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Cause, Cure, Prevention of Black Algae Stains on Asphalt Roof Shingles InspectAPedia®  -     How to identify & prevent black algae growth and stains on roof surfaces Algae stains and black marks on asphalt roof shingles: causes, effects on roof life, cure & prevention Types & photographs of organic felt asphalt roof shingle defects & failures

Here we discuss the causes of algae or "black fungal growth" on roof shingles, the effects of these conditions, and how to cure or prevent them. We provide a list of type of stains and colors that appear on roof shingles and on other roofing materials. Also see Black Algae for more photos of black algae stains on roofs. And see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES and see STAINS on ROOF SHINGLES. To remove and prevent black algae stains on roofs, see BLACK STAIN REMOVAL & PREVENTION.

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. These defects occur on organic-mat or 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.

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Black algae stains on asphalt singles

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.

Some black stains on asphalt roof shingles are caused by a black algae (sometimes misnamed 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 bleed-through stains emanate from single points regardless of sun or shade exposure (probably the cause of stains in the photo at above left).

The more faint black stains on the roof 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.

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 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.

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

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.

Since black 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 Bleed-Through 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.

Free Testing Offered to Identify Roof Stain Material: see ALGAE STAIN TEST METHODS for a description of both free 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? 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.

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ASPHALT ROOF SHINGLES
  ASPHALT SHINGLE DEFINITIONS
  ASPHALT SHINGLE FAILURE TYPES
  ALGAE STAINS ON ASPHALT ROOF SHINGLES
  ALGAE STAIN TEST METHODS
  ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES
  BLACK STAIN REMOVAL & PREVENTION
  BLISTERS on ASPHALT SHINGLES
  CRACKS in FIBERGLASS SHINGLES
  CUPPING ASPHALT SHINGLES
  CURLING ASPHALT SHINGLES
  FISHMOUTHING ASPHALT SHINGLES
  GRANULE LOSS from SHINGLES
  HAIL DAMAGED SHINGLES
  LADDERING & STAIR STEPPING SHINGLES
  LIFE / WEAR FACTORS in SHINGLES
  MECHANICAL DAMAGE of SHINGLES
  MOSS & LICHENS on SHINGLES
  ORGANIC FELT SHINGLE DEFECTS
  SPLICE DEFECTS on ASPHALT SHINGLES
  STAINS on ROOF SHINGLES

• Shingle Shield™ are zinc strips that are inserted under the shingle tabs of individual shingles to reduce moss, lichens, and algae growth on asphalt roofing - see shingleshield.com
• StainhandleR are zinc strips that are inserted under the shingle tabs of individual shingles to reduce moss, lichens, and algae growth on asphalt roofing- see stainhandler.com
• Zinc-Shield® - zincshield.com and Z-stop™ zinc roofing strips - z-stop.com, are roll-out zinc strips intended for installation near the ridge of a roof to reduce moss, lichens, and algae growth on roofs
• 04/09: thanks to William M. Norman, P.E., S.E., Keeler-Webb Associates, 486 Gradle Drive, Carmel, IN 46032 for opening discussion regarding the legitimacy of extractive bleeding as a term to apply to asphalt roofing material. Mr. Norman suggests that many (not all) black stains on asphalt roofing may be due to algal growth. We will report progress in this discussion as updates to this web article.

List of All 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.

• Algae caused black stains on roof surfaces - Some photos of black algae stains are at Black Algae Stains on Asphalt Shingles. For details see ALGAE STAINS ON ASPHALT ROOF SHINGLES. Algae can produce black stains on asphalt roofing products. Also see  ALGAE, FUNGUS, LICHENS, MOSS COMPARED.
  
• 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.
• Black stains on building interiors: such as on carpets, drywall, or trim: see STAIN DIAGNOSIS on Indoor Surfaces and then STAIN DIAGNOSIS on Indoor Surfaces and also see STAINS on Indoor Surfaces: PHOTO GUIDE
  
• 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.
• Chimney stains on roofs - see see SOOT STAINING - Black Soot Stains on Roof Shingles Around Chimneys and see Proximate cause roof shingle stains
• Debris stains on roof shingles - see Debris Staining - Black Debris Stains on Asphalt Shingle Roofs
• Extractive bleeding stains on asphalt roofs - see Bleed-Through and see Roofing material defect
• Lichens growing on roof surfaces - Lichens are not the same growths as moss or algae, and are difficult to remove without damaging the roof surface. See Lichens on Roofs for details about how lichens can cause roof damage, see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES for a guide to preventing these roof growths and stains. Also see   ALGAE, FUNGUS, LICHENS, MOSS COMPARED.
• Mold or fungus growing on roof surfaces or on other building surfaces - see ALGAE STAINS ON ASPHALT ROOF SHINGLES. See FIND MOLD in BUILDINGS, HOW TO for a guide to looking for mold contamination in or on buildings, and see WHAT MOLD LOOKS LIKE for a guide to just what mold looks like on building surfaces. Be sure also to see Stuff that is Mistaken for Mold to avoid becoming excited about harmless substances that people think are toxic mold. If you know your building has a mold problem, see ACTION GUIDE - WHAT TO DO ABOUT INDOOR MOLD.
• Moss on roof surfaces - see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES. 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
• Tree-caused stains on roofs - see Proximate cause roof shingle stains
• 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.

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