You are mixing a few things here, flat roofs have nothing to do with brutalism in particular or Le Corbusier - they existed long before both. Maybe you meant modernism in general?
Anyway, for anyone wondering about all these flat roofs in modern architecture I recommend watching this video:
Why Do Architects Insist on Using Flat Roofs? - Stewart Hicks https://www.youtube.com/watch?v=QW0ydAMVQ2w