Western Australian builders do not have a strong grasp of roof plumbing details.
Architects need to thoroughly understand different roof products with respect to their weather performance. For example flat slate roof tiles are significantly less weather resistant than those with deep profiles. Flat tiles are not as strong as deep profiled ones.
Roof plumbing details not otherwise covered by acceptable solutions in the Australian National Construction Code should be specifically detailed within design documents.
An example is a valley gutter between roofs of different slopes as with dormers projecting out a main roof. Where no special details are provided the builder builds as if it were a standard valley gutter. It is likely to overflow into the flatter sloped roof space.
There are buildings that use secret gutters along parapets wall/roof edges without aprons. Secret gutters are not suitable as a single barrier to protect internal living spaces.