Tim,
IMO
The construction can be in lumber, I-joist, steel bar joist, poured concrete, to name a few. The bottom line is what spans do you need for the design and geographic location for specific load calculations. As far as drainage is concerned it usually is on the low side of the roof pitch. but seriously it depends on the finished look of your design. Do you want gutters to show or not? that dictates how the drainage will be designed weather external or hidden.
regards,
Michael