Algae-Resistant Shingle

An algae-resistant shingle is an asphalt shingle that includes a proportion of granules coated with copper or, less commonly, zinc compounds. When rain and dew wet the roof, these granules slowly release metal ions that inhibit the growth of blue-green algae, the cause of the black or dark-brown streaks seen on many shingle roofs in humid climates.

The streaking problem

The streaks are usually caused by the cyanobacterium Gloeocapsa magma. It thrives where dew, humidity and shade keep the roof damp, and it often appears first on north-facing slopes. In very humid regions, one manufacturer reports that streaks can appear within about three years of installation and spread over time. The algae are generally regarded as a cosmetic problem rather than a cause of structural damage, although dark streaks can raise surface temperature slightly.

How the protection works

Copper-coated granules are blended in with the normal colour granules so they are not visible. One manufacturer states that its algae-resistant products use at least a 10% blend of copper granules. The protection is preventive: it aims to stop algae from establishing rather than to remove existing growth. Over decades the copper is gradually depleted.

Algae warranties are usually separate from the main shingle warranty and much shorter, often 5-10 years in older products and longer in some current lines. They typically cover only Gloeocapsa magma, not moss, lichen, mould or other growths.

Alternatives and maintenance

• Strips of bare zinc or copper installed near the ridge release ions when rain runs over them and can reduce growth on the slope below.
• Existing streaks can be removed with dilute bleach-based cleaners applied at low pressure.
• High-pressure washing is discouraged because it strips granules and shortens shingle life.