooling and thermal movement: Their effects on built-up roofing (United States. National Bureau of Standards. Technical note), William C Cullen, Superintendent of Documents, U.S. Govt. Print. Off (1963), ASIN: B0007FTV2Q
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
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