How To Remove and Prevent Black Algae Stains On Asphalt Shingles

You have probably noticed that asphalt shingle roofs often have unsightly dark streaks, especially if you live in a humid part of the country. People usually think these streaks are caused by things like defective shingles, mold or mildew, or dirt, but in reality, it is caused by Gloeocapsa Magma, which is a blue-green algae that is spread by airborne spores.

This algae does not necessarily harm your roof, although it does affect the appearance of your roof and thus, diminishes your home’s value.

You won’t likely see theses stains just below the metal flashing on your vents or chimney because galvanized sheet metal has a copper and zinc coating that is toxic to algae. When it rains, the algae is washed away by a trace amount of these metals, which inhibits the growth of algae.

Shingles that are resistant to algae

Algae-resistant shingles are being manufactured in recent years, as roofing manufacturers have started mixing copper granules into their shingles. When you are replacing your roof, be sure to ask for algae-resistant shingles, especially if you live in a humid part of the country.

Cleaning with chemicals

Although algae stains are likely to return, they can be cleaned. Cleaning your roof occasionally will not likely harm it, although pressure washing (which has erosive effects) and using harsh chemicals regularly can shorten the life of your shingles.

Products like Wet & Forget Outdoor, which is a mixture of TSP (trisodium phosphate), bleach, and water, are available that are specifically designed to remove algae from your roof. A more environmental-friendly option is to use oxygen bleach instead of chlorine bleach. It doesn’t work as quickly or dramatically though.

To clean your roof, you will need:

• Your choice of cleaning product
• A pump sprayer
• A garden hose and spray nozzle
• Sturdy ladder
• Rubber gloves
• Eye protection
• Shoes that are slip resistant
• Clothing that can be ruined

Safety precautions

Please take adequate safety precautions—it is always dangerous to work on a roof, especially so when the roof is wet.

You should always use safety equipment, such as slip-resistant shoes and safety ropes, and avoid working on high-pitched or steep roofs. Always protect your eyes and skin with gloves and goggles when working with harsh chemicals.

To avoid killing or damage nearby plants, wet them with water before and after bleach application to allow the chemicals to run off. You may also want to consider covering your plants with plastic to protect them.

Cleaning your roof

You don’t want to clean your roof when it is too hot or sunny out because your cleaner will evaporate too quickly—a cloudy day is best.

If possible, contact the shingle manufacturer for recommendations on the best cleaning product for your shingles. Always follow the manufacturer’s instructions for whichever product you choose.

The basic formula for cleaning shingles is:

• ¼ cup heavy duty cleaner, such as trisodium phosphate (TSP)

Never use an ammonia-based cleaner, as it is harmful when mixed with bleach

• 1-quart bleach
• 1-gallon water

Add ingredients in a pump-sprayer, mix, and apply to your roof. Allow the mixture to sit for 15 minutes, then rinse with hose.

Algae prevention

You can install a strip of metal (2-4 inches) coated with copper or zinc just below the ridge on each side of the roof to keep algae from returning.

Galvanized sheet metal is less expensive than copper, although copper is more toxic to algae. Galvanized and copper rolls are available from your local metal suppliers or online in a variety of sizes (thickness and width). Zinc strips to prevent roof algae are available as well.

Using roofing screws or nails and rubber washers, narrow strips of sheet metal can be attached directly to the roof. However, installing wider strips is a little more involved. Using a putty knife, loosen the self-sealing tabs on the top row of shingles and slip part of the sheet metal underneath. Then, nail the sheet metal into place.

Not only will this method help prevent algae, but it will also help prevent moss growth.