Pressure washing a roof is very dangerous and ineffective. When pressure washing a roof at 3500-4000 PSI, you run the risk of damaging the tiles, cement, or granules on an asphalt roof. This can leave thousand of dollars in damage if a roof needs to be replaced. Pressure washing does not kill the algae inside the pores of the tiles and/or shingles. Because of that, the algae will grow back within months of it being pressure washed.
Soft washing is very safe and effective. Soft washing a roof allows the chemicals to get into the pores and kill the roots of algae, leaving the roof algae free for years. We apply a mix of chlorine, water, and a surfactant at a very low pressure of 70 PSI compared to 4000 PSI.