I added a second layer of mache to the test roof yesterday. This was a smooth mache mix which -after it dries in a week or so- will be touched up with joint compound and then painted. Although the layer turned out well enough it was frustrating to build. Partly this was because I was mixing mache in small quantities (about a gallon at a time) and partly because I didn't start with a good technique for spreading the mache, especially around the edges. I used my hands again to push and mold the material, a process that is both very time consuming and leaves behind marks of the hand working. To use this material at a larger scale it will be necessary to involve hand tools (like trowels) to spread and mold the mache. My next experiment will be in this area.
I'm still working out how to handle edges. In this case I will sand/shave off the uneven mache along the edges once the material has dried. Definitely looking for a better solution.
A couple of images of the roof with the second layer.