If you are considering doing this as a professional, then you will appreciate getting it done quickly and efficiently….. then you should look at at least 2,000 psi…….
I have both a 1,000 psi steam cleaner ( diesel boiler, electric piston pump ) and a gernie …3,000 psi diesel powered ( 10 hp lombardini ) high pressure water blaster…..
These units cover most all jobs, the steam cleaner is very good for lichen build-up on terra cotta tiles etc.
The 3,000 psi can make light of most tasks including brick cleaning and driveways……for roofs keep the jet half a meter back or you can break tiles…….
Both these machines are aged and different techs are now available so do your homework on what is available…..do not get anything less than 1,000 psi and flow rate of 6 litres per minute as these are just toys…..
happy hunting…..
later, Jim ( Contract Builder for 34 years )