Shingle Stains - Long Dark Streaks

For years, I've thought the long dark streaks I see on roof shingles was due to stains form trees. Then I notices stains where there were no trees. So I thought perhaps it came from rusted staples. Then I saw too many to make that the root cause.
Someone now tells me "heat". That seems questionable since the patterns I see don't support irregular heat under the same roof.
Just what does cause these dark streaks draining down from under shingles as though they are leaking something dark. I've seen it on old and not so old roofs.
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Mold or mildew. Tom
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In a straight line? Then bleach is the simple solution (6% prox)?

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As Trajen's reference sez, it could also be algae. Tom
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And I've seen colonies of lichen(sp?), also. Tom
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Mold, mildew, algae, and lichens will form on the roof in areas that are last to dry after a rain, usually slightly low areas oriented along the water flow lines on the roof. Bleach may or may not be good for the roof, depending on your roof material. They make an algicide/mildewcide with zinc in it, that you dilute with water then spray on using a pump sprayer or hose end sprayer. I use a zinc strip at the top of the roof, that dissolves a tiny bit every rain. Seems to work, and spray is kind of messy, and likely more toxic after runoff. Google for the word association "zinc strip" and "roof" and "algae", all together. Check both Groups as well as the Web google.
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Most black mold algae streaks on roofs can be removed with a product called Shingle Shield. I believe you can find this product at www.egutter.com
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Check this out, it may be your answer.....http://www.chrisdhilton.com/newsletters/roof_stains.htm

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