Ferrocement is an excellent, well tested building system used throughout the world. This post explores two earthbag structures on our Projects pages that have ferrocement roofs.
For small projects, simple curved shapes of ferrocement make a good roof. See EarthDome House. In this project they used rebar and mesh for the roof and recycled polystyrene for roof insulation. The author provides sufficient detail to enable DIY builders to build a roof like they did.
For large structures, see the method used on the Large Arizona Shop. They built arching trusses fabricated from 5/8″ rebar. Trusses were covered with expanded metal lath over stock panels, wired on using safety wire. The roof was coated with 1/2″ non-shrink construction grout.