FAQs About the Scotchgard™ Algae Resistant Roofing System

Following are questions and answers that provide detailed, helpful information about algae staining on asphalt shingle roofs.

What causes black streaks on asphalt roof shingles?

Unsightly black stains on shingles are often mistaken for fungus, mildew, dirt or oil, but are actually caused by a hardy type of algae. This common problem has increased during the past 20 years, indiscriminately settling on rooftops from coast to coast. Dark algae streaks are visible when algae cover the normally vibrant colors of white, cobalt blue or hunter green roofing granules.

How do algae grow?

The primary type of algae found on rooftops - Gloeocapsa magma - thrives in humid environments, often appearing first on the north slope of a roof where extra shade and moisture support its growth. Algae can derive mineral nutrients from fillers such as calcium carbonate in asphalt shingles. Increased use of moisture attracting calcium carbonate fillers cause asphalt shingles to retain moisture longer, thus promoting algae growth.

Why are algae black?

The organism Gloeocapsa produces a dark-pigmented sheath to protect itself from ultra-violet rays. By the time the black streaks are noticeable, the algae usually have been growing for months or years - the result of generations of dead algae cells built up over time.

Are algae harmful to shingles?

There are no proven damaging effects of algae accumulation on shingles. However, the dark algae streaks are the number one appearance-related problem reported to roofing contractors. Consumers commonly mistake the dark stains for faulty shingles, leading them to complain to the contractor or manufacturer about a shingle's performance.

How do algae stains spread?

When environmental conditions are right for algae development, the problem rapidly expands, covering an entire roof. Once algae are present in an area, airborne spores are carried from rooftop to rooftop, infesting whole neighborhoods. All roof types, including wood shakes, concrete and clay tile, and asphalt shingles are susceptible to algae infestation.

What methods are available to clean an algae-infested roof if a homeowner isn't ready to replace the roof?

Cleaning is the most commonly used homeowner defense against algae build-up on asphalt roofing shingles, but at best, it's only a "quick fix." Typically, roofs must be cleaned frequently to minimize obvious staining. Not only are repeated cleanings costly ($400 - $700), but some cleaning methods can damage a roof and shorten the life of the shingles. Excessive scrubbing or power washing loosens the granules - and in essence, the color. It also deteriorates the asphalt once the protective granules are lost due to sunlight, causing further degradation to the shingle. Potent bleach solutions also can damage shrubs and bushes, and can be caustic to the homeowner or roof cleaner.

What is the most effective asphalt shingle product available to inhibit algae growth and prevent the black staining?

Look for the Scotchgard™ Algae Resistant Roofing System logo on quality shingle brands. The Scotchgard™ Algae Resistant Roofing System means protection. Many shingles feature algae resistance. But your assurance of the optimal long-term protection from granule discoloration due to algae can only be achieved when the precise level of granules with Scotchgard™ protection is uniformly distributed across the surface of the shingle in combination with standard colored granules.

How can a homeowner be sure of the performance of an Algae Resistant (AR) product?

Consult with a quality professional roofing contractor about asphalt shingles with built-in Scotchgard™ protection. The Scotchgard™ Algae Resistant Roofing System logo on quality shingle brands is your assurance of the optimal long-term protection from algae staining.

Do asphalt shingles with built-in Scotchgard™ protection cost more than regular asphalt shingles?

Generally yes, but many experts and homeowners agree that it's much less expensive to prevent algae growth from the outset. The price difference is minimal when one faces the accumulated cost of frequent roof cleanings. The extra cost associated with built-in Scotchgard™ protection refers to the materials only, because the labor cost to install these shingles is the same as it is for regular shingles.