Soft washing is a cleaning method that uses a low pressure washing technique, along with a mixture of algaecides, bleach, surfactants and water, to safely clean exterior surfaces of organic stains, mildew and algae.
During roof cleaning, the focus is on killing and removing the algae or moss growth. Chlorine bleach, the same kind you use to whiten laundry or scrub your toilet, is enough to kill off moss and algae spores. The recommendation is a 1:1 ratio of bleach to water to clean a roof. They also advise against using a pressure washer on asphalt shingles, as the force of the water can damage them.
After being allowed to sit on the roof surface for up to 20 minutes, the cleaning solution should be rinsed away with clean water. A few rain showers should be enough to wash any remaining algae off the roof. The person cleaning your roof might also use a leaf blower to remove any leftover pieces of moss.