Moss often grows on roofs because leaves that fall on the roof from overhanging tree limbs retain moisture. Copper sulphate works well to remove moss on roof shingles, especially when the moss is growing and rain is unlikely. You can use copper sulphate alone or a mixture of zinc and copper sulphate.
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http://mistralni.co.uk/articles/?p=163
To read the full article click the link below
http://mistralni.co.uk/articles/?p=163