shooter had to go there days before and scouted it out. Still cant believe he got that close. Even CNN is roasting the SS for this.
Looking at the GE image again, there appears to be a low-roofed connection between the buildings - maybe 8 feet tall judging from the doorways.
Lots of wood pallets on the loading dock. He could have dragged one of those over to the 8-foot tall flat roof and used it as a ladder and then pulled himself up. And then go up another 3 feet to the gently sloping roof. Two trees are also between the sidewalk and the flat roof section so he could be mostly hidden as he climbed up.
But it sure seems he must have planned it out to some degree, and knew (or lucked out) how the stage would be set up.