Could you say 'The cats are above on the roof.'. I know it's bad grammar but I assume it's still possible..?
That's what I wrote but it's marked as wrong. To me "the cats are above, on the roof" seems fine though.
Upstairs implies the proper levels of the living space of the house that's reached by the stairs, which doesn't usually include the roof. Though actually, there are buildings which do use the roof like that, so in that context, where the stairs reach a roof garden or structure, it would make sense.
You can't really say that in English. The cats are up on the roof, or the cats are on the roof.