Are you noticing black streaks appearing on your shingles? They might look like they are "running" down the roof. These streaks are unattractive, but luckily, they have a pretty simple explanation -- they're caused by algae! Here's some helpful advice for getting rid of the algae and associated streaks and then keeping it from coming back.
Getting Rid of Algae
Though there are commercial cleaners made for removing algae from the roof, you can also use a simple mixture of bleach and water for this purpose. Fill a large, 5-gallon bucket almost all of the way up with warm water. Then, add about 5 cups of bleach to the bucket, and stir it well. Fill a large, heavy-duty spray bottle with this mixture, and then climb up onto the roof.
Start at the top of your roof and work your way down. Spray the shingles with the bleach water, making sure you apply enough to completely dampen the shingles. Refill your spray bottle as needed, and keep on spraying until the whole roof is soaked.
There's no need to rinse the bleach off the roof -- it will slowly evaporate on its own. It should kill the algae rather quickly, though it may not remove all of the streaks. To get rid of the streaks, you can scrub the roof with a long-handled broom dipped in plain water a few days down the road.
Keeping Algae from Growing Back
Algae needs two conditions to thrive: moisture and shade. To keep the algae from coming back, be sure to have any trees that are overhanging the roof trimmed back. This way, your roof will dry off quickly after a rain storm, and the sun will reach it more easily.
If there's no way to "unshade" your roof -- such as if your home is placed next to a taller building -- you'll have to turn to other measures to keep the algae from returning. You can have your roofing contractor install zinc or copper strips along the peak of the roof. Whenever it rains, the water will rinse metal ions down over the shingles, and this will deter algae growth. They can also replace your shingles with special, algae-resistant shingles with built-in copper granules.
If climbing up on your roof to fight algae sounds dangerous or like too much work, then contact a roofing company, such as Allen Roofing & Construction Inc. Many offer algae removal services and may even be able to do this on the same day that they install your copper or zinc strips.