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It depends on the age of the roof and condition its in.  Those natural "chemicals" leach out of the wood over time mostly due to the sun, not rain.  General soils and trees are acidic by nature, which also adds to its deteriorative state.  Most older roofs are going to require some sort of penetrating, pigmented protection from UV rays.  Most people up her opt for a clear coating, which in my opinion, does not last as long or serve the complete purpose of protection.  A clear water sealant could be replaced every season depending on it's (homes) location to the elements.