And for what its worth, this is what was done to my house at some point in its past, before I moved into it. It appears the old layer of shingles may have even been left on (although I'm not completely sure).. .they just framed over it, insulated, decked it, and singled over all that. Kind of a "double roof".
Insulation is the key. I don't even have any kind of air conditioning (air conditioner, swamp cooler, etc) in my place, and it rarely gets so hot that I wish I had one.