Algae and roofing shingles
Algae develops on roofs everywhere where there is substantial humidity in the air. As for Europe and United States it concerns majority of the territory, therefore your roof is never 100% algae-safe here and it is advised to use algae-resistant roofing shingles.
Algae itself is not that dangerous, but it causes darkening of the roofing material. As it usually does not attack the whole roof, it causes awfully looking unattractive spotty roofs.Fighting the algaeThe roofing manufacturers (or their R&D labs) know that copper or zinc ions kill the algae and prevent it from growing, therefore couple of years ago the chase begun, to develop such material that would be able to release slowly copper or zinc ions to help reduce the discoloration of shingles covered by algae.Zinc and copper granulesAs the result, the roofing shingles manufacturers started to produce so called algae-retardant roofing materials that contain zinc or copper granules releasing the metal ions to the shingles surface and stopping the roof darkening.
Quite problematic about this method is the capability of the algae-resistant roofing materials to release the copper or zinc ions slowly enough to achieve the long-term effectiveness (do that over the whole life-span of the shingles).
To slow down the chemical processes, ceramic-coated copper or zinc-fortified granules are used that are plainly distributed across the whole surface of the shingle.
Such treated roofing shingles are more expensive but from the long-term point of view they can save your money you would spend on regular chemical removal of algae and cleaning of the roof.