Alga is an airborne plant material that can attach to your asphalt roof and cause dark stains as it grows and lingers. While the algae are primarily a cosmetic issue, the stains can become quite an eyesore especially if your home's roof is highly visible from the ground or curb.
Luckily, the problem is fairly easy to treat for yourself if you can safely access your roof. If you have a high roof and lack the proper safety equipment, call in a roofing company to remove the stains instead. Stains aren't worth the risk of ou falling off a roof.
How can you remove algae from your roof, treat the stains, and prevent the algae from returning to do more cosmetic damage?
(Carefully) Use a Bleach-Water Rinse
You can remove the stains from your asphalt using a diluted bleach mixture that includes about two parts water to one of the bleach. Load some of the mix into a large spray bottle that you can easily carry onto the roof. Spray the diluted bleach directly onto the stains. Try to avoid wetting the shingle too much as you don't want the solution to risk pooling under the tile and causing water damage.
Allow the solution to set on the stain for a few minutes before rinsing off any excess bleach water with a hose. Make sure the diluted bleach isn't going straight over the side of your house and onto the ground around your foundation as the bleach can cause foundation damage over time. In other words, try to aim the solution down towards your gutter system, which should carry the bleach runoff safely away from your house.
Scrub with Stiff Brush
Did the bleach fail to remove all of the algae from your roof? Use a handled, flat-headed broom with stiff bristles to remove the stubborn algae. You want to start with a lighter hand and then intensify pressure but don't press so hard that you risk removing the protective rough upper layer of the asphalt shingles.
Did the stiff brushing remove the algae but stains remain? Follow up the brushing with another carefully done rinse with the bleach-water mixture. Stains still lingering? You might need to bite the bullet and call your roofing contractor to discuss replacing the stained shingles.
Mount Copper or Zinc Strips
Want to prevent the algae from taking hold again in the future? Ask your roofing contractor to install copper or zinc strips at the peak of your roof. Rainwater will carry minerals from the metal down over any existing algae and kill the organic material before it has a chance to cause any staining.
Your roofing contractor (like Emerald Roofing) should be able to install the strips in a subtle way that partly uses your existing shingles as camouflage. If you have a taller roof where the ridge isn't visible from the ground, the camouflaging isn't as important since no one will see the metal regardless.