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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
ICE DAM PREVENTION
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 DORMER TYPES - PHOTO GUIDE
ROOF INSPECTION SAFETY & LIMITS
ROOF JOB PROBLEMS, RESOLVING
ROOF LEAK DIAGNOSIS & REPAIR
ROOF NOISE TRANSMISSION
ROOF REPLACEMENT SNAFUs
ROOFING FELT UNDERLAYMENT REQUIREMENTS
ROOFING MATERIALS, Age, Types
ROOFING TILE SHAPES & PROFILES
ROOFING UNDERLAYMENT BEST PRACTICES
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
Roof algae stains: algae and occasionally fungal growth discolors roof shingles. This article explains methods for cleaning off and preventing future roof stains due to algae or "black fungal growth" or green algae 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.
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Regardless of the cause of a roof stain, moss, or lichens, avoid aggressive roof cleaning methods (like scrubbing with a stiff brush or power washing) that might damage the roof surface. Before deciding to remove a roof stain, you should
Second Step of Shingle Stain Removal: Test the roof stain as well as your stain cleaning method on a small area of the roof
Identify what the roof problem or stain or material is: algae staining, debris staining (probably can be removed or cleaned gently) versus moss or lichens growth (probably will damage the roof during removal).
How to identify algae staining on roofs: there is a simple test of applying a light spray of a diluted mixture of trisodium phosphate [use a TSP substitute, not TSP which is an environmental contaminant] and chlorine (with water) in the proper proportion will determine weather or not this is algae quite readily. If it is asphalt pigment, it will not lighten.
While we have not confirmed its presence, some black stains on roofs and roof gutters might be due to black molds or sooty molds, for which the USDA has published How to Recognize & Control Sooty Molds. Black mold growth on leaves, on roof debris that has fallen from a tree overhead, or on an aluminum roof gutter, is not a predictor that any of the species discussed in the USDA article will appear as black "sooty mold" growth on a roof shingle.
Step Three of Shingle Stain or Debris Removal: clean the roof using the gentlest possible method
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.
Power washing or brushing to clean a roof? it is possible to remove moss or lichens from a roof surface by gentle cleaning using a soft brush or a garden hose. Power washing risks damaging shingles by blowing mineral granules off of the shingle surface (as does unnecessary foot traffic on a roof).
NRCA's James R. Kirby notes that if a roof's appearance is not acceptable and cleaning it is the preferred method of improvement, asphalt shingles should be washed with a mild solution of chlorine bleach or mild detergent, gently applied with a sponge or hand-held sprayer. The surface then must be rinsed thoroughly with water. Kirby continues to warn that cleaning off roof algae is only a temporary fix - it will reappear unless we take steps (discussed below) to prevent future algae growth.
Watch out again: As we discuss at Power Washing Roofs we do not recommend power-washing asbestos-cement nor any other roofing. In addition to damaging the roof surface, power-washing an asbestos-cement roof can create an environmental hazard.
Watch out again: Don't do what one of our readers suggested: she sprinkles a mix of Tide laundry detergent over her stained roof then pours bleach onto that surface. That's a dangerous maneuver for obvious reasons: slipping off the roof, ruining the shingles, staining building siding, killing plants and grass below.
However our direct inspection of roof shingle surfaces where moss or lichens have been removed using any method, even non-abrasives such as installing copper or zinc strips whose salts eventually kill those organisms, usually shows that there has been shingle granule loss and even exposure of the shingle substrate where the lichens or moss root action had affixed some of the mineral granule roof shingle covering. See MOSS & LICHENS on SHINGLES.
Furthermore, unlike the simple removal of black algae stains or debris stains from a roof, trying to mechanically remove moss or lichens is likely to also remove roof surface materials, damaging the roof and reducing its life. Where mineral granules are lost from an asphalt roof surface, such as due to lichens damage shown at ALGAE, FUNGUS, LICHENS, MOSS COMPARED, there is a real risk of increased deterioration rate or reduced roof life because of loss of the protection of the mineral surface coating. In some cases the result may be actual holes in the roof shingles and leaks that can damage the structure below.
Be very 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.
That's why we prefer to use the most gentle method possible to remove moss, algae, lichens, or stains from roofs, and usually we leave moss and lichens alone except to take steps to reduce further growth of these organisms - discussed below at Remedies.
Some Roof Cleaning Chemicals may reduce or remove roof stains from algae or other causes. Be cautious: some chemicals intended "kill" the moss or lichens risk also damaging the shingles, staining building siding (such as using bleach) 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. Some deck cleaning sprays such as StainhandleR roof and deck cleaner are also sold for use as cleaning solutions for stained roofs .
Roofing manufacturers can provide shingles specifically made with an alternative type of mineral granule that resist algae growth. Earlier roofing manufacturers tried an algaecide coating to prevent algae growth. In the late 1980's manufacturers began using zinc granules but zinc was discontinued out of environmental concerns for using a heavy metal on roofs.
Zinc can also emit a "white rust" that builds up as a white stain or oxide. By 1996 manufacturers were using several coatings on asphalt shingles to reduce algae growth, including cuprous oxide coated granules mixed in with other granules on the shingle surface. The percentage of copper-coated granules (4% up to 10%) affects the resistance of the roof to algae.
Is algae the same as mold on roof shingles? Kirby's 1996 article confused algae and fungal growth on roofs as equivalent, which may be ok since the same chemicals that resist algae growth may also resist fungal growth and stains. We discuss removing sooty black molds on buildings below. By the way, most of the roof surface samples we have tested (pro-bono in our forensic laboratory) have been shown to be algae, not fungal and not extractive bleeding stains.
Shingle product names that include an "AR" suffix or "3M Algac Block" are products manufactured and warranted to be algae-resistant (OPINION-DF: "algae resistant" doesn't mean "algae proof", no?).
See Causes of Roof Stains and Catalog of All Roof Stains for the causes and types of stains that occur on building roofs. Our complete list of types of black stains on building roofs is at Catalog of Black Roof Stains.
Removing Sooty Black Molds on Plastic or Painted Surfaces (not specifically roofs)
The USDA publication we cited above includes suggestions for removing sooty black molds (the author considers Cladosporium sp. and Aureobasidium sp. as sooty black molds though they may in fact be brown or black and may not easily wipe off as "sooty" molds). Quoting:
How to Prevent Algae or Fungal Growth on Roof Surfaces
Black stains on asphalt shingles caused by algae is avoided by some of the new roof shingle products which include chemicals to retard algal growth.
Cutting back overhanging tree branches for more sun can also help reduce this problem.
Zinc or copper strips tacked along the ridge will produce (in rain) metal salts which will kill algae, lichens, and moss as the zinc salts wash down the roof. New shingles offered by some manufacturers include algae resistant chemicals in their mineral granule coating.
Whatever cleaning is done on a roof surface, make sure the process does not cause more harm than good to the roof and to the environment around the building.
NRCA's James R. Kirby also noted that metal fixtures (galvanized steel or copper, and in our experience even aluminum) retard algae growth on roofs, and he, too recommends installing zinc or copper strips at the ridge to control algae growth, but warns they may not be effective over the entire roof slope because concentrations of zinc, copper, or other metal salts are more diluted further away from the strips. Installing multiple strips can thus be a more effective solution.
Installing copper, zinc, or other metal strips along the ridge of an existing roof will slowly kill off algae, moss or lichens as rainwater washes over the metal and down the roof surface. Fungal stains may also be reduced; extractive bleeding stains may be unaffected unless simply being whitened by zinc salts.
This method is also suitable for both prevention of future or further moss or lichens growth on the roof and for gently treating an otherwise fragile old roof.
Our photo, left, shows an absence of moss or lichens on this wood shingle roof found in Key West, Florida - notice that the "clean" wooden shingles are all below the cupola and its flashing?
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. See our discussion A Brief History of Algae-resistant AR-Shingles and What They Are Made-Of
For an extensive catalog of methods for preventing algae, moss, or lichens growth on roofs see How to Prevent Moss, Lichens & Algae Growth on Roofs
The differences among algae, lichens, and moss on roofs are described at ALGAE, FUNGUS, LICHENS, MOSS COMPARED where we also show what moss looks like under the microscope. And at ALGAE STAINS ON ASPHALT ROOF SHINGLES we show what roof shingle algae looks like under the microscope.
Also see TREES & SHRUBS, TRIM OFF BUILDING
Frequently Asked Questions (FAQs) about cleaning algae stains off of roof surfaces
Questions & answers or comments about roof algae stains and black marks on asphalt roof shingles: causes, effects on roof life, cure & prevention.
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