I was wondering what you guys would have written, as there can be several scenario's. The roof is a model. Just a 'sheet' of tiles, that I can hover over a roof, angle and cut at wil. Not very highpoly, but just good enough for decent roofs. I tried a bumped texture, but was only good enough from above. I doubt if forced displacement will work good enough, and then I'd have to separate roofs from buildings... tedious.
