On the face of it, it would seem very strange if the roof (which doesn’t look accessible) were part of the top floor’s lot, and not the common property of the building. Even in the internal area of the domes are part of the top floor lots, it’s no different to the internal area of a room under the roof of any top floor apartment.
We had a similar thing in our body corporate – some owners queried fixing a roof leak into a top floor owner’s lot – seemingly not realising the same roof sheltered their own lots!
It’s even stranger given there had clearly been a sinking fund for the issue, and that VCAT still sent to mediation rather than just issuing a determination. Could make interesting reading if the case notes are made public, perhaps there’s some nuance present that isn’t public yet.
If I were being asked to cough up 100k, I’d just put the furniture in that room into storage, let the water through, and see how long the owner underneath lasted before agreeing the roof is common property!