Firstly, is the builder an expert in roofs or is he just giving an opinion.
Speak to the strata manager to get the committee to engage a footing expert for a report. If that fails you may need to engage your own expert.
With the experts report, raise it with the committee that based on the experts report the roof needs to be replaced.
Give them two months do do something.
If after two months nothing is happening then got to NCAT under section 232(2) and seek orders for the roof to be replaced. Section 232)2) is the clause that the OC has failed to carry out their duty.
The other order you want is that a special levy is raised to fund the roof repair