I can answer this one Robert, no we havn't tried it on the roof yet, mainly because i remember the customer saying, when we spoke to him, that all the rainwater was harvested into an underground tank and he thought that no chemicals or solutions could be used. I aim to find out more from him next time we are there as i presume the harvested rainwater is not used for drinking water (would be some filtration plant if so!). If we do try it, we will post results up.Ian did you try this out on a roof?
If you did what was the result ?