Go to the box on the helipad and hide on the side closest to the stairs.
Crouch in front of the part that has a mini-box on it.
Hold the left trigger and lean out to the right, now release all the buttons and now press forward on to the box!
You are on the top of the box.
Follow the fence around until it leads you to the roof.
Common Myths: You do not have to be Elite and you CAN be killed while glitching on the forbidden lands.