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There is Mold on my Roof and I Don't Know Why. Help!

Answered by Jeffrey ~ March 25, 2010 ~ No Comments

The asphalt shingle roof of my Redding, California home is flowering with mold. What causes this? Is it dangerous? What can I do about it?

Alison K. ~ Redding, California

Jeffrey Anderson

Hi Alison, What you are seeing is more than likely algae rather than mold, and it can be fairly common on roofs. It is more of a cosmetic issue than anything else, and is more visible on some shingled roofs than others simply due to the color of the shingles.

A good way to get rid of algae is to spray your roof with a 50/50 combination of bleach and water, then rinse it off. Do not use a pressure washer, as that can harm the shingles. Before you spray your roof, make sure you wet down any plants, shrubs, or flowers below, as the bleach can be harmful. When you are finished with the roof, hose all of the vegetation off again.

You can prevent the algae from coming back by having a roofing contractor install zinc or copper plates near the ridge of your roof. A small bit of the plate will be exposed, and rain water should pass over the plate, and that helps prevent the algae from growing. I'm sure that roofing contractors in the Redding, California area are probably familiar with the problem. Depending on the pitch and height of your roof, you may want to let them handle the washing down of the roof as well.

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