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The Spokesman-Review Newspaper
Spokane, Washington  Est. May 19, 1883

Ask the Builder: Treat shingle streaks with soap, copper

Tim Carter Tribune Content Agency

DEAR TIM: My newer roof is turning black. It’s got ugly black streaks that look like someone poured used motor oil on my roof. What is causing the problem? Are the shingles defective? Is there a way to clean my roof and salvage it? Once clean, is there a way to prevent the ugly stains ? – Carly S., Chilmark, Mass.

DEAR CARLY: You and millions of other homeowners are victims of black roof streaks because a living organism is eating your roof. But don’t worry; your roof will go bad long before Gloeocapsa magma stops gorging itself on the tasty shingles. The black stains are merely the dead cells of the tiny, pesky microscopic creature.

Gloeocapsa magma is often referred to as algae. That’s technically incorrect; it’s a type of cyanobacteria, or photosynthesizing bacteria. It thrives in humid climates like yours. If you travel to just about any part of the United States that gets a fair amount of rainfall, you’ll observe quite a few roofs sporting these horrid black streaks. Many houses have lots of roof visible from the ground, so your pain is shared by many homeowners just like you.

Years ago shingles were made with felt paper as the base material. But in the 1970s manufacturers replaced the felt paper base mat that made up the foundation of a typical shingle with fiberglass. They were marketed as a revolutionary product, but the manufacturers were only telling part of the story.

They quickly discovered they had a problem using fiberglass. The thin mats wouldn’t absorb enough asphalt to meet the minimum requirements of weight per square. It was decided the best way to add weight was to blend powdered limestone in with the liquid asphalt. It worked. The shingles got up to weight much like a prizefighter.

However, the scientists at the shingle manufacturers didn’t look around at limestone buildings in the Southern U.S. Most of them had black streaks. If the scientists did look, they must have assumed it was dirt. It wasn’t. It was our friend Gloeocapsa magma. It turns out these bacteria love to eat limestone.

Your shingles may be defective. I just recently had to replace my own 40-year-guarantee asphalt shingles that only lasted 12 years. However, the roof bacteria were not the cause of deteriorating shingles or granule loss.

The good news is you can clean your roof and make it look like new. If you’re able to safely get on a part of the roof that’s stained, try mixing up some soap and water. Just use regular liquid dish soap you probably have in your kitchen. Apply it with a hand-pump sprayer and work in the shade when the roof is cool.

Once you’ve applied the soap, go ahead and scrub the shingles with a traditional scrub brush. This brush will not harm the shingles. Rinse the roof with clear water and check your results. I’ve had far better success using oxygen bleach using the same method as described above. Oxygen bleach rapidly restores the roof to its original look. Do NOT use chlorine bleach, as it will kill any vegetation that it comes into contact with around your home.

A few decades after the new fiberglass shingles were brought to market and roofs started to turn black, the manufacturers came out with a cure. They noticed that some roofs, or parts of them, were NOT turning black. Anywhere there was copper or zinc above the roof, the shingles looked great. Copper produced better results than zinc.

This is nothing new, as it’s been known for hundreds of years that copper is a natural biocide. This is why copper plating was put on the hulls of many clipper ships and warships to prevent barnacle growth that increased drag on the ships’ hulls. Copper is a great flashing material for roofs, and each time it rains, some of the copper atoms erode away and wash onto the roof, killing the bacteria.

You can purchase shingles that have copper in the colored granules, but some manufacturers skimp on the amount and there’s not enough copper to give great results. I know, as I had asphalt shingles with bacteria-protection granules on my own garden shed, and they turned black within two years.

The best thing to do is to install strips of copper high up on your roof at the ridge line. I’d leave at least 2 or 3 inches of copper exposed to ensure the black roof stains never appear again. The copper will weather quickly to a gorgeous dark bronze or green color, adding a distinctive touch to your home.

Carer’s columns are archived at AsktheBuilder.com. You can also watch hundreds of videos, download Quick Start Guides and more, all for free.