Having been in the business for 21+ years, I would recommend that you contact a local sunroom dealer. As you will see by the picture below, a structure such as the one you are trying to build can be achieved by using the light weight structural roof panels used in that industry.
CraftBilt Manufacturing from the Philadelphia, PA area is a manufacturer of such product and we've been utilizing their products for 16+ years. They have dealers throughout the US and Canada so I suspect they have one near you. Perhaps a visit to
www.craftbilt.com will help you locate such a dealer.
They manufacture 3", 4 1/2" and 6" panels in both an expanded polystyrene and honeycomb......the thicker the panel the greater it capacity. The panels are typically 3 ft wide and go up one at a time making this a relatively easy and very quick solution. The difference between the two types of panels pertains to their use in the sunroom industry......it will not be an issue in your case.
Being from Grass Valley, Ca snow loads will be important to take into consideration. As I am writing you from Canada, snow loads are something we deal with for each project and the CraftBilt products serve us well and that leads me to believe that you should be very pleased with the finished product.
I hope this helps you down the path to achieving your goal.
Kind Regards,
PS.....more pictures can be viewed at
http://www.gaudetsaluminum.com/shade_structures.html