I did a calculation a while back and came up with about 750 pounds for my roof (11-window International CE 300), not counting the removed headliner. 16 ga. steel sheet is 2.5 pounds per square foot, and the roof is about 8' x 27.5" inches per window, or about 18 square feet per window. So my 11-window bus has about 202 square feet of sheet for about 500 pounds. The rear cap plus the cab area is maybe another 50 square feet and 125 pounds (625 total).
The hat channel ribs are equivalent in material to 1.5" square tube with 16 ga. walls, which is about 1.25 pounds per foot, so each 8' section of rib is about 10 pounds; with 13 ribs that's another 130 pounds, or about 750 pounds total for my roof (I don't think the rivets are a whole lot, maybe another 10 pounds or so). The two longitudinal stringers are maybe another 50 pounds and the hatches don't weigh much at all, so maybe 800 pounds is a better estimate. A 40-footer RE or FE would probably be closer to 1000 pounds (or maybe a little over that).
Edit: whoops, I screwed up - my roof is 20 ga. (1.5 pounds per square foot) not 16 ga., so my roof would be more like 600 pounds than 800. Pretty remarkable that something this light is as strong as it is, but it shows that the shape of the material is a lot more important than the amount of it.