To provide a method for manufacturing a covering, which enables a lightweight main curved beam with high durability and workability to be easily worked while keeping the whole dimensional accuracy uniform and which can enhance productivity of the covering with the built-in main curved beam.

A composite board material 110 is formed of a material having a plastic foam reinforced with inorganic fibers. The composite board materials 110 are fixed in the state of being stacked one by one while being deformed so as to have a curvature radius smaller than a preset and predetermined curvature radius. After a main curved beam 11 is manufactured by stacking the predetermined number of composite board materials 110, the main curved beam 11 is immediately detached from a regulation member 2.