Photograph of shingle algae or fungus stains on a roof (C) Daniel Friedman Cause, Cure, Prevention of Black or Green Algae Stains on Asphalt Shingles

  • ALGAE STAINS on ROOFS - CONTENTS: How to identify & prevent black algae growth & stains on roof surfaces. How to identify & prevent green algae growth & stains on roofs. Algae stains and black marks on asphalt roof shingles: causes, effects on roof life, cure & prevention
  • POST a QUESTION or READ FAQs about the cause, cleaning, cure or prevention of algae staining on building roofs

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Causes of roof stains by algae:

This article explains the causes of dark colored algae (sometimes mistaken for "black fungal growth") on roof shingles, the effects of these conditions, and how to cure or prevent them.

Algae growth is common on roof surfaces of several materials including asphalt shingles, concrete and clay roof tiles, and even slate. Often algae that is identified as green when studied microscopically, also looks "black" or quite dark on a roof surface. We provide a list of type of stains and colors that appear on roof shingles and on other roofing materials.

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Black (or Brown or Green) Algae Stains on Asphalt Roof Shingles

Extractive bleeding roof stains (C) Daniel FriedmanMisnamed as "mildew" or "fungus" by some writers, algae staining on asphalt shingles usually in shaded areas or on the more-shaded roof slopes- characterized by black staining fairly uniform over shingles, but appearing specifically in areas of the roof shaded by nearby trees.

[Click to enlarge any image]

Black roof algae: Some black stains on asphalt roof shingles are caused by a black algae, typically Gloeocapsa magma. These black roof algal stains are sometimes misnamed or misidentified as a black fungus or even a "mildew") .

Algae is more likely to grow on shaded roof sections (probably the cause of black stains on the lower roof in the photo at above right), while asphalt shingle bleed-through stains more often appear to emanate from single points regardless of sun or shade exposure (probably the cause of stains in the photo at above left).

Article Series Contents

As NRCA's James R. Kirby [at REFERENCES] pointed out back in 1996, black or dark-green streaking on asphalt shingles is most likely caused by an airborne algae, Gloeocapsa that tends to grow in warm, humid environments, but can occur throughout most of the U.S. and Canada.

Kirby notes that algae on asphalt shingles is a cosmetic or aesthetic issue, not a roof performance (durability) problem, and it is generally not necessary to remove and replace asphalt shingles strictly because of algae growth and the associated discoloration.

The more faint black stains on the roof in our photograph of roof stains at above are probably an algal growth.

Photograph of black algae growth on roof shingles (C) Daniel Friedman

The black stains on the lower left roof in our photo immediately above are in an area that is shaded from sunlight and are almost certainly restated to that fact: algal growth. You may notice faint algae stains on the upper roof to the left side of each of the roof dormers.

Below we take an ever-closer look at algae growth on roof surfaces.

Closer Look at Dark Algae & Green Algae on Roofs & Under the Microscope

Algal growth on roofs also appears as green stains in many climates and locations. And keep in mind that some black roof stains, when studied in by an expert, may also be identified as actually green algae, even though it looks black on the roof surface.

Green & black algae stains on a brown asphalt roof shingle (C)

Above: green and black stains on this asphalt roof shingle [green and black arrows in the photo above] were examined under the microscope to confirm the presence of algae as shown in the photographs below.

According to algae expert Michael Guiry, (Algae Base at REFERENCES) some of these “black algae” are (as identified in the lab), blue-green or even lichenized algae, while others are a diverse group of coccoid algae (like our microscope photo below) that are turning out be be very diverse.

Green algae from an asphalt shingle roof (C) Daniel Friedman

Green roof algae under the microscope

Our photo above shows green coccoid algae (tentative identification: Synechococcoideae) from an asphalt shingle roof, magnified 720x. (Coccoid algae are algae that appear as individual non-motile cells with a cell wall and lacking a flagella

Here is a similar sample [photo] from a different home.

When black or green stains on a roof surface are more uniform and cover a wide area that does not originate at one or more single pinpoints in the shingles, this is likely to be a black algae growing on the shingle surface. Some writers refer to the black staining as a fungal growth - probably inaccurately in many cases.

Green coccoid type roof algae (C) Daniel Friedman Green coccoid type roof algae (C) Daniel Friedman

Above: two more examples of spherical clusters of green coccoidial roof algae from an asphalt shingle roof in a northern climate. Our first photo is of globules of conglomerates of coccoidial algae cells (720x) while the second photo of roof algae shows individual cells and cell chains (1200x).

On the roof surface and to the naked eye, these algae may appear as a dark brown or even black discoloration, showing up more abundantly on more damp, more shaded areas of a roof. Often, but not always, we will find moss as well as algae on the same roofs where algae growth is present.

The presence of moss, lichens, and algae together on a roof should come as no surprise since the conditions favorable to one are favorable to all three, and also because lichens itself is a symbiotic combination of an algae with a fungus. (Berdahl 2008).

There are many genera/species of algae. AlgaeBase lists more than 125,000, and it is likely that quite a few different genera/species of algae appear on different types of building surfaces. Here is a photo of a different green algae identified in our lab from a tape sample collected from a concrete surface [image] outdoors.

Watch out: don't waste much time arguing with your roofing manufacturer over cosmetic green or even most black algae stains on roofs: the warranty coverage for algae-resistant AR roofing shingles is limited, as we discuss at ROOF SHINGLE ALGAE STAINS AND ROOF WARRANTIES

What Causes Black or Green Algae Stains and Algae Growth on Asphalt Shingles?

Extractive bleeding roof stains (C) Daniel Friedman

Why does algae grow on roofs? Algae is naturally in the air in most areas of the country except probably the very dry. It grows on building surfaces where it finds food (lime, paint, other) and moisture; it is retarded but perhaps not entirely retarded, by sunlight and heat.

The increase in occurrence of Gloeocapsa magma algae growth on asphalt shingle roofs in the last decade or so may be due to changes in the composition of asphalt shingles: limestone added to shingles to add weight also makes the surface attractive to Gloeocapsa magma.

Black algae stains may be mistaken for but are not "extractive bleeding" - a product cosmetic defect. We need a closer look.

Black algae stains on asphalt shingles tend to be fairly uniform over the stained area, appearing more on shaded and more damp roof slopes. When stains on the roof surface are more uniform in pattern and when they cover a wider 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.

At BLACK STAIN REMOVAL & PREVENTION we discuss getting rid of and preventing these stains.

Tips for Distinguishing Algae on Roofs from Other Stains

Since black or green roof algae is more likely to grow on a shaded, cooler, or more damp roof surface, it is less likely that you'll find black algae stains uniformly on all roof slopes on a building. Black algae staining on an asphalt shingle roof is actually dead organic debris from an algae. The algae most often found on black-stained roofs is Gloeocapsa magma (some writers refer to it as "GM" algae).

Roof consultant William Norman has pointed out that the absence of staining below the dormer in this photo is compelling evidence that the stains are due to algal growth, not extractive bleeding.

To remove and prevent black algae stains on roofs, see BLACK STAIN REMOVAL & PREVENTION.

See EXTRACTIVE BLEEDING on SHINGLES for more on black stains that may be due to extractive bleeding on asphalt shingles. Some roofing consultants, including Mr. Norman (above) assert that many of the black stains on asphalt shingle roofs are due to algae growth and that extractive bleeding may not be a current roof problem.

Other dark roof stains from specific sources include soot from chimneys (brown/black, around a chimney and down-roof from the chimney), and debris staining on roof shingles or tiles where organic debris such as leaves has been left in place over time.

Free Testing Offered to Identify Roof Stain Material: see ALGAE STAIN TEST METHODS for a description of both free forensic microscopy lab tests and do-it-yourself roof stain tests that can help decide on the cause, clean-up, and prevention of algae stains on roofs.

Does Roof Algae Growth on Asphalt Shingles Affect Shingle Life?

Comments & Opinion on three claims of harm caused by roof algae:

1. Roof algae causes shingle granule loss? We have read in roof-cleaning-product sales literature that black algae reduces roof shingle life by "eating" at the shingles (loosening mineral granules and exposing the shingle substrate).

We agree that granule loss is associated with reduction in shingle life, but inspecting black algae-stained roofs in the U.S. we have not seen a difference in granule loss between the black algae-stained area and the non-stained areas. Contact us if you have different experience.

2. Roof algae causes shingle tab lifting? We have also read in roof cleaning product sales literature that black algae on roof shingles is a cause of shingle tab lifting. We have not observed this phenomenon nor have we found technical reports of this defect nor an explanation of its causal mechanism.

3. Roof algae releases harmful toxic spores on, around, and into homes? No. This is nonsense. Except when the ground is covered by snow, outdoor air has varying amounts, often very large amounts of all kinds of particles, including mold spores and rarely, algal spores.

Algae on the roof likes being there and hangs-on for dear life. It grew from random algae spores that landed on the roof and found a hospitable site.

Outdoor airborne particles indeed can enter a home through open windows and doors, and if building occupants are allergic to specific particles (typically certain pollen grains, some molds, other dust particles) they may suffer. Having inspected and tested airborne particles in and around nearly 3000 buildings we have never once found a significant level of algal particles in indoor air.

OPINION: As we said about green algae on roofs, the presence of these black "fungal" or algal stains on the roof is sometimes an indicator that this roof area is in a shaded spot (look for shade from trees or nearby buildings) where you may want to be alert for development of moss or other roof problems, but the level of damage from the black algae is probably low, even less than that caused by lichens and certainly less than that caused by moss.

To remove and prevent black algae stains on roofs, see BLACK STAIN REMOVAL & PREVENTION.

Roof Shingle Algae Stains & and Roof Warranties

Watch out: Algae resistance provisions of asphalt roof shingle warranties are limited in coverage and scope. (Also see WARRANTIES for ROOF SHINGLES)

Here are some major roofing manufacturer's warranty statements

Using OWENS CORNING ROOF SHINGLE WARRANTY [PDF] (4/30/10) as an example, note that [quoting]

Roof Shingle Warranty Exclusions [include the following]
Discoloration caused by algae, fungi, lichens or cyanobacteria (unless covered under the section “What About Algae Resistance” that follows)

What About Algae Resistance

If the Shingles that you purchased were not specifically labeled as “Algae Resistant” (“AR”), then any discoloration caused by algae is not covered by this warranty as explained in the section above “What is Not Covered.” However, if you did purchase AR shingles, they are covered under this warranty for a period of five (5) years commencing on the date of installation (“AR Warranty Period”) against discoloration caused by algal, fungal, lichen, or cyanobacterial growth (“Algae Growth”). If such discoloration occurs during the AR [Algae Resistant] Warranty Period, you will be entitled to the following remedy:

Remedy for Algae Growth – Should the AR Shingles exhibit discoloration due solely to algal, fungal, lichen, or cyanobacterial growth during the AR Warranty Period, your sole and exclusive remedy, and Owens Corning’s sole obligation, shall be for Owens Corning to pay you an amount equal to the reasonable cost of cleaning such growth off of the affected AR Shingles.

A later 2008 OWENS CORNING ROOF WARRANTY [PDF] provided to us by a client whose Owens Corning algae-resistant shingle roof was just two years old when an extensive green algae growth was observed, provides similar constraints as the 2001 warranty above, but extends the warranty period to ten (10) years.

An Owens Corning representative informed us (by telephone O.C. to DF, 04/2010) that the algae-resistant portion of a roof shingle warranty is also limited to one genera/species of roof algae, apparently excluding green algae staining.

Watch out: don't try cleaning asphalt shingle roofs without checking with the manufacturer about their recommended methods.

For example using power washers (not recommended), and some chemicals may damage the shingles and reduce the roof life.

Other chemicals such as bleach may also cause staining on other building areas and can be unsafe if not handled properly. To remove and prevent black algae stains on roofs, see BLACK STAIN REMOVAL & PREVENTION.

OPINION: algal growth on asphalt roof shingles, unlike moss and lichens growth (MOSS & LICHENS on SHINGLES), is a cosmetic concern and is not likely to affect roof shingle life.

See WARRANTIES for ROOF SHINGLES for more about roofing product warranties and for roofing product manufacturer claims & contact information.

Roof Staining Algae Types, Identification, Growth Research

Green algae from an asphalt shingle roof (C) Daniel Friedman

Our photo shows green coccoid algae (tentative identification: Synechococcoideae) from an asphalt shingle roof, magnified 720x. (Coccoid algae are algae that appear as individual non-motile cells with a cell wall and lacking a flagella.

Common algae that form on roof surfaces, including asphalt shingles and roll roofing include the algae shown above and Gleocapsa magma, a species that generally appears black on the roof surface but under some lighting conditions even on the roof Gleocapsa magma can appear as a blue-green color as might other algae genera/species.

On a typical asphalt shingle or roof roofing surface, roof algae exude a chemical that dissolves carbohydrates that the alga then absorb as nutrients.

In my [DF] OPINION algae on roofs are a concern only as a cosmetic or staining issue. Unlike both moss and lichens that also grow on roofs, inspecting roofs in North America as well as in Europe since the 1970's I have not once found roof deterioration or damage that I could ascribe to the presence of algae.

Unlike algae, moss and lichens do damage roofs, both by holding water on the roof and by the mechanical damage to the roof's protective coating or mineral granules caused by both their roots and in freezing climates by the freezing action of water trapped within the roof coating by those invaders. See LICHENS on ROOFS - for an example.

Watch out: black and green algae genera/species may both appear as dark black or brown on roof surfaces, and the green color of the algae shown above as well as in other photos in this article series may be apparent only under the microscope.

You can see evidence of particles from the very same roof shingle algae stain samples appearing as black, brown, and green at ALGAE STAIN TEST METHODS.

The green algae clusters shown in my microscope-photo appeared as both green and black stains when examined in photos and by the naked eye set upon the Minnesota roof where the samples were collected by a professional roofer in 2017.

Green algae under the microscope may look green, brown, or black on a roof shingle (C)


Continue reading at BLACK ROOF STAIN REMOVAL & PREVENTION To remove and prevent black algae stains on roofs, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES to compare algae, moss and lichens on roofs

Or see ROOF SHINGLE ALGAE STAINS AND ROOF WARRANTIES -roof stain warranty limitations


Or see STAINS on STONE, STUCCO DIAGNOSE & CURE for additional examples of diagnosing, cleaning, and preventing stains on building materials and artifacts.

Or see these

Roof Algae Lichens Moss or Stain Articles

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BLACK or GREEN ALGAE STAINS on ROOFS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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