The roof material is called Polygal; it is plastic with 1 cm by 0.5 cm channels. There is even a double channel version available if you needed more insulation. I ran the channels vertically but seems like you could run in any orientation. There is an outside and inside to the panels, so pay attention when you remove the protective cover. The outside has UV protection of some sort. It is held in place by screws drilled through aluminum brackets. The screws have a neoprene washer to provide a water seal. Puckett Plants & Green Houses (http://www.puckettgreenhouses.com/) did a nice job of preparing the panels and brackets, although the shipping cost was almost as much as the material. The brackets or holders are aluminum H-channel to join two (2) panels together and J-channel for the ends.
There are other roof materials but I have been very impressed with how easy Polygal is to work with and how well it functions. During kiln operation, you can feel the heat come through the insulated walls while the roof is cool to the touch. It also seems very durable as it got whacked by a significant hail storm this spring with quarter-sized hail with no apparent damage.
I also put self-adhesive weather stripping between the frame and roof to reduce air leaks although the stripping did make it a little more difficult to slide the roofing panels into the brackets. I beveled the edges very slightly with a utility knife and the panels slid into the brackets a little easier.