Responsibility for the roof and basement

Hello
I have had a good look through the forum but haven't found the answer to my query so hoping someone can help.

There are 3 flats in my building, and the top flat has access to the roof terrace. This area is demised to the leaseholder. Can someone clarify the responsibility for a demised roof terrace - is it for just the terrace, ie the decking, railing etc or everything about the roof including the felting, ensuring it is water tight etc?

Likewise if the basement is demised to the ground floor flat and there is an issue with water seepage, is this the leaseholder or freeholder responsibility?

Which flat would have responsibility for the main roof of the house, the top flat, or the freehold?

Thanks in advance