Hi, I thought this type of query was discussed elsewhere on the forum recently, and can see only discussion on balconies 2/1/19 where there may be a point re your issue.
If I have this straight, thought I read elsewhere here that went something like: if a part of common property is up for repair that it’s the whole building’s responsibility. That presumes that the rooftop is one which can be used by others (but isn’t at present due to your special case), therefore all must pay for repairs to it.
Stands to reason in my experience, as a roof presumably is for everyone’s unit, not just one.
Others will know more.