I'm sorry, but that is a load of twaddle. You're seriously worried about green streaks on the roof? Oh no, our PVs have some lichen growing along the bottom edge, what am I to do, they've only been there twelve years and we're in the wet and windy South West. We're going to be building a flat roofed extension as well, there's no hope. Perhaps we'll be alright though, as technology has moved on and there are now roofing materials that allow flat roofs to be built safely.