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What Causes Streaks on Roofs?


As home inspectors, we are often faced with the question from a potential home buyer about what the ugly black streaks on a roof are, and more importantly how do they get rid of them?

Many people believe the streaks are caused by dirt or pollen building up on the roof, but more often than not the culprit is actually an algae that is growing on the roof and feeding off the roof materials.

The algae is most commonly Gloeocapsa Magma, a blue-green algae that travels and propagates through airborne spores.

The algae doesn’t typically do much damage to the roof, but it does look unsightly. And many people argue as to how to deal with it.

There is an old fix involving a mix of bleach and water – one part bleach and two parts water. The mixture is sprayed on and then using a telescoping brush (the type used on cars for car washes works well), you gently massage the affected areas, washing away the algae.

Just be aware that the bleach can kill vegetation along the side of the home beneath the roof as it rains down. Covering the vegetation with a tarp is a good way to avoid killing the vegetation.

Definitely don’t use a pressure washer because it can damage the shingles. It is recommended that you clean on a cloudy day so the mixture doesn’t evaporate quickly in the sun.

And there are also chemical mixes that can be purchased in hardware stores as well.

How can you prevent future growth?

There are newer shingles that are algae resistant that can be installed.

But on an existing roof, you can install strips of either copper or zinc along the roof ridge. When it rains, the ions from these strips flow down the roof with the water and are actually toxic to the algae.

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