Lithium metal batteries are the most promising candidates for next generation portable devices where higher energy density is necessary. However, dendrite growth on top of Li anode that has highest energy efficiency hinders practical application of lithium metal batteries. In this regards, employment of protecting layers such as tough membranes or solid electrolytes can be candidates to inhibit dendrite growth and to achieve a long-life cycling performance. Herein, we propose a novel analytical method that measures shear strength of the protecting layer in the out-of-plane direction. Since the lithium dendrite grows in the vertical direction from the anode surface, the shear modulus in the thickness direction of the protective film is an indispensable method. In particular, the stress can be calculated from the axial force that is measured by a fixed solid probe during thermal expansion. Similarly, the thickness change is measured directly from the thermal expansion. The obtained stress and strain can also be converted to shear modulus. This method is applied to a series of poly vinyl alcohol (PVA) thin protecting layers of varying salt contents. The expansion force and thickness change are measured to be reduced with the increase of salt ratio for declining in the elasticity of the thin layers. Initial capacity and cycle performance of lithium metal batteries with PVA protecting layer are improved proportionally with the increase of stress and strain. The newly introduced analytical method demonstrates the measurements of mechanical properties in the thin protecting layer relevant to the direction of dendrite growth.