Algae stains on a roof (C) Daniel Friedman Cleaning & Prevention of Black Algae or Green Algae Stains on Roofs
     

  • BLACK ROOF STAIN REMOVAL & PREVENTION - CONTENTS: How to clean off & then prevent black algae growth and stains on roof surfaces. Algae and occasionally fungal growth discolors roof shingles. How to identify & prevent black algae growth and stains on roof surfaces. Advice about roof cleaning methods: chemicals, brushes, washing, power-washing roofs
  • POST a QUESTION or READ FAQs about roof algae stains and black marks on asphalt roof shingles: causes, effects on roof life, cure & prevention
  • REFERENCES

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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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How to clean off & prevent algae, moss or lichens formation on roofs

Photograph of mossy growth on a worn out asphalt shingle roofRegardless 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

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.

First Step of shingle stain removal: Assure that the roof shingles are sound and thus not likely to be damaged by any cleaning procedure.

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.

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. [PDF] 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 .

Do-it-yourself Roof Stain Cleanup for Algae Stains on Shingles

TSP Substitute (C) Daniel Friedman1 teaspoon of trisodium phosphate equivalent + 8 tbsp bleach in 1-1/2 cups water. (Oxygen bleach is readily available [Sodium percarbonate], which can be mixed with about anything that isn’t reactive to oxygen and it is not toxic.)

In greater proportions, Mr. Nomran reports using 1 cup of trisodium phosphate, one gallon bleach and 4 gallons water on his own roof and ... "it has worked just fine, spraying it on lightly, then rinsing thoroughly with water about 15 minutes later."

TSP substitutes such as the product shown (at left) are available at building supply stores and hardware stores.

A Brief History of Algae-resistant AR-Shingles and What They Are Made-Of

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 Algae 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:

Sooty molds can be indirectly controlled by reducing populations of sucking insects that excrete honeydew. Outdoor furniture can be hosed down with water during periods of honeydew excretion, particularly during drought. The growth of sooty mold fungi is inhibited by preservatives used in treated wood in rustic outdoor furniture. Here is the recipe for a good cleaning solution for removing sooty molds from plastic or painted surfaces:

Formula for Roof Cleaning Solution

Powdered household detergent
Household liquid bleach
Trisodium phosphate
Water
1/3 cup
1 quart
2/3 cup
3 quarts

Be sure to wear rubber gloves when cleaning with this solution. [Note that trisodium phosphate is no longer used as a cleaning agent because it is an environmental contaminant. However substitute products may be available. Be careful not to add a cleaning agent to bleach that can cause production of toxic chlorine gas.

Remedies for & Prevention of Black Algae Staining on Shingles

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.

Shingle Shield™ and StainhandleR and Z-stop™ are example of a zinc strip products intended for use in reducing roof growth of moss, lichens, algae.

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.

Wood shingle roof (C) Daniel FriedmanTrim 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.

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?

Readers Offer Other Remedies for Cleaning Algae Stains off of Roof Surfaces

Two Florida (USA) Roof Cleaning Companies have offered their opinions about alterative approaches to cleaning stained roof surfaces.

Note: InspectAPedia.com is an independent publisher of building, environmental, and forensic inspection, diagnosis, and repair information provided free to the public - we have no business nor financial connection with any manufacturer or service provider discussed at our website.

Chuck Bergman Roof Cleaning Port Charlotte, Fl USA Roof Cleaning Suggestions

We agree that granule loss is as said:

You wrote one of the better articles I have read on roof cleaning! I have been cleaning roofs since 1989 here in Florida.
I now clean them with what is called "SoftWashing" applying a mixture as specified by many roofing manufacturers, as well as the Asphalt Roofing Manufacturers Association. Prior to that, I pressure washed them. I know for a fact that black algae does loosen the granules. On many roofs there would be long streaks of black, especially on homes that had zinc strips installed. The strips would work, IF a roof surface was 100% flat and even-it's not.

So, you get really clean roofing in a slightly low area {lets call it valleys} and very blackened roofing in the higher areas, that don't hold the zinc. { No doubt due to where the rafters are? } When I pressure washed down the roof, the areas with the algae would lose about 20% of their granules, while the clean areas, perhaps 5% { as you said, pressure washing is not the correct method to clean roofing! I didn't know any better in 1989, 1990 and most of 1991 } I saw that scenario over and over- 4 x the granule loss in the algae infested areas.

The blacker the section, the more loss- I'd say up to 33%. I hope this info is of assistance to you. I agree with your info, other than this point and of course, the copper and zinc do not keep a roof clean-only portions. Please read what I have accumulated, as to manufacturer's statements and the ARMA bulletins given below. here- www.bergmanpressurewashing.com/Manufacturer_Specifics.html
Please feel free to use my info.

I only ask that you give me credit for what you may get from me. I will also be happy to work with you on any further articles or at perfecting this one.

Thanks! Chuck Bergman Roof Cleaning Port Charlotte, Fl (Aug 23, 2012) Website: www.bergmanpressurewashing.com

The links below all say to clean roofs with the method I use! They all also say specifically "DO NOT PRESSURE WASH"

  • ARMA Roofing Manufacturer Association Cleaning Bulletin http://www.asphaltroofing.org/algae-discoloration-roofs
  • Watch out: The ARMA also says not to allow anyone to apply any roof coatings. This is a common sales gimmick. Usually pressure washing companies-not roof cleaning companies-will pressure wash your roof and then sell you an after coating, which they want to reapply every 2 years. Names like Roof-A----. Not wanting to name any specific one. Read the ARMA warning below.

    http://www.asphaltroofing.org/sites/default/files/tech-bulletin/tb_227.pdf
  • CORNING Roofing Manufacturer Click number 1 "algae growth" http://roofing.owenscorning.com/homeowner/thingsyoushouldknow/needaroof.aspx
  • GAF Roofing Manufacturer http://georgoulisroofing.com/wp- content/uploads/2012/01/Algae_Staining_on_Shingled_Roof_Surfaces_-_Steep_Slope_Techn-43-808-v3.pdf

- Chuck Bergman is a roof cleaning company operator in Port Charlotte FL

Paul at Pelican Exterior Cleaning Inc in Pensacola Florida Roof Cleaning Suggestions

Forget the TSP and use swimming pool chlorine (not Clorox) that you can buy at your local swimming pool outlet. It comes at 10% strength (sodium hypochlorite (Sh)), the active chemical in bleach, but be sure to verify the Sh volume with the retail outlet you choose to buy from.

Dilute the chlorine down to .5 - 3% strength or 5% to 30% if you include all the buffers/liquids that compose a gallon of swimming pool chlorine.

Watch out:

  • ALWAYS add chemicals to water.
  • NEVER add water to them in their undiluted state because if you do and you accidentally get some splash back, you may subsequently get a concentrated dose of chemical on you.
  • Always wear the appropriate gloves and safety glasses.

We often use bibs made of PVC material to prevent chemicals from ruining our clothes and getting on our skin. Use as little chemical as possible and test test test.

Be patient and give the chemical a chance to work before you add more to your mix. Our chlorine detergent is mixed as follows (assuming 10% Sh).

For a 1% mix we start with 9 gallons of water, then add 1 gallon of swimming pool chlorine, then add 8-10 ounces of regular (not antibacterial - don't use that with bleach) liquid dishwashing soap. Stir the mix thoroughly and gett'er done!

A mix over 1% Sh should only be used on roofs. When it comes to working with chemicals the least amount you need the better. - Regards, Paul @ Pelican Exterior Cleaning. Inc. 8 October 2014

Watch out: use of any chemicals on building exteriors includes not only the safety risks outlined by contributors in the article above but also stains on siding (from bleach left thereon) and dead plants if you use bleaches or cleaners in too-high concentrations.

- Paul is the operator of Pelican Exterior Cleaning Inc in Pensacola Florida

Preventing Roof Algae

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.

Also see TREES & SHRUBS, TRIM OFF BUILDING

 

Continue reading at DEBRIS STAINS on ROOFS or select a topic from the More Reading links shown below.

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

Suggested citation for this web page

BLACK ROOF STAIN REMOVAL & PREVENTION at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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