I had to work on my roof this spring. The weather here in the Northwest makes moss grow everywhere. I neglected my roof for a couple of years. The moss was growing thick.
Removal begins with getting rid the big growths of moss. I have a tar shingle roof like so many homes in America have. This type of roof cannot be pressure washed. Doing so would blow off the granules embedded in the tar. This will ruin your roof right then and there. You can pressure wash a metal roof, but moss does not grow on metal roofs. Wood shingles can also be pressure washed, but the PSI of the washer should be reduced. Having no other option, I removed the moss by hand. You do not have to get everything, just enough not to waste the poison used in the next step.
Moss plants can be very small. On my roof they cling to the flat parts of the roof. Removing all of it mechanically is impossible. The common approach is spraying poison. You can buy special poison for this task at the home center. I prefer and used a bleach/water solution, the kind used for washing clothes. Simply load your sprayer and spray the area you want to clean. Best do this on a hot day when the moss is 'thirsty'. After only 5 minutes you will see the moss starting to die.
The bleach method is incredibly effective. The only spots the moss keeps growing are the spots you missed. The drawback of the method is it's environmental impact. Bleach will kill all plants and probably most small animals. Don't get the overspray on your roses. Don't forget to cheack the location your gutter vents to. The next time it rains the bleach solution will wash down from the roof into the gutter. The water might end up in a stream. If this is concern for you, consider the poisons sold at the home centers. They are typically bio degradable.
Moss prevention can be done using zinc or copper strip. Nail or screw narrow strips a couple of feet below the crest of your roof. Rain will wash metal particles down the roof killing all moss on the surface. My roof is at the end of its life. I decided against spending money on it. My next roof will be a metal one.