It is possible to prevent or at least reduce the amount of moss on a roof by installing a zinc strip under the edge of the ridge cap on the roof. It is also possible to buy some composition roofing materials with zinc granules compressed into the shingles. The presence of zinc (or copper) on the roof changes the chemical composition of the rain water flowing onto the shingles. This change in the composition of the water reduces the likelihood of the growth of moss, lichens, and similar organisms on the roof surface.
Where moss has already started to grow on the roof, it is necessary to treat the moss with commercially available liquids or granules which are specifically formulated for the control of moss on roofs. Some, but not all, of these products are quite toxic and require careful attention to application instructions.
Once the moss has been killed (usually takes a few weeks), it is possible to use a soft broom to sweep the dead moss from the roof. However, the removal of the dead moss is not nearly as important as killing the moss.
Damp, humid climates can encourage the growth of fungus on asphalt shingles. The result is unsightly green and black streaks that are especially noticeable on light-colored roofs. But, there is hope! You can use shingles that are "fungus resistant". Shingles become fungus resistant with the addition of copper granules to the shingle surface.