House Washing consist of killing off organic growth such as algae, mold and mildew with chemicals safe for your home and environment and then removing them along with any dirt, dust and other debris. A simple pressure washing does not kill the organic growth. It may appear to be removed but the organic growth is still alive and will return much faster than effectively killing the growth first.
Roof Washing is similar to the house washing but for shingled roofs it requires a much stronger mix of chemicals to penetrate the asphalt and kill the organic growth. In this procedure the chemical is left to dwell in most situation and not rinsed as we do for the siding of the house.