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I have never had the pleasure of having a customer ask me to paint a roof! Up here in New England, the weathering on roofs is not really severe (aside from the occasional tree dropping on them), so most roofs live up to their warranties, 20 years or more useful life. However, in more tropical climates, the elements are more hostile, and the plant life more aggressive!
A company out of Miami known as Somay Products, Inc., http://www.somay.com, has a very informative web site about painting in general. One product they market is a roof coating called Roof Mastic, which they claim will do everything for your roof except paint itself on! You can contact the company as to the availability of the product in your area, or maybe you can order direct. Let me know your experience with this product if you decide to try it.