After installing a roof last year for a customer on one of his income properties we were called back to install a roof on his residence. He had Crimmins Construction, a well respected contractor from Wayland install a new front porch on his house where we followed up and installed a new roof.
The house is in a wooded area so there was a lot growth on the old roof. The Certianteed Landmark shingle has a 10 year streakfighter warranty which protects against streaking and discoloration caused by airborne algae. Certainteed accomplishes this by mixing special copper coated granules in with the colored ones. Rain and moisture diperse the the copper in what is called an environmentally safe cuprous oxide wash. The wash inhibits growth of blu-green algae. Some roofs such as this also have moss on them. Moss needs organic matter to grow. So the best way to prevent moss growth is the roof clean of all organic matter and to cut back over hanging limbs. If your roof is not that old and there is algae growth there are cleaner that can be used. The best way is to contact your local roofer to find out your options.