This has caused me to give some amateur engineering thought to this topic. Essentially the roof system becomes one big beam, with a plywood top and a plywood bottom. with a 1.5" spread, the roof probably does not need much in the middle, as the tension and compression strength is provided by full coverage plywood to accept those strains. If the ribs are 3/4 wide by 1.5 inches tall, it seems the major concern is that they be true when installed, and they should remain that way.
And, the only reason I would use 3/4 wide is so I could hit them with a brad nailer... otherwise, 1/4" ply every 6 inches would probably work. This thinking is based on the cardboard interior of those cheap doors that are very rigid. The only real load on the roof is snow in the northern areas.
And one of those cheap doors spans more than 5 feet. If I was not building a conventiional curved tear, but something like a weekender, I might even consider using those doors as a roof system. Put some cpes on each side, and it would make a very rigid roof.... but, no standing on it.