When you start the level, go back up by the helicopter, and by the stairs there is 2 electrical boxes, go to the left one and press LT (left trigger) to get up against the electical box.
Then once you are up against it press to the left with the left analog stick while holding LT then your guy should be floating in the air [dont let go of LT].
Then once you are floating press up on the left analog to look up over the boxes (although you are already over it) then quickly press down on the left analog stick and you should be up on top of the boxes.
Once on top of the boxes just folow slowly on top of the railings until you have walked up on top of the roof.
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