A.Paul Fisette responds: Scrub the surface with a mixture of 1/3 cup household detergent, 1 to 2 quarts of household bleach, and 2 to 3 quarts of warm water. That should kill the moss. The detergent helps wet the surface, and the bleach kills the growth.
To prevent the moss from returning, you have to create conditions that won't support its growth. It's not possible to keep all dampness off the roof, but you can enable it to dry out faster by cutting back overhanging trees. You can also poison the food source of the moss by applying finishes that contain zinc, or a solution of copper...
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