What sort of a roof? Slates like? Check the back of your van, there's bound to be one in there somewhere. If Not i'd try a good hardware. Its really a wire brush with a nub on it, you'd need isn't it? And a scraper of any kind just for the agitation. Rub on the acid, scrubb with the brush, scraper for the corners or where needed, dwell, pressure wash off and repeat. Whats the issue? You are the minister for this type of stuff. I should be asking you. I have a picture on my site of a roof I done, its a sloped roof showing the before and after if thats what you had in mind.