A blue-green algae, called gloeocapsa magma, is very common in the Atlanta area because we have a climate that is very warm and humid during the summer and looks like a “black mold.” It does not harm your shingles, but it certainly looks bad.
If your roofing shingles are at the end of their normal life, you could replace them with special new shingles that are algae resistant and have copper or zinc built directly into the colored granules. The concept behind algae resistant roof shingles is actually quite simple. Copper or zinc, which is a main ingredient in this new innovation of shingles, stops algae before it attaches to your home. The metal isn’t visible, but its presence is certainly felt by unsuspecting algae as it is unable to survive on your roof’s surface. With use of these new products, your entire roof becomes an unwelcoming host to algae. Expectedly, these shingles will cost a few extra bucks, but are well worth the extra cost when you factor the expense of algae removal. The copper or zinc present in algae resistant shingles is activated further by rain, which distributes the algae fighting components even further along the roof’s surface. Rain generally promotes moisture on the roof, which serves as a breeding ground for algae. Thankfully, that’s no longer the case because of algae resistant roof shingles. This is an expensive solution if your shingles are still in good shape.
The least expensive method is to clean your roof with something that will kill the algae. There are many products on the market that you can purchase to do the job. It is recommended to use a standard pump type garden sprayer. The back-pack type is the most convenient to use. There are many different solutions you can use. Try mixing 1 part bleach to 3 parts water. Adding in tri-sodium phosphate (TSP) will help clean the roof at the same time. When using bleach, it is always advisable to wear protective gear. First, soak all of your shrubbery with water and cover to prevent damage from the bleach. Then soak the roof with the solution and allow it to soak in for 15 minutes. Rinse completely to prevent the bleach from damaging the shingles. Let the roof completely dry. If the algae is still present, repeat the process. As a last resort, try increasing the bleach to a 50%-50% concentration, being protective of the foliage underneath that will also receive the solution when you rinse it off the roof.
Pressure washing the roof is not recommended because it can blow off the granules that protect the shingles from the harmful rays of the sun or lift and damage the shingles that could result in roof leaks. If you do use a pressure washer, be sure to use very low pressure and wash from the rear of the shingle or from the ridge down to prevent blowing water under the edge of the shingle, lifting and damaging the shingle.
Unfortunately, the algae will probably return next year. To prevent the algae from returning, you could install a six inch strip of copper or zinc coated sheet metal under the row of shingles near the ridge. Rain running over the metal creates a chemical reaction that will kill the algae. This is often visible at the chimney or under the furnace vents where they meet your roof.
This information was reprinted from Christian Building Inspectors, Inc. newsletter.