• Hometalker
  • Dalton, GA
Asked on Jan 5, 2019

How do I get algae/fungi off my asphalt roof?



My roof, 10 years old and facing north with deciduous trees all around, has gotten dark streaks that I attribute to algae or fungi. The roof itself is in good shape. Any suggestions?

4 answers
  • Kelli L. Milligan
    on Jan 5, 2019

    I don't know what will harm the roof more, having it cleaned or the algae. Hopefully there's a roofer on home talk.

  • William
    on Jan 5, 2019

    The black mold-like stains and streaks that appear on roofs, particularly light-colored asphalt shingles, is actually a blue-green algae (Gloeocapsa magma). Commonly found in climates with warm, humid summers, it does no damage to the roofing, but it certainly does looks bad.

    The less expensive solution is to spray wash the roof with a 50 percent mix of water and bleach to get rid of the algae. (No pressure washers, please. They're likely to damage the shingles.) Just be sure to wet your foundation plantings first, and rinse everything in clean water when you're done. Plants don't like bleach, and wetting them with plain water first protects them. You can also cover them with tarps or plastic sheeting, which will protect the soil around your plants as well. Make sure that you create a wide radius around your home to be safe.

    Allow the solution to soak into the shingles for at least 15 minutes. If they are severely molded, you may want to give it a little longer. Next, rinse the shingles thoroughly with a garden hose at low pressure. Start from the top shingle and rinse downward to prevent water seepage.

    To keep the algae from coming back, insert 6-inch-wide strips of zinc or copper under the row of shingling closest to the roof peak, leaving an inch or two of the lower edge exposed to the weather. That way whenever it rains, some of the metal molecules will wash down the roof and kill any algae trying to regain a foothold on your shingles.

    You can probably see this same principle working on roofs in your neighborhood. Look for chimneys with copper flashing; the areas directly below the flashing will be free of any algae stains.

    The strips also work on roofs suffering from moss buildup. Just scrub it off first with a brush, then bleach as above.

    • Jeanette
      on Jan 5, 2019

      Thank you for your detailed advice, William! I live in north Georgia, where we have lots of humidity. The copper strips make sense. Wish I had known about it when I had my roof installed.

  • BarbB
    on Jan 5, 2019

    Just spray it with "Wet & Forget" (Amazon or Lowes) it doesn't get rid of it immediately but over a month or so it will disappear and stays gone for quite awhile. Follow package directions Really easy and won't damage anything. Works anywhere algae or moss is a problem. (Their shower spray works wonders too)

  • K
    on Jan 5, 2019

    Go to an aquiruim shop or someone who's in pond specialty.=)

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