Given the roof is defined as part of your lot and not common property, you are individually liable for its maintenance and potentially for consequential damage to other parts of the building. This makes your situation atypical in strata ownership.
The challenge is that most building insurance assumes structural components like roofs are under the body corporate’s remit. You will likely need to obtain bespoke coverage from a broker specializing in complex or non-standard strata property. Consult a solicitor with expertise in Victorian strata law before proceeding with the purchase. This is not a risk you want to walk into blindly.