my guess is that the roof is not to stop rain, but to help keep the sun off the bus. Having an air space between the upper and lower roof helps to significantly reduce temperatures in a vehicle.
I am pretty sure range rover or land cruiser used this concept in some of their hot weather vehicles prior to the common use of air conditioning in vehicles.
having a difficult time finding the photo i want...but here is an aftermarket gizmo designed to do exactly that
Linear Mode
