Okay, I'll try to explain this as good as I can without screenshots. Follow the stairs up to the roof as normal, come out of the window and go between the two raised section you're trying to get onto. Stand on the slanted section between them on either side of the hole and sprint jump from there to the top edge of the windows you come out of to get on the roof. From there slowly move forward until the wall starts to push you away from the building. Then sprint jump and rapidly move the mouse in the direction of the top of the roof. It takes a little practice, good luck.
Here's some screenies