How to Remove and Prevent Black Algae Stains on Asphalt Shingle Roofs
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.
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.
Algae Resistant Shingles
In recent years, roofing manufacturers have begun mixing copper granules 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.
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 Wet & Forget Outdoor. 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.
Here’s what you’ll need to clean your roof:
- Pump sprayer
- Garden hose with spray nozzle
- Safety glasses or goggles
- Rubber gloves
- Safety rope
- Old clothes
- Slip resistant shoes
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.
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)
- 1 gallon water
- 1/4 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)
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.
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.
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