Having one center post holding up a ridge beam was a common design in England during certain time periods. These frames are still standing. The center post is called a "Crown post" and it can hold up a structural ridge beam or a beam that runs from gable to gable under the collar beams, thereby holding up the roof and reducing the thrust at the plate. With collars you are also supporting the rafters near the sag point and this would be a good thing considering the load of your roofing materials.
Your design needs to be looked at very carefully as there are many factors in play.
Jim Rogers
