The only way you can tell for sure whether or not the roof is common property is by checking your strata plan which should have been part of your purchase contract.
However, the chances are that it is common property as the roof is such an important part of maintaining the structural integrity of the building.
I doubt very much if the building warranty will be of any use to you unless the building is less than six years old and lower than three storeys in height.
So make your claim but remember the repairs will be done to the Owners Corp’s standards, not necessarily yours, so be prepared to compromise if you want the terrace to look as good as it might. By compromise, I mean put some cash in to get your choice of tiles laid, rather than theirs, if that’s an issue for you.