A surface of uniform thickness is made from a mesh by converting it to roofs - look for "Mesh to Roof"
Copy the original mesh and hide it to protect it for further editing and put the new roof parts on a unique layer to simplify future selection.
Oh wait "Mesh to Roof" is a Goodie, so you must download it special - Look under the Help menu for the link taking you to the Graphisoft website for Goodies.
I know.
Dwight Atkinson