To provide the building method for a big roof by a rational bending construction method, in which jacks are not required at all or are hardly required at the top section of a bending working platform and which has excellent profitability and workability.

The big roof to be built is ground-assembled in block roofs. The block roofs are constructed on mutually separate supporting members 4 and a bending working platform installed among these supporting members 4. Cables 12 are disposed in tension among the lower end sections of bundle materials 10 and both end sections of the block roofs while the bending working platform supporting the block roofs is disassembled gradually. Accordingly, since the whole block roofs are moved downwards while the bundle materials 10 are moved downwards, tension at the first stage is introduced automatically to the cables 12. Roof finishing materials 16 are laid gradually on the upper sections of the block roofs. The bundle materials 10 are further moved downwards by increasing their own weight of the block roofs by laying the roof finishing materials 16, thus introducing designed tension Ts to the cables.