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If the shingle or tile roof has gutters and they are badly stained and you want to get them sparkling clean, here is the method...you must apply your detergent to a dry gutter system.  We use a potasium hydroxide based soap.  Others work too, the secret is to lightly brush the soap on in sections.  Do not let it dry, so work with the weather.  Naples FL and Ft. Myers FL can be hot and sunny, so it dries faster.  Go back to the start and put a bit more elbow grease into it and you will notice the streaks come right off.  Again, work in sections and DONT let it dry or you will have to repeat the process.  After the 2nd brushing rinse immediatley without getting untreated sections wet with rinse water, ie spray the opposite direction.  We use telescoping wands for higher areas.  Unless the gutters have been seriously neglected, we have found that this method restores the paint to almost new.  White paint sometimes takes a bit more work or a second application.