When you are in any Hospital, in the room to the far left hand side where you normally have to open the door and can find loot (Opposite to the staircase) there is this one spot when facing the wall, to the right hand side of the door, that you get a ladder climb icon. You can exploit it and when you click to climb it, it glitches you trough the wall to the side ladder that leads you up onto the first level's roof.