Your roof can be washed safely from the ground these days without the danger of having people walking around on your shingles risking injury or harm to the shingles with blasting water or boot marks . Your shingles have tiny small granules on them that will come off somewhat if its power washed. They will dry and cur up sometimes much quicker by doing this and it is hard to make it look uniform We let our special chemicals sit on your roof and do not rinse them off. All the mold, mildew, moss, lichen and algae will all run off your home when it rains also will dry up and blow off in the wind within a few weeks or months safely without wrecking your shingles. Last year alone I did over 200 roofs and did not have any complaints or issues all year.