Clean and remove all the dirt and oxidation found on the surface of the roof. If there is a stubborn stain, grease or any sign of mildew, use a scrubber or a rag with a mixture of bleach and water to clean it. After washing the surface, let it dry out for some time. The method of repair usually depends on the type of roof material and the type of contamination, e.g. oil, grease, etc., which needs to be removed.