The Mock Turtle

I need to clean my roof as black bits are coming off it. Can I just get up there and blast it with a hose or a scrubbing brush, or do I need to do something more technical please?

What kind of black bits? Bits of the roof?

Oh, I don't know! I just thought it was dirt. I will get up there and have a look.

I got a Man in and he blasted it with his hose It turned out to be just moss, thankfully.