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

Featured, Home Improvement, Roofing by Danny Lipford


Algae Stains on Roof

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 (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
  • 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

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

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

    where do i buy some trisodium phosphate?

  2. Dot Moran Says:
    August 13th, 2008 at 22:42

    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?

  3. Omprakash De Says:
    September 10th, 2008 at 11:52

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

    Thank You

  4. Terry Reed Says:
    September 20th, 2008 at 02:21

    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

  5. Zan Says:
    August 17th, 2009 at 15:40

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

    Thanks, Zan

  6. Official Comment:

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

    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.

  7. sembatya habib Says:
    September 14th, 2009 at 06:15

    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.

  8. Official Comment:

    Ben Erickson Says:
    September 14th, 2009 at 07:58

    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.

  9. rita Says:
    April 6th, 2010 at 05:24

    Please let me know how to remove green algae from tiles on the ground and on cement. thank you

  10. Official Comment:

    Ben Erickson Says:
    April 7th, 2010 at 10:58

    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.

  11. Mark Trine Says:
    April 26th, 2010 at 11:32

    After putting down the zinc strips what adhesive or sealant should be used to cover nail heads and hold shingles to the zinc strips?

  12. Official Comment:

    Ben Erickson Says:
    April 27th, 2010 at 12:46

    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!

  13. Steve Rock Says:
    May 19th, 2010 at 18:52

    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?

  14. Official Comment:

    Ben Erickson Says:
    May 20th, 2010 at 08:15

    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!

  15. Arly Sutroa Says:
    August 29th, 2010 at 16:13

    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

  16. Official Comment:

    Ben Erickson Says:
    August 31st, 2010 at 10:50

    Hi Arly,
    Good point, thanks for the tip!

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