What Causes Black Roof Streaks And How To Remove & Prevent Them

What Causes Black Roof Streaks And How To Remove & Prevent Them

What Are Those Black Streaks On My Roof?

When you're roof is in good repair, it goes largely unnoticed. But when it's the eyesore of the neighborhood due to unsightly black streaks, it seems like you can't go outside without looking up. What are those black streaks? Are they damaging to your roof? And most importantly, can they be removed?

The Black Streaks On Your Roof Are Colonies Of Algae

Gross, but true. A strain of airborne algae called gloeocapsa magma, or GM, establishes colonies on roofs with newer asphalt shingles. Why newer? Because today's asphalt shingles contain limestone, and the algae's favorite food source is calcium carbonate - one of the major minerals that make up limestone. Asphalt shingles made 30 years ago did not contain limestone and roof streaks were nonexistent on roofs then. So the algae feed on the limestone granules in shingles, grow to form large colonies, and develop a dark UV coating that protects them from the sun. When it rains, the water runoff spreads the colonies further down the roof. Hence the black streaking.

Is GM Harmful To Your Roof?

Experts don't seem to agree on whether the black algae are actually harmful to the structure of your roof. While common sense dictates that if the algae are feeding on granules in the shingles, you should expect a faster breakdown and loss of granules, there is no evidence of this actually happening. Even the Asphalt Roof Manufactures Association (ARMA) has stated that "There is no scientific evidence that algae is damaging to asphalt shingles."

How To Get Rid Of GM

Harmful or not, the fact remains that roof streaks are aesthetically unattractive and afflicted homeowners often want to know how to get rid of it. While it is possible to clean your own roof using a mixture of bleach, water and trisodium phosphate (TSP), it is a dangerous job and generally one best left to the pros. Using a pump sprayer, they will apply the cleaning solution to the stains, let it soak in, then remove with a low pressure washer. This will temporarily lighten the algae but not actually kill it or prevent future staining. Scrubbing or using a high-pressure power washer on an asphalt shingle roof is NEVER recommended as it could cause serious damage.

Preventing GM Stains

Shaded homes in humid areas will almost always be affected by black algae. While annual professional roof cleanings can help keep the colonies in check and maintain your home's curb appeal, the algae will always come back. The only lasting solution is to make your roof inhospitable to the algae by making it toxic. After roofing companies began noticing that the streaks were generally missing below metal attic vents and chimneys with metal flashing around them, studies found that copper and zinc were actually toxic to the algae. When it rains, trace amounts of these minerals run down the roof below the metal, creating an algae-free zone. By having a qualified roofer add strips of copper or zinc coated galvanized metal along the roof's ridges, you can prevent future growth and streaking. If you're in the market for a new roof, today's shingle manufacturers have begun adding copper granules to asphalt shingles to make them naturally resistant to algae.

ProTek Construction Can Fix Roof Streaks

If you're embarrassed by ugly roof streaks on your Naperville home, call ProTek Construction for a free onsite evaluation and estimate. We can perform regular cleaning to lighten the algae, or take preventative measures to make your roof algae-resistant. Contact us at 331-444-7380 throughout the Chicagoland area to schedule a free roof evaluation. Mention code CLEANROOF10 when scheduling to receive a 5% discount on an approved roofing project.