I've just picked up some opinions here and there from people I feel know a bit about this. They say to not use power washing on shingle roofs because it removes the grit from shingles. they say to not use it on siding because it can loosen it from the side of the house. It has been recommended to me to wash my house with a car wash brush attached to a garden hose. There are soaps for different types of siding. I get the impression that power washing is both dangerous to our homes and overrated.