The active ing in spray and forget is sodium percarbonate. Its not going to stick to the roof or prevent anything. It turns mildew orange when it kills it and the rain rinses away the dead algae. It has no residual that I know of. I could be wrong however...

AC

I have used it, although not on a metal roof, but on a shingle roof and also brick. It worked as advertised, just took a long time. I another case it did not work, which could of been my fault in dilution or something. I was mainly playing around with it....