Attach the nozzle to your pressure washer. Most washers have a nozzle rated for concrete, otherwise use a 3,000 PSI-rated nozzle. Plug the pressure washer in and attach it to your water source.
Turn on the pressure washer then position yourself at the peak of the roof. You will need to wash in the direction of the roof's natural drainage. Put on your safety goggles before starting.
Make even passes over the roof with the pressure washer. Start at the top and work your way down, paying special attention to debris that is caught in the valleys of your roof.
Survey the roof after your first pass to verify that the concrete is cleaned evenly and that there is no remaining debris. If areas still appear dirty or drab, move closer to the area make 1 to 2 more passes with the pressure washer.