You use a lower pressure and higher GPM than pressure washing. There are different ways to do it some people use a 12 volt pump with a flow between 5 and 10 GPM along with either a downstream injector or a mixing valve and separate tanks to inject SH (Sodium Hydroxide) and a surfactant (soap) into the wash stream. Other people use gas engine driven pumps the same as a pressure washer but the nozzle at the end of the line takes down the pressure of the water emitted from the pump so you don’t damage the house. This is the most non-destructive and really the only safe way to “pressure” wash a house or roof. You can also use it on sidewalks and driveways. The SH kills organic matter and the soap breaks up dirt and grime. It is applied to the house and sits for several minutes then washed off with a stream of water.