Algae or moss on roof

[donnamatrix]

Noticed on our garage roof - that is pretty much in the shade - that we have some algae or moss or some green glop "growing" on the shingles. Egad. It's always something! What's the best way to eradicate this problem other than installing copper across the top of the roof. I don't want to get up on the roof and think it's too high and pitched to just spray something from a leaning ladder. There is no way to scrub off what I would spray on. Any suggestions are welcome.