When constructing a steam generator, buckstay modules are joined with tube wall units to form buckstay and tube wall modules while the boiler steel structure is being erected, and they are temporarily placed within the boiler steel structure level by level. Then an external part of the boiler top main grid is constructed whose beams are flush with the tube walls of the buckstay and tube wall modules. Then the boiler wall is mounted by aligning and welding the individual tube wall modules from top to bottom. Simultaneously, the internal components are constructed on the boiler base and hoisted to installation height. Preassembling buckstay and tube wall modules eliminate the need for inserting buckstays or tube walls from above through the boiler top main grid. For this reason, the roof opening can be smaller than usual. The inner boiler top main grid therefore does not require its own main beams, which saves a substantial amount of steel.