Roof Algae: What Is That Stuff On My Roof?

July 8, 2009 | Filed under: Articles, Misc

As asphalt and fiberglass shingles age, they can sometimes develop streaks of discoloration. This is especially prevalent in damp climates or shaded areas. While these streaks are often seen as a sign of aging of the roof, they are actually biological growth. As dirt accumulates on standard shingles over time, it gets stuck in the nooks and crannies of the aggregate surface. That can then become a host for biological growth known loosely as “roof algae”. In areas where there is not a lot of moisture or where the roof gets frequent and regular sunshine, the algae does not thrive. But in shaded or damp areas, it can quickly overtake the surface of the roof, spreading vertically down the roof from the point where it first takes hold.

In worst cases scenarios, over time, the roof algae will even take on a green mossy appearance.

One benefit to many of today’s residential roofs is that they can be installed over old shingles. This is a “green” benefit as it avoids tearing off old shingles and disposing of them in landfills. However, when this is done, many folks wonder whether it is okay to install metal roofing over the biological growth on their old shingles.

It has been my observation over the years that this is not an issue. I always suggest a layer of underlayment over the old shingles. One purpose this serves is that it isolates the old shingles and any biological growth on them from moisture. This will cause the biological growth to harmlessly die.

Over my nearly 30 years in the industry I have been involved at least peripherally with tens of thousands of installations of metal roofing over old shingles, including those with algae, moss and fungus growth. Never have I later regretted that the old shingles were not removed.

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