you should definitely design your structure, or any structure for that matter, with at least a 2 foot overhand at all sides. Unfortunately, people frame things with a small (16 inch or less) overhang. Sure it costs more, but the price far our weighs the protection your building will get from the elements as well as provide a great area to get artistic with exposed rafter tails and nice stained pine soffits. If the design you are trying to use as a reference does not have good overhangs, either redesign the frame, or use 2x4 sleepers across the entire roof system to get additional framing for it. Cheers!