What to do about dark streaks on roof shingles

Q: Lately I've noticed dark streaks on my roof shingles as well as other roofs in the neighborhood. The roof is about 12 years old and is in good condition. Can you tell me what the streaks are and if they will damage the roof?

A: The streaks are most likely algae stains and will not harm the shingles. I turned to an engineer who works in the shingle industry for some answers as to why this is happening.

“Per our conversation this morning on Algae Resistant (AR) Shingles: When you purchase an AR Shingle, the roofing manufacturer adds in a special copper coated granule from the granule supplier to the blended mix of colored granules. Some manufacturers may add more or less copper coated granules for added resistance against the algae and black fungus. Finding the supplier with the highest mix of AR coated granules is a tightly held trade secret. The reason you see the formation of green algae and black fungus in shingles today verses 40 years ago is that the roofing manufacturers now add filler to the asphalt. It’s the filler (crushed limestone) that absorbs and holds moisture. As a result, the filler acts as a perfect storm for hibernating and breeding fungus. The amount of filler some manufacturers add can be as much as 65 percent with the industry average being around 50-55 percent. In recent years there has been a push to reduce filler, but it is a very expensive trade off.”