Take a section 162 complaint against the Owners Corporation for failure to maintain and repair common property. Yes, I know you are taking action against yourself but you need to let NCAT decide if something needs to be done.
Don’t let the “I’m Alright Jacks” bully you. Make sure they know that their instransigence is likely to cost the OC even more than it should.
By the way, pebbles are a simple and effective way of dissipating heat from roofs