Cleaning my roof

I don't use one on the roof. There's a risk of damaging something and it's not really much more effective then just washing it with a brush. A pressure washer is a pretty powerful tool, even on low settings, and damage can be done in the blink of an eye. There's a chance that you can drive water into a place where it shouldn't be. It can turn little cracks and voids into big problems. The only time I'll use a pressure washer on the roof is if there's a problem that's best solved by it. For general maintenance I use a brush, soap, and a hose.