We get 1600 psi (at the tip, with a 0 degree nozzle) and without hauling around a bulky hose to boot!
It's got to be a HUGE roof if we can't reach from the ladder, then we bring in the lift.

probably be just as far ahead to use a pressure washer

Here is another super pump I have always wondered about http://www.mmscusa.com/pdfs/chemical_pumps.pdf
Flow can be varied on tyhese just by changing the speed of the pump.
They are capable of up to 1000 PSI, nothing to go wrong but gears and seals.
No valves, springs, diapraghms, etc, etc, etc!
BIG bucks,  but if found used, may be the LAST pump for roof cleaning you may ever buy ?
Even as thr gears wear, the pump ill till put out flow/pressure
If the pump is big enough, even crippled by worn gears, there will probably still be enough left to clean roofs for years!
Kinda like a big block Chevvy 427 V 8, even running on 6 cylinders, it will still smoke the tires, LOL