Roof:I'm also baffled by 20d, but your others:
5a: 1st 3 words of clue give the 2, then take the "sides" of "casino" and say what they are
21a: 1, 3, 1, from 1st word, 2nd word, next 3 words of clue
28a: 3, then "manage" surrounding a modern kind of sign.
I hope that sheds enough light?