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To just KILL Roof Algae, simply spray it with some Copper or Zinc Sulfate mixed with water.
4 ounces of copper sulfate per gallon of water will kill any plant growing on a roof, and provide lasting protection against re growth for awhile.
Be sure and wet ALL plants and grass down before application.
It can be applied with a pump up sprayer.
The Roof Algae will die, and so will roof moss, and roof lichens!
Eventually, the rain will wash the dead plant matter from the Black Algae away, but it does take a long time.