Prevent and Clean Black Streaking on Roofs

A red tile roof with algae and mold growth.
What You'll Need
spray stain remover solution
scrub brush
garden hose
protective eyewear
rubber gloves
work boots
copper or zinc strips
galvanized flashing

Housetops are often left unnoticed. That is, until there is a problem. Because of their location, they're often an afterthought, however, roofs can quickly become unsightly or damaged if they are not checked regularly for problems. One issue that can lead to roof damage is black streaking caused by algae.

Caused by tiny organisms that get their food from minerals in the shingles, algae create unattractive streaks and stains. While these stains may not cause actual damage to shingles, they do leave a home looking less than appealing. A combination of bleach and water can be used to scrub the algae from the shingles. Once the stains are gone, the shingles will then need to be rinsed with clean water to prevent any damage from bleach residue.

Causes of Black Streaking on Roofs

Black streaking on roofs is a common problem. While the streaks do not cause a great deal of damage, they can cause the shingle to age prematurely. The streaks are caused by a type of algae known as Gloeocapsa magma. Over time this algae accumulates, developing a black outer coating which causes stains on roof shingles. The algae feed off of the limestone within the roof shingles. With each passing year the algae will grow and become more and more noticeable. As it grows, it will also trap moisture in the shingles, causing them to age sooner than they should.

A messy roof with branches and leaves on top.

Branches and dead leaves can build up on a roof and create the perfect environment for algae growth. It’s important to regularly remove leaf and branch debris from your roof in order to prevent the algae from growing.

Black Streaking Prevention

You can prevent the problem of black streaking by re-shingling your roof with algae-resistant shingles. Modern shingles contain a protective coating that prevents algae from growing on their surface. However, if you do not have the budget or time to replace the shingles, you will need to ensure that your roof does not provide an environment for algae growth.

Consider installing copper or zinc strips or galvanized flashing along the top of your roof. When the strips become wet from rain, a solution is created which will run down the shingles and keep any algae from growing. By installing galvanized roof flashing, you will also be able to build an environment that will be toxic for future growth of this potentially damaging organism. The flashing will release metal particles that will protect your shingles from algae growth. Install the material under the shingles at the top of your roof. Remove the self-sealing portion of the shingles and carefully slide the flashing under them. Use a hammer and nails to secure the flashing in place.

A roof with red tiles and windows.

Unfortunately, neither the strips nor the flashing will destroy any existing algae; they will only prevent future growth. You will need to remove the existing algae by hand in order to eliminate the black streaking altogether.

Black Streaking Removal

In order to remove the black streaking from your roof you will need several items: a spray stain remover solution, a scrub brush, a garden house, protective eyewear, rubber gloves, shoes with tread, a bucket, and a ladder. Be sure to have a friend or family member with you to ensure that you are safe while your climb up and work on your roof.

Watch the weather forecast and wait for a day of cool weather before beginning work on your roof. A day of hot temperatures will cause the cleaning solution to evaporate and will prevent it from doing its job in removing the stains. Place your stain remover and scrub brush into a pail. This will make it much easier to carry them up to the roof. Keep in mind that the work on your roof will involve the shingles becoming wet—the spray cleaner and the garden hose will both create a slippery surface for working. It's vital to be particularly careful as you move about on your roof. If you are uncomfortable with heights or have issues with mobility, consider hiring a professional to do the work for you.

A man spraying off dirt from a red tile roof.

Put on your protective eyewear, shoes, and rubber gloves before you begin your way up the ladder. Have your friend hold the ladder for you as you climb up to roof with your bucket in hand. Once you've reached the roof, spray your stain remover solution onto the stained areas. Allow the solution to sit for about fifteen minutes on the problem areas. Gently use your brush to scrub the algae off of the shingles. Be careful scrubbing your shingles; the scrubbing process can loosen the granules on the shingle surface and lead to cracking and breakage. Once you have finished scrubbing the shingles, use your garden hose to rinse away the algae and the cleaning solution from your roof—any residual cleaner left on the shingles can lead to premature aging of your shingles.