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Mold and Roofs

You may have noticed streaks on the roof of your home or on the homes of others. And you may have wondered what caused them.  Most likely you were told that they were from stains from mold growing on the roof.  Oddly enough, barring another obvious source, those streaks are likely caused not by mold, but by algae called Gloeocapsa magma, which leaves black streaks as it dies off on the shingles.

At one time, roof shingles were made of asphalt and cotton (which absorbed the asphalt).  These were then changed to asphalt and fiberglass shingles, which were ultimately lighter than the asphalt and cotton shingles and did not last as long.  To add weight and strength to improve the fiberglass shingles, limestone was added to the shingle-making recipe.  Unfortunately, the added limestone turned out to be a food source for the algae that eventually makes the streaks on the roofs.  Shingles are now manufactured to be algae resistant but there are countless roofs that were constructed before these improved shingles were made available.

Algae streaks can be cleaned away by the “do-it-yourselfer” with a mixture of bleach and water, or professional roof cleaners can be called to do the job.  Pressure washers should not be used as they can power away the surface of the shingles, which can significantly decrease their lifespan and void their warranty.  Widely available at home improvement centers and relatively inexpensive, copper or zinc strips can be added just under the first row of shingles at the peak of each side of the roof.  Traces of these metals will wash down over the roof during rain showers and will inhibit the growth of these algae.  A wet roof can be a slippery and dangerous roof.  No actions should be taken without first planning for safety.

Does actual mold ever grow on a roof?  Given the right circumstances of food and moisture, it is possible for mold to grow anywhere.  Mold prefers temperatures between 40 and 100 degrees Fahrenheit and most roof shingles are exposed to temperatures outside of either end of this range. In these circumstances, roof mold problems are more likely to occur beneath the shingles, in the sheeting on which the shingles lie, and in places where there is a great deal of sun exposure yet cold winter temps.  With these conditions, mold often grows on the sheeting that is visible in the attic.  Hot air rising to the attic, heating system ductwork running through the attic, or bathrooms that are vented to the attic can all cause condensation when they meet the cold sheeting of the roof.  There then are the ingredients for vigorous mold growth: moisture, food (the wood), and proper temperature.  Mold can also grow on joists and other wooden structural components involved with the roof.

When there is question as to the existence of mold anywhere within a roof, a professional mold mitigation company can be consulted for further information and assistance.

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