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ROOFING INSPECTION & REPAIR
AMERICAN CEMWOOD ROOFING
BEST ROOFING PRACTICES
BUILT UP ROOFS
CATHEDRAL CEILING VENTILATION
CERTIFICATIONS for ROOFING CONTRACTORS
CHIMNEY FLASHING Mistakes & Leaks
COLD WEATHER ROOF TROUBLE
DECKS, ROOFTOP CONSTRUCTION
EPDM, RUBBER, PVC ROOFING
EXTRACTIVE BLEEDING on SHINGLES
FIRE RETARDANT PLYWOOD
FLASHING on BUILDINGS
FLAT ROOF MOISTURE & CONDENSATION
Green House or Solarium Roof Leaks
HEAT TAPES & CABLES on Roofs for Ice Dams
ROOF ICE DAM LEAKS
MASONITE WOODRUF FIBERBOARD ROOFING
NOISE CONTROL for ROOFS
PLASTIC ROOFING TYPES
PVC, EPDM, RUBBER ROOFING
ROOF ARCHITECTURAL STYLES - PHOTO GUIDE
ROOF CLEANING RECOMMENDATIONS
ROOF COLOR RECOMMENDATIONS
ROOF INSPECTION SAFETY & LIMITS
ROOF LEAK DIAGNOSIS & REPAIR
ROOF NOISE TRANSMISSION
ROOF REPLACEMENT SNAFUs
ROOFING FELT UNDERLAYMENT REQUIREMENTS
ROOFING MATERIALS, Age, Types
SADDLE CONSTRUCTION at CHIMNEYS
SNOW GUARDS & SNOW BRAKES
STANDARDS for ROOFING
STRESS SKIN INSULATED PANELS
TEST LABS - ROOF SHINGLE
TREES & SHRUBS, TRIM OFF BUILDING
TRUSSES, Floor & Roof
UNDERLAYMENT REQUIREMENTS on ROOFS
VENTILATION in BUILDINGS
WALK-ON ROOF SURFACES
WARRANTIES for ROOF SHINGLES
WORKMANSHIP & ROOF DAMAGE
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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Misnamed 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.
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).
As NRCA's James R. Kirby 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 left are probably an algal growth. The black stains on the lower left roof in our photo at above right 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.
According to algae expert Michael Guiry, (Algae Base) 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.
To remove and prevent black algae stains on roofs, see BLACK STAIN REMOVAL & PREVENTION. See STAIN DIAGNOSIS on BUILDING EXTERIORS or STAIN DIAGNOSIS on BUILDING INTERIORS and STAIN DIAGNOSIS on STONE for additional examples of diagnosing, cleaning, and preventing stains on building materials and artifacts.
Green roof algae: Algal growth on roofs also appears as green stains in some 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.
Our photo (left) shows green coccoid algae (tentative identification: Synechococcoideae) from an asphalt shingle roof, magnified 720x. Here is a similar sample from a different home. (Coccoid algae are algae that appear as individual non-motile cells with a cell wall and lacking a flagella.)
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.
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 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.
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.
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.
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? 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. 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.
Watch out: Algae resistance provisions of asphalt roof shingle warranties are limited in coverage and scope. (Also see WARRANTIES for ROOF SHINGLES)
Using Owens Corning's shingle warranty (4/28/10) as an example, note that [quoting]
A later 2008 Owens Corning Roof Warranty 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.
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