How Do Algae Resistant Shingles Work?

Asphalt shingles can be home to algae in humid environments.

Dark streaks on an asphalt roof are typically a result of the Gloeocapsa magma algae, which spread quickly with airborne spores. To combat this problem, roofing manufacturers developed shingles that incorporate copper granules, which are toxic to algae.

Problem

Algae thrive in moist, humid environments. According to 3M, algae can be seen on roofs in 80 percent of the United States. The Pacific Northwest and Southeastern and Middle Atlantic states experience the greatest algae problems.

History

The problem of algae on rooftops began when roofing manufacturers changed the way shingles were made. Previously, asphalt was applied to a thick felt mat that absorbed the asphalt well and created a waterproof barrier. When manufacturers began using fiberglass as a foundation for the asphalt, they found it did not absorb the asphalt as well and created shingles that were too light. Powdered limestone was then added to the liquid asphalt mixture to give the shingles more weight. Unfortunately, limestone is a wonderful growth medium for algae.

Solution

Roofing manufacturers used these findings to develop a shingle that contained copper, a metal that kills algae. Copper granules are uniformly distributed along the surface of these asphalt shingles, thus creating a shingle that is resistant to algae growth.

Considerations

Rooftops affected by algae seem to have no problems in areas near metal flashings, such as those around chimneys and vents. This is because the copper and zinc found in the galvanized metal of those flashings kill algae. When rainwater runs off these metal areas, trace amounts of minerals are washed down the roof and prevent algae from growing in those areas. These findings aided the invention of algae-resistant shingles.

Significance

While algae growth is generally harmless to roofing material, it can be unsightly. Homeowners often resort to harsh cleaning methods to maintain the appearance of their homes, and those chemicals can shorten the lifespan of their roof. The incorporation of copper granules into asphalt shingles has restricted the infiltration of algae on rooftops and reduced the need for damaging algae-killing methods.

References

• 3M: Scotchgard: Algae Resistant Roofing System
• Danny Lipford: How to Remove and Prevent Algae Stains on Asphalt Shingle Roofs
• Ask the Builder: Roof Shingles with Copper; Algae Resistant