What causes dark, mildew-like stains on a fiberglass roof only three years old? Can the stains easily be removed and how can it be prevented?

The roof has what is known as Gloeocapsa magma, a very hardy algae. They have probably been up on the roof since day one. The streaks and discoloration is an accumulation of their dead, dark colored cells. The algae was deposited on the roof by wind and/or wildlife.

Older shingles used an asphalt saturated felt paper and did not have this problem. When roofing companies switched to fiberglass, the shingles contained lots of ground up limestone. It is used as a filler to increase the weight of the shingles. Gloeocapsa magma loves to eat limestone.

This algae can be prevented from growing on roofs. All you need to do is introduce copper or zinc into the picture. Gloeocapsa magma hates copper or zinc. These metals are a very effective poison.

Install long, seven inch wide strips of copper underneath the last full course of shingles at the top of the roof. Slide the copper up under the shingles approximately five inches or so. This will leave approximately two inches exposed to the weather.  Rain will wash small amounts of copper down the roof which will poison the algae.

When getting ready to install new shingles, you need not do this. Many fiberglass shingles are available with colored granules that contain copper. There are enough granules present on each shingle to kill the algae. The copper is slowly released over time. Many of these shingles are guaranteed to kill the algae for the life of the shingle.

The existing stains can be removed safely by using one of several treated wood deck cleaners that will do the job without hurting people, the roof, gutters, and any vegetation below. Look for deck cleaners that contain bleach. Do not use cleaners that contain chlorine bleach. Oxygen bleach is a non-toxic powder that mixes with water. Do not use a pressure washer. Apply the oxygen bleach solution to a cool roof surface on an overcast day. Do not let the solution dry. If windy or sunny, be sure to continue to apply a light spray of the oxygen bleach solution. After it has soaked for 30 minutes, the solution can be brushed with a stiff broom or brush. These procedures should be done by a professionals.

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