I need to replace my old tar shingles and am considering putting a metal roof over them. Complicating matters I would like to do this by nailing 2X2's through the tar shingles to the old roof joists then put horizontal strapping across the 2X2's to provide the base for the steel roof. In doing so I hope to create sufficient air space between the steel and the old roof to control ice build up in the winter and allow me to stuff insulation up under the old roof so that I can create cathedral ceilings inside the building. Has anyone heard or done such a project? Wil the space created by the 2X2's and strapping be sufficient for air flow under the steel?