A roof needs to shed the rainwater, especially in heavy storms. With metal roofs this can be helped a lot by installing “valleys” which are v-shaped tracks that channel the water into the drains. There’s a post by a Seattle roofing company ( there’s a lot of rain in Seattle )with a good pic at the Valentine Roofing Blog. It looks like it’s metal shingles on the roof they show. In Sydney we use corrugated sheets a lot which are also good channelers.