Face the leap of faith (seen here: http://www.goonskate.com/video.php?id=5 ) and follow the wall to the left until you reach a square (like 2m x 2m) section that doesn't have fence around it.
Jump up on that, it can be tricky. Once you're up, set a session marker. Then do tic-tacs to build up a bit of speed and jump over the fence. You'll have to use the grab glitch to land because the roof is pretty far below.