If the roof is in bad condition i.e. badly flaking and bubbling paint then its best to get it stripped off. This is a more expensive procedure but will look a lot better and last you at least twice the amount of time than a roof re paint.
All of the above procedures are followed other than the stripping. A coat of paint stripper is applied to the roof surface using an airless spray gun. After the appropriate amount of time for the stripper to work has passed then the surface is pressure cleaned as above to remove all paint. In some cases the roof will need two coats of the stripper (depending on the age and amount of paint on the surface)