All roofs are different in structure, size and orientation. Ideally, the orientation of the solar panels placement in the Southern Hemisphere will need to face a Northerly direction to receive optimal energy, though if the roof structure does not allow for optimal placement of the solar panels then there are other options which are considered for the loss of efficiency, for example adding a roof mount or extra solar panels. Your roof also needs to be free from shade from trees, mountains or other buildings.
Most roof structures are stable and reliable for solar installations. A structural engineer will have to look at the roof to ensure it is suitable for installment.