Roof, glassing for skyscrapers

[AlexJK]

What is kind of glassing used in skyscrapers especially in the roof construction?
Of course, there may be various materials like triplex glass, different cases require different solutions for glassing, but what is the most effective, safety and the least expensive? What is the material which combines described qualities the best way?
And one more necessary question is about how to clean roof's glassing. Are there some water system or machinery cleaning by movable robots?
Who has the deal with these questions?