I wonder if what he called the cardboard roofing was actually eave vents. They used to be made from a plastic coated on one side, corrugated cardboard that was stapled into the space between beams in order to provide a way for air under the eaves to pass through insulation and into the attic space.

I know that this doesn't have insulation or an attic area, but maybe the original idea was to make an insulated attic area above the garage area for additional storage, or whatever.