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How to Explain Roof Algae to a Homeowner

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The spring and summer seasons are a time when homeowners start to notice unsightly dark stains on their roofs that they sometimes call fungus. While it could be from dirt or bad shingles, the most common cause is blue-green algae that can affect the appearance of a home. If you get a call from a homeowner, there are some things you can share with them to help make their roof and home looking clean and neat again.

Stains and streaks don’t necessarily mean a roof needs replacing, but it’s important for a contractor to assess the state of the shingles. If they are worn out, then discuss replacement options with them.

If the shingles look fine, then there are solutions to clean the roof. One option for cleaning the roof is to use a special mixture.  That mixture is 4 gallons of water, 1 gallon of bleach, and 1 cup of TSP (Tri-Sodium Phosphate—or acceptable phosphate-free substitute) found at many home centers. Apply this mixture with a garden sprayer, let it sit on the roof surface for up to 20 minutes, then rinse with low-pressure water.  Be sure to protect any shrubs below. Remember, the roof surface will become slippery when wet. Tell the homeowner this should only be done by a roofing professional who knows what they are doing, since getting on the roof is dangerous, especially when it is wet and slippery.

Do not power wash the shingles to clean them.  Some roof cleaning companies offer this service. However, it is not recommended as it may dislodge granules, which can lead to premature shingle failure.

However, cleaning will not ensure those ugly roof stains will not return. If a homeowner wants a more permanent solution, then suggest installing a strip of copper- or zinc-coated sheet metal on the side of the roof below the ridge. Copper or zinc retards the growth of algae, which is why stains are not usually present below metal roof flashings.

Algae can grow in moist, shaded areas, so if a homeowner has trees hanging over their roof, tell them to trim them back to let the sunshine warm and dry the roof.

If a new roof is in order, then suggest GAF Shingles with StainGuard® Protection to help ensure the beauty of a roof against unsightly blue-green algae.*

*StainGuard® Protection applies only to shingles with StainGuard®-labeled packaging. See GAF Shingle & Accessory Ltd. Warranty for complete coverage and restrictions.

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  1. Fred sieling

    1st roof in picture is either very worn out 90 lb, or the wrinkles could imply underpayment. Normally stains and streaks will tend to be on northern exposures- does not dry as well. Trimming overhanging branches can help a little but if roof on a bright day is not dry by say 10 am you can get growth easily.
    Also stains and streaks are not actual algae, but cyanobacteria ( used to be called blue green algae a simpler hardier organism than algaes. Under the right conditions lichens and or mosses can also grow

  2. Stevie Mak

    Great article, I have seen many people use a power washer on their shingles, now i know to tell them otherwise. Another idea is to go the route of a metal roof, it does cost more up front but will last much longer than shingles and with less maintenance.

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