How to Remove and Prevent Black Algae Stains on Asphalt Shingle Roofs

Black stains on an asphalt roof caused by algae.

If you live in a humid area of the country, you’ve probably seen unsightly dark streaks on asphalt shingle roofs. Though often attributed to an accumulation of dirt, defective shingles, mold, or mildew; the most common culprit is actually a blue-green algae known as Gloeocapsa Magma that is spread by airborne spores. While algae growth does little harm to roofing, the stains don’t help the overall appearance or resale value of your home.

Areas Susceptible to Algae Stains

You may also have noticed that algae stains are absent directly below the metal flashing around chimneys or roof vents. This is due to fact that copper and the zinc coating on galvanized sheet metal are toxic to algae. Every time it rains, trace amounts of metals are washed down the roof, inhibiting algae growth.

Absence of algae under metal flashing

Algae Resistant Shingles

In recent years, roofing manufacturers have begun mixing copper granules, such as those from 3M Scotchgard, into roofing products to produce algae resistant shingles. If you live in an area susceptible to algae growth, be sure to specify this type of shingle when replacing your roof.

Chemical Cleaning

Algae stains can be removed by cleaning, though they usually return. While an occasional cleaning might not harm your roof, repeated use of harsh chemicals, or the erosive effects of pressure washing, can damage or shorten the life of asphalt shingles.

There are several products on the market specifically designed to remove algae stains from roofs, such as Spray and Forget and Moss Out!. A mixture of trisodium phosphate (TSP), bleach, and water will also remove stains. Oxygen bleach lightens stains as well and is less harmful to the environment, but it doesn’t produce as immediate or dramatic an effect as chlorine bleach. To learn more about oxygen bleach, go to The Laundry Alternative, Inc.

Roof cleaned with oxygen bleach and chlorine bleach . Galvanized vent prevented algae growth.

Roof cleaned with oxygen bleach (left) and chlorine bleach (center). Galvanized vent (right) prevented algae growth.

Here’s what you’ll need to clean your roof:

Cleaner
Pump sprayer
Garden hose with spray nozzle
Safety glasses or goggles
Rubber gloves
Safety rope
Old clothes
Slip resistant shoes
Ladder

Safety

Working on a roof can be dangerous, and it becomes even more hazardous when wet, so be sure to take adequate safety precautions. Avoid working on steep roofs, wear slip resistant shoes, and use a safety rope where needed. When working with harsh chemicals, be sure to protect your skin and eyes.

Bleach can damage or kill other plants in addition to algae, so wet down any grass, shrubs, or other plants where runoff will occur before and after applying bleach. Covering bushes with plastic after wetting them down helps as well.

Cleaning

It’s best to clean your roof on a cloudy day to prevent the cleaner from evaporating too quickly. If you know the manufacturer of your shingles, contact them or check their website for specific recommendations on cleaning. When using a product made for cleaning roofs or oxygen bleach, follow the directions on the container.

While the recipe for cleaning a roof with bleach can vary, the basic formula is:

1 quart bleach (6% sodium hypochlorite)
¼ cup TSP (trisodium phosphate) or other heavy duty cleaner (don’t use a cleaner that contains ammonia or is not recommended for mixing with bleach, as it can result in poisonous chlorine gas)
1 gallon water

Pour the ingredients into a pump type garden sprayer, mix well, and spray.

After wetting down the roof with the solution, allow it to remain on the roof for about 15 minutes before rinsing off with a hose.

Prevention

To keep algae from coming back once your roof is clean, install a strip of copper or zinc coated sheet metal along each side of the roof just below the ridge. To be effective, 2”- 4” of metal should be visible on the roof.

While copper is more toxic to algae, galvanized sheet metal is much less expensive. Both copper and galvanized metal are available in rolls of various widths and thickness both online and from local metal suppliers. You can also purchase zinc strips specifically made to eliminate roof algae from Z-Stop.

Narrow strips of sheet metal can be attached directly to your roof using roofing nails or screws with a rubber washer. For wider pieces of sheet metal, loosen the self-sealing tabs on the top row of shingles with a putty knife, slip part of the sheet metal under them, and nail it in place under the shingles with roofing nails.

The methods used to eliminate and prevent algae on your roof can also be applied to moss.

Further Information

Tags: , algae, roof, Roofing, stain

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30 Comments on “How to Remove and Prevent Black Algae Stains on Asphalt Shingle Roofs”

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vicki meister Says:
August 2nd, 2008 at 10:14 am

where do i buy some trisodium phosphate?
Dot Moran Says:
August 13th, 2008 at 10:42 pm

There is a large pond on another persons property across the road from our house. Since these algae spores are air borne, could that contribute to the discoloration on our roof?
Omprakash De Says:
September 10th, 2008 at 11:52 am

What % of Zinc Oxide ,I have to add in a Cement bag to prevent gorworth of moss on side work.

Thank You
Terry Reed Says:
September 20th, 2008 at 2:21 am

Found a service called SHINGLE RENEW has a better recipe and tells where to buy cheap,safe chemicals.Informative how to on Cleaning asphalt shingles yourself…http://www.ShingleRenew.com
Zan Says:
August 17th, 2009 at 3:40 pm

Thanks for the help. How long before you see an effect on the shingles with the TSP soulition.

Thanks, Zan
Ben Erickson Says:
August 19th, 2009 at 11:27 am

TSP is a strong cleaner (it’s available at home centers and paint stores), but since you aren’t scrubbing your roof with it, it mainly acts to reduce the surface tension and allow the bleach to soak in without running off the roof. It’s the bleach that actually kills the algae. The effect of the bleach should be visible right away, though it will become more pronounced once it has had time to dry. Oxygen bleach, on the other hand, can take several days to a week before the difference is very noticeable.
sembatya habib Says:
September 14th, 2009 at 6:15 am

i want to clearly know how to remove and prevent algae, moss on clay roofing tiles; the procedure,the cleaner, and other tools so that we can start this project in uganda.
Ben Erickson Says:
September 14th, 2009 at 7:58 am

Hi Sembatya,
The above article on removing algae from a room can also be applied to removing moss. Additional information on removing moss can be found in our article How to Remove Moss from a Roof. Good luck with your project.
rita Says:
April 6th, 2010 at 5:24 am

Please let me know how to remove green algae from tiles on the ground and on cement. thank you
Ben Erickson Says:
April 7th, 2010 at 10:58 am

Hi Rita,
Try spraying the algae with a mixture of 1 part bleach to 4 parts water with a little detergent added to break the surface tension. Make sure the detergent doesn’t contain ammonia and the directions say it’s okay to mix it with bleach. Mixing bleach with ammonia, or cleaners containing ammonia, can result in forming poisonous chlorine gas.
Mark Trine Says:
April 26th, 2010 at 11:32 am

After putting down the zinc strips what adhesive or sealant should be used to cover nail heads and hold shingles to the zinc strips?
Ben Erickson Says:
April 27th, 2010 at 12:46 pm

Hi Mark,
If you buy a product like Z-Stop, mentioned in the article above, they include nails with a rubber washer on them to prevent leaks. You can also buy roofing nails with rubber washers on them that are used for metal roofing. If you use a wider strip of stock copper or galvanized sheet metal, slide a couple of inches of the metal under the shingle tabs and attach it with roofing nails beneath the tabs so they’re hidden (a dab of roofing cement on each one would be a good idea). Good luck with your project!
Steve Rock Says:
May 19th, 2010 at 6:52 pm

While the article identifies TSP as trisodium phosphate, the photo shows a package of Red Devil TSP/90 Heavy Duty Cleaner which, based on Red Devil’s site, is actually sodium metasilicate, pentahydrate (ref: http://www.reddevil.com/pdfs/msds_0261_0265.pdf ). Will this also work? Is it preferred over trisodium phosphate?
Ben Erickson Says:
May 20th, 2010 at 8:15 am

Hi Steve,
I don’t know if the TSP/90 cleaner pictured works better than trisodium phosphate or not, but it did do a good job of cleaning the algae stains off the roof we tested it on. Thanks for the feedback!
Arly Sutroa Says:
August 29th, 2010 at 4:13 pm

If you use ferrous based nails for nailing zinc strips, the iron will oxidize the zinc in the strips and the strip will corrode and eventually detach from the roof. Hot dipped galvanized nails will slow down the process but eventually you will have to reinstall the zinc strips. A non-metallic or plastic/rubber coated metal fastener would be preferred to reduce the zinc corrosion
Ben Erickson Says:
August 31st, 2010 at 10:50 am

Hi Arly,
Good point, thanks for the tip!
michael matulaitis Says:
October 5th, 2010 at 2:15 pm

Hi Danny,
Do you know of a product that I can spray on my roof shingles so that the little stones won’t wash away?
I did not have a ridge vent, and my roof is failing on on the south side. I am going to re-roof the south side and
install a ridge vent. I would like to protect the other side from further erosion of the aggregate.
Mike
Asphalt Shingle Roofing and Repair | Repairing Water Damage Says:
October 20th, 2010 at 11:13 am

[...] H&#959w t&#959 Remove &#1072&#1495&#1281 Prevent Black Algae Stains &#959&#1495 Asphalt Shingle Roof… [...]
Ralph Says:
November 10th, 2010 at 11:40 am

I have run into what looks like black roof algae on a fiberglass deck below a kynar-painted metal roof (the roof is OK) in a coastal North Carolina environment. It cleans up readily, but comes back in 6 to 8 weeks. This is the first time I have run into this problem. Have you run into this before? If there is a cleaner with residual algea resistance, it would be interesting. I’m guessing that the other choice is to paint the deck with an algae resistent paint.

Thanks,

Ralph
Carolina Allison Says:
November 22nd, 2010 at 11:52 pm

What do I need to remove algie from cement? Is there anything I can do to prevent it from comming back? How do I remove it from wood, privacy fence?
Thank you for your help!
Ben Erickson Says:
November 30th, 2010 at 9:14 am

Hi Carolina,
The same methods used to clean the roof in the article above can be used to clean your cement and fence, or you could use a pressure washer to remove it. Good luck with your project!
Bob East Says:
January 28th, 2011 at 12:38 pm

I think the use of bleach is not only hazardous but it’s environmentally friendly. The bleach and TSP will wash into the surrounding ground and on it’s way can damage gutters and vegetation. I prefer the use of a product called Spray & Forget which you spray on the roof or any surface contaminated with algae or moss and it does a great job. It isn’t instantaneous like bleach and can take months but it’s the safest way to eliminate the problem
Marv Says:
March 10th, 2011 at 2:47 am

After discovering that I had this “stuff” on my roof last summer, I spent a dozen or so hours doing internet research to find what to do about it : First , I did ~ 25% of my Garage roof – THE WORST AREA ! – using the homemade brews described, followed by a modified pressure washer WASH using VERY LOW PRESSURE and a SPREADER WAND to keep from concentrating the stream in small areas : a 20′ x 30′ area took several hours to to do the actual cleaning ( the tool , equipment & supply mixing took just about the same time ! TOTAL TIME WAS A FULL HALF DAY !

THEN , I wanted to use the COPPER or ZINC metal protection routine ; but found the COST WAS FAIRLY HIGH ! That was UNTIL I remembered that CONGRESS DEVALUED THE MOST BASIC PART OF OUR MONEY TO MAKE IT CHEAP TO PRODUCE !

The American PENNY is a COPPER-COATED-ZINC-SLUG ! SO , I collected several dollars worth of PENNIES , I cleaned them with a QUICK soak in DILUTED BLEACH, and quickly rinsed them with a water bath ; then distributed them along the top row of shingles @ ~6″ spacing : SO FAR THIS IS CLEANING THE UNTREATED ROOF AREA FROM LAST YEARS EFFORT QUITE GOOD !

ALSO , the ACTUAL COST WAS ~495-PENNIES : $4.95 !

What WAS a pain was waddling along, placing them !

My next development will PROBABLY be to expand the technique to using (drilled) PENNIES as “washers” on roofing nails ; even though the existing Pennies are still right where I placed them ( I have a rather shallow slopped roof) : Even though this is a rather LABOR INTENSIVE approach , the COST IS VERY CHEAP – compared to purchasing Zinc strips – which I ALSO now have !

I will do another (back-side) roof area this year ; using PENNY-WASHERED-ROOFING-NAILS for tacking down the Zinc Strip(s) ; even though they come in 50-Foot Lengths , it is NOT a good Idea to put them down that way ; the Zinc is VERY SOFT , and will tear itself loose from the Nails due to Heat-expansion-contraction if NOT cut to 5′-10′ Lengths.

ZINC PRODUCT : “MOSS BOSS” from Menards INTERNET SALES – NOT from their local store ! IT IS ~ 100%-PURE-ZINC ; NOT GALVANIZED CRAP METAL THAT WILL RUST & STAIN WHITE SHINGLES ! I verified their claim of Purity by MELTING a thin strip using SOLDERING equipment – it IS soft as Lead !

If you want to try this Product , be PREPARED to deal with VERY SOFT METAL ! If you have problems working with Aluminum Foil, you may want to avoid this! IT IS THICKER & STRONGER THAN ALUMINUM FOIL ; just making a POINT !

By Summer, this year, I hope to have a WHITE ROOF , again !
(With small DOTS nailed inconspicuously near the ridge)
///MRB
Ben Erickson Says:
March 10th, 2011 at 7:33 am

Hi Marv,
Thanks for the tips on how to remove black algae stains from a roof!
Bill Says:
October 16th, 2011 at 4:27 pm

A good efficient roof cleaning formula is 1 gal Chlorox, 1 gal water, 4 oz of dishwashing detergent (that doesn’t contain ammonia and is safe to mix with bleach) and 4 oz of sodium silicate (same as sodium metasilicate, often available in bags at auto parts stores sold as radiator flush). Mix the soap and water well first then add the bleach. This will cover 200 sq. ft. Spray the black stains first then the entire roof so the stains get a second dose. DON”T RINSE!!! The bleach degrades into salt which kills microbial growths and the sodium silicate does the same. They’ll get reactivated by high humidity, dew, fog or rain to eliminate more growth. A few hard rains will flush off the stains.
The sodium silicate naturally adheres to the roofing and is slowly released to deter regrowth for awhile. Once stains redevelope and stay then it’s time to recoat the entire roof since this dark algae can develope anywhere upon it.
This job goes much faster if you can borrow or rent a 12 volt diaphram garden sprayer with a large tank and a slew of hose (preferably on a reel) Be sure to wear rubber boots plus gloves and stay upwind while applying the solution.
I’m a housepainter and have been using this formula and method for decades on customers’ homes. It works faster tha Spray & Forget at much less cost. S&F could be used as an preventative treatment after the stains been removed, tho I’d mix it with water in a tank before application, That’ll provide a more uniform coverage opposed to the inefficient hose-end sprayer that dispenses product in a scattered fashion so more product needs to be sprayed to get the desired results. (ie: 50 sq. ft. vs 100-150 sq. ft. when premixed)
I wouldn’t bother with metal strips. You’d be better off dissolving copper sulfate in water then applying that to the roof to deter any growth. You could use that instead of bleach/soap for stain removal. Costs much less and isn’t as nasty yet does take a bit longer for the rain to remove the crud. Use 1 oz CS and 4 oz Ivory Liquid per gallon and treat the roof as stated above. The crud will turn brown and get washed off by rain as it decays into dirt. The copper also will deter regrowth for awhile, too. Copper isn’t dangerous to use. It’s a needed nutrient for plants and animals. Our bodies naturally slough off any excess copper we consume in foods or random contact while plants welcome it since the mineral is usually in short supply from dirt. I also treat customers’ roof with copper sulfate for cleaning and stain prevention since it works great. Haven’t done mny in the past few years due to our Texas drought. All this funky stuff needs moisture to develope and grow. We’ve been so dry little developes and what does get’s whiped out by the sun’s UV rays. Even existing nasty stains on homes have nearly disappeared. The sun turns them to dust and our rare rainfalls washes off the debris. Then it stays dry as a bone to deny any regrowth. There are some “professional” roof cleaners in drought stricken areas that started claiming their straight bleach treatments will keep a roof clean for 5 years gauranteed. Boy howdy will they be in for a surprise when humidity levels and annual rainfalls return to normal! Now if you have a steep pitched roof then I’d use one of them since they’re equiped and insured. Their product (an old painter’s trick) does work quite well plus gives immeadiate results that consumers rather have instead of waiting awhile. Just be ready for them to hem and haw around when you make a warranty claim since they’ll be dealing with a slew of those.
Ben Erickson Says:
October 16th, 2011 at 7:39 pm

Hi Bill,
Thanks for the helpful tips and feedback on cleaning your roof!
Bill Says:
October 16th, 2011 at 9:12 pm

Hiya Ben,
It’s not rocket science if one does their homework a for the cause, the common materials used to get rid of the crud and practical methods to get rid of them. My rain-rinse methods save customers the cost of rinsing and me the liability of possibly washing off granules (which naturaly come off due to weathering).
I mostly deal with dark Algae (Gloeocaspa Magma) here in the south. It thrives on the calcium carbonate that’s steadily extracted from the granules by mildly acidic rainfall. New roofs take awhile before enough comes out to support GM growth. Once it starts coming out GM will be a constant problem, Using products that will deter their growth helps delay future treatments, GM is the same crud that turns concrete (and composite decking) black so keeping these surfaces clean of te crud too, will help prevent it’s spread back onto your and neighbors’ surfaces. This is why neighbors should act in conjunction in having these surfaces cleaned since it reduces the amount of GM spores to be broadcasted across neighborhoods.
Perhaps these DIY’s that frequent this site could encourage neighbors to do the same on a given weekend? They could chip in on a sprayer rental to reduce costs plus lend a hand. It’s always wise to have a “groundman” present when a worker is on the roof in case something happens.
Heck, if you drain and flush your vhicles’ cooling system then you can use the old antifreeze and flush on your flat concrete! Just add a few gallons of bleach, douse the cement and rinse away 30 minutes later. It’ll put the whammy to the crud and allow easy removal with a hose or pressure washer. Once done and dru then treat the surfaces with copper sulfate to deter regrowth. These surfaces can be retreated with copper every two years to deter regrowth, Can do that when retreating the roof.
Tom from Cobb Says:
November 15th, 2011 at 11:58 am

I have very bad stains on my roof and have tried the bleach + TSP mixture as well as Spray and forget. ( The Spray and forget was used first with no results then tried the bleach / TSP mixture ) I have done this exact to the instructions given and the stains are still there. My roof is steep and difficult to walk on. ( I think it’s a 10/12 pitch ) Could it be these solutions are just running off to quickly and not having time to soak in or did I wait to long to address this. These stains have been on the roof for aprox. 5 years.

Thanks for any recommendations
Gary Says:
November 20th, 2011 at 6:30 pm

After researching different opinions on the matter, I tried what the shingle manufacturers (all of them) suggest. Something like 50 50 bleach/ water with some tsp. Not much help!
So, today I used a 10% bleach/water and a little dishwashing detergent, then pressure washed it. Fantastic results!
Granted these 10 year old strip shingles (designed to last 20 years) did lose their adhesion to one another but I know after one sunny day, they will reseal.
I went back over it again with the bleach solution and left it, for good measure.
As we all know bleach will kill mildew.
I have been a residential builder for 30 years.
I am looking into installing some zink strips. I am worried that any metal fasteners will cause corrosion. The rubber washered nails are designed to be used with some corrugated roofing and they work well there to seal out water. They will do nothing in respect to sealing out water on a shingle roof. The rubber may create a barrier between the bottom of the head of the nail, but what about the shank?
Any fasteners/protrusions left uncovered are chanceing leakage.
I am luckey enough to have a very walkable roof, so was not too difficult.
Good luck all.
Ben Erickson Says:
November 20th, 2011 at 9:38 pm

Hi Gary,
Thanks for the feedback on your roof!

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