I haven't used illustrator in years so I've forgotten the name of the tool you would use. I think it's a scatter brush I could well be wrong. It's basically a tool that places vector objects along a line. Just make, say 6 roof tiles and load them into the brush, set the brush to place the objects randomly and draw the lines - the brush will place the objects along the lines...sorry if this is not much help but I'm sure someone who uses AI will be able to explain it better.