The basic criterion of a moisture-resistant roof is that the accumulation of moisture must not be heavy enough to cause condensation on the roof. Should the moisture condense, the roof structure’s insulation could be damaged, and its effectivity in protecting your home against the outdoor elements could be reduced.
Note that even if the roof was well constructed, a combination of UV radiation, age, history of leaks, temperature variations, and gradual sagging could increase drainage defects. For this reason, it is highly vital to have the roof inspected and maintained regularly. Also, the contracted designer could determine if your roof will need replacement in due course.