Remove and Prevent Black Algae Stains on Asphalt Shingle Roofs

Living in a humid area puts you at risk for having dark streaks on asphalt shingle roofs. While this is caused by dirt, defective shingles and mildew, the most common cause is blue-green algae: Gloeocapsa Magma, which is spread by airborne spores.

algae stains on roof

Even though they are harmless, the impact they have hinders the resale value of your home and the appearance. Algae stains are usually absent below the metal flashing near chimneys or roof vents, due to algae growth being inhibited by trace metals. Copper and zinc on galvanized sheet metal are toxic to algae, halting growth.

Algae Resistant Shingles

Copper granules are now mixed into some roofing products to create algae resistance shingles. If your area is prone to algae growth, specify this type of shingle when replacing your roof.

Chemical Cleaning

By cleaning, algae stains can be dealt with. The bad news is, they will return. Occasionally cleaning is fine for your roof, but the repeated use of harsh chemicals can cause damage and shorten the life of asphalt shingles.

Wet & Forget Outdoor is a popular brand used specially for algae stain removal. A mixture of trisodium phosphate, bleach and water can also remove the stains. Oxygen bleach is a less harmful choice to the environment and lightens the stains, but does not produce an immediate impact like chlorine bleach.

If you want to clean your roof yourself, you will need these materials:

  • Ladder
  • Cleaner
  • Old clothes
  • Safety glasses or goggles
  • Rubber gloves
  • Safety rope
  • Garden hose with spray nozzle
  • Slip resistant shoes
  • Pump spray

Safety

It’s dangerous to work on a roof, which is why you should take precautions before working on your roof. Wear slip resistant shoes, avoid steep roofs, and use a safety rope if needed. Protect your skin and eyes when using harsh chemicals.

Wet down any grass, shrubs or plants if using bleach, as the bleach can kill more than just algae. Cover bushes with plastic after wetting them down.

Cleaning

The best day to clean your roof is on a cloudy day, as it prevents the cleaner from evaporating too quickly. You can find our the manufacturer of your shingles if you don’t know already, and find out what’s best to clean the roof with. Follow the right directions if the product is directly made for cleaning roofs.

Despite most bleach products to clean roofs varying in formula, the most basic formula is:

  • 1 quart bleach (6% sodium hypochlorite)
  • 1 gallon water
  • 1/4 cup TSP (trisodium phosphate) or other heavy duty cleaner (No cleaners that contain ammonia as it can result in poisonous chlorine gas when mixed with bleach)

In a pump-type garden spray, pour the ingredients, mix and spray. Wet down the roof wit the solution, let it sit for about 15 minutes, then wash it off with a hose.

Prevention

To prevent algae stains from coming back, look into installing a strip of copper or zinc coated sheet metal just below the ridge alongside the roof. About 2 to 4 inches of metal should be visible on the roof for this to be effective.

Copper is toxic to algae, but galvanized sheet metal is less expensive. Both are available in rolls, varying in widths and thickness from either online or in store locally.

With the help of roofing nails or screws with a rubber washer, you can attach narrow strips of sheet metal to your roof. Wider pieces of metal require loosening the self-sealing tabs on the top row shingles with a putty knife, slipping part of the metal under them and nailing it under the shingles with roofing nails.

While this article is about the removal of algae, these methods can be used for moss as well.