Min thickness of Aly used I thin was 3/8" Usually 1/2" and up. I would have said that and have never been told weathewise it was not safe to walk on, In terms I would say temperature while welding/operating. Temps if the plate is welded say at 54degrees point a on the plate to point b on the stiffener when the 2 pieces of metal reached the same temp you had movement.
When big portions of plating, garbage room to door comes to mind, the plates area around were heated