in the early 60's GMC and MCI built double roofed buses that where shipped to places like egypt and the middle east where its all desert. the purpose was to be able to air condition them. with out the air gap between the roofs they found it almost Impossible to keep them cool. having said that, what your wanting to to isn't that. we build tiny houses on bus chassis. we strip the bus body off right behind the driver and replace it with a house. we use metal studs and framing, with metal on the roof. we use a bonding agent from 3M to bond all the pcs. together instead of rivets or screws, the reason is that the bonding agent will absorb the road vibrations and stay together where most other fasteners will work lose over time. if you want to lay a roof over the bus roof as has already been mention you have to watch your weight to prevent a top heavy un-drivable coach unless you put training wheels on it or a active suspension. it can be done and its not that problematic. need to address a couple of issues and your off. using metal studs is a great way to do it and cover it with metal tin or aluminum i have also sean a person who bonded wood shingles right to the metal roof of the bus. it was a very interesting and different look but worked well for many years and it didn't leak also added insulation value to the roof he used the same 3M product to bond the shingles to the metal where the sides meet in the middle of the bus he ran a ridge row.