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.