Definitely a power washer with a simple green solution added to the resevoir. The power washer usually comes with two different spray nozzles as well. Depending on how dirty, or how much mildew there is on your deck. You can mix and match nozzles to make your job that much easier. I am getting ready to clean my deck and this is the method that I am going to use. If you do not own a power washer you can go to a Home depot store and rent one for a day.