Since no one else has mentioned this, I have heard that pressure washing a shingle roof will also shorten the life span of the shingles because power washing removes some of the protective top coating. I think steam cleaning is a safer way to clean a shingle roof. [green]
Steam cleaning the roof will heat up the shingles too much and two things will happen. The grit will come out and the shingles will melt. I've had shingles stretch and melt when doing roof repairs just from regular hot summer days.