Expanded Polystyrene foam (EPS) is a chemically inert and 100% recyclable material that is lightweight and has a good compression strength per weight ratio; however, its current construction use is mostly limited to insulation or landfill. The key concept of this paper is to develop an EPS composite to create an ultra-lightweight long-span sustainable roofing structure by integrating the minimum necessary structural tension layer with a certified fire protection system. The authors present this concept in the following four steps, 1) EPS composite structural specimen test, 2) structural optimisation of the reversed displacement model, 3) discretisation with developable surfaces and 4) CNC hotwire rapid prototyping and assembly in scaled prototypes. The Cloud Arch is an economical, material-efficient, thermally insulated, quickly assembled ultra-lightweight construction that eliminates the need for formworks for long-span structures. It can be applied to many types of column-free spaces, such as in factories, gymnasiums, markets and cafeterias.