We do not use a pressure washer on your slate roof. We use a biodegradable cleaner that safely cleans your roof. Because we use professional soaps, we are able to clean with much weaker concentrates. Other companies use too high of a concentrated cleaner and some even use a method which involves a “low pressure” rinse. What they don’t tell you is that try walk on your roof using a a machine that is attached to a pressure washer. We don’t have to walk on your roof to get amazing results, along with a 5 year limited warrant.