I would not take a chance on the roof. Instead of pc I would mix sand, Portland and latex paint {colored or not} and use a criss cross of window screen several layers thick and then paint the mixture on. This has shown to last 20 years in tropical Africa . If you need more info write me and I will dig up a link to the instructions for you.
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Forrest Charnock
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Forrest Charnock
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