"John Gregory" wrote in message
...
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.
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.