Let's settle the roof safety issue. It's a steel roof, it has at a minimum, 2x4 stringers underneath it on truss rafters. It is designed for snow load. NO ONE is going to fall through it. There is ZERO safety issues with a steel roof. Watching these guys walking around on it, I can tell it's built substantially better than the average pole building as the spacing on the stringers is close enough that they are not watching to walk on them specifically.
As a remodeling contractor who has built, repaired and replaced many roofs in my 40 year career, the chances of falling through a roof are near ZERO. It simply does not happen except in the movies. In all that time, I have gone through a roof with my foot one time, because it was a shingle roof that was in very bad shape, I was inspecting it knowing it was bad and very cautiously making my way around it. The shingles were intact over pulverized rotted plywood.
Back to the snipers on that roof. According to some sources, there was supposed to be a either a sniper or an observer on that roof watching which is why the sniper teams hesitated on shooting the assassin. That individual or individuals left the roof because it was "too hot"