The fight against algae

Manufacturers have a long history of trying to curb algae growth on shingles

In 1993, three of the 14 U.S. asphalt shingle manufacturers listed in the first edition of NRCA's Steep-slope Roofing Materials Guide offered algae-resistant shingles. Algae-resistant shingles filled a niche in the steep-slope roofing market, appealing to roofing contractors and consumers located in hot, humid areas of the U.S. Availability typically was restricted to the Southeastern and Gulf of Mexico coastal regions of the U.S.

Presently, nine manufacturers supply asphalt shingles to the U.S. roofing market. Eight of them offer a variety of algae-resistant shingle products that are readily available throughout the U.S. and Canada.

The growth of algae-resistant-labeled asphalt shingle sales can be interpreted as an indication the steep-slope roofing market is receiving products that provide effective protection against unsightly microbial discoloration. That, in essence, is the position of asphalt shingle manufacturers. Asphalt shingle manufacturers and algae-resistant system suppliers say this claim is based on evidence collected from long-term product exposure in weathering farms and positive feedback about product performance.

All asphalt shingle manufacturers that currently offer algae-resistant shingle products assert they possess empirical data—or have access to algae-resistant granule suppliers' long-term shingle exposure results—that support claims about the performance of their respective products. This evidence is not available to the public. In addition, no recognized standard is available for objective evaluation of long-term resistance to microbial staining of roof covering materials.