I tried it and found that unless I "do get close" it won't eliminate the brush and scrubbing. So, why drag the hose and nozzle up onto the roof, spray with soap, then crawl down, turn off the pressure washer and climb back up onto the roof to scrub? Just take the bucket, soapy water and scrub brush the first time and save the power washer for the sidewalls and awnings.
If you do use a power washer on the roof, be very careful about overspray getting into the refrigerator vent and separating the DICOR sealant from the TPO roofing..... Don't ask me how I know