How do you clean a roof exactly? Well, it depends. There are all different types of roofs, ranging from clay tiles roofs, cedar shake roofs, composite roofs, etc. There's also other factors like roof pitch (how steep is your roof?), how many stories the building is, etc. I'll explain how to clean each roof type.
I'll start with composite shingled roofs since they're most common. An estimated 85% of all American houses are built with a composition roof because of their performance, as well as their price. The correct way to remove moss safely and efficiently from a composite roof is by hand using a wire brush. It's recommended to remove approximately 80-90% of the moss to avoid removing too much of the composition granules off of the shingles. After moss removal, blow off all lose moss/debris with a gas powered leaf blower. If you're having your gutters cleaned too, I personally would clean them before treating the roof with a zinc sulfate, this will help me avoid getting zinc sulfate all over my gloves/hands which can be unhealthy. This zinc sulfate will kill any remaining moss, as well as preventing it from growing back faster. Never pressure wash this roof type! Yes, maybe it's an easier way to clean the roof, but it is very damaging to the roof! Pressure washing a composite roof removes too much of the composite granules embedded into the asphalt shingle, causing the asphalt layer underneath to be exposed. When the asphalt is exposed to rain and sun, over time it'll start to crack and form holes. Cracked shingles falling apart leads to roof leaks and spending money on repairs. So pressure washing a composite roof is only a good decision if you want to pay to replace it sooner.
How does one clean a clay tile roof or a cedar shake roof? Well, pressure washing these types of roofs is actually the best way to clean them. It's safe, efficient, and it'll make your roof look amazing. Yes, you can try removing the moss the same way as a composite roof by hand with a wire brush, but it's more difficult and time comsuming.