Provide yourself with a safe environment before you embark on lichen removal. Wear good skid-proof boots and a safety harness tied to the chimney. If you slip, you won't fall off the roof.
Set your ladder up on a flat area. Run a hose line from the faucet to connect to the pressure washer. Connect the hose that came with the pressure washer up to the roof. Turn on the pressure washer and the water.
Spray a small test area on the roof with the wide sprayer nozzle. A newer roof will be able to take the pressure washing better than an old one. Asphalt roofs also seem to fair well without any damage.
Hold the nozzle 12 inches from the roof and move it across a small space evenly. Turn off the machine and check for any damage, such as broken or chipped tiles, pulled up shingles or exposed tar paper. If there is damage you will have to remove the lichen manually. If there is not, proceed with the washer in slightly overlapping strokes.
Remove the lichen manually with a hose and hose end sprayer. Moisten an area of the roof and scrub with a long handled scrub brush. You don't need to apply any chemicals or products. Hard work is sufficient and more environmentally friendly. For persistent areas of lichen, use a spatula to chip off the lichen. Use knee pads while crawling around on the roof to save your legs.