Push a bristle scrub brush mounted on a pole down the roof to scrape away the algae. This will be relatively easy, as algae plants do not develop roots. Do not push up the roof, as this may damage the bond between shingle layers.
Remove remaining pieces of algae with a power washer. Avoid using chlorine bleach as a roof cleaner, as chlorine bleach may remove the natural color from the shingles, accelerate metal gutter corrosion, and kill ground vegetation.
Place a copper strip, available at most roofing supply stores, under a row of shingles at the top of the roof. Expose at least four inches of the copper from the edge of the shingles. When it rains, the copper molecules will wash down the roof and create an environment that is poisonous to algae.