We propose TensionFab, a novel technique for creating shape-changeable room-scale structures. This method employs easily available planar materials (plywood, MDF), cuts them into multiple 2D shapes, and then connects the pieces manually to create a unified structure capable of 2D and 3D deformations. Constructed TensionFab structures are characterized by easy achievability of target surfaces, substantial structural strength, shape changeability, time and material savings, and easy storage and transportation. In this paper, we introduce the basic principles of shape-making, module combination, and structural characteristics of TensionFab. We also developed a design assistance tool to allow users to automate design based on the target shape. We evaluate the design results for shape validation and structural performance. Finally, we validate the approach with actual construction and propose a variety of application scenarios. Overall, TensionFab is an efficient strategy for spatial design and structural organization. This paper contributes to research and exploration in the HCI room-scale interaction field.