Make sure you have a decent, long, clean feed hose, and good pressure/volume of water to feed it.
I think you will likely have one guy carrying the washer on the ridge of the roof, and one guy handling the spray wand, so an extra long high pressure hose would not be required.
I would pre-spray the roof with whatever chemical cleaner you like with a garden sprayer, let it soak and pressure wash it off from the ridge down. A push-broom can help also.
All that said, i would not send a crew up on a roof the is not leaking. You probably are not going to like the result, and even if you do, it may not last long.
Do the carport first.