This study deals with calculating and comparing the bending moments of two types of corner joints for commercially produced wooden sashes. The triple tenon and mortise joint was compared to a dowel joint on the sash of a window profile IV 92, made of spruce glued laminated timber. On the testing machine, the maximum force in the angular plane of a window sash under compression or tension mode was applied, and the measured values were converted to the bending moments. A significant difference between the bending moments for the mortise and tenon joint, and dowel joint were determined. The dowel joints achieved 167 Nm in a tensile test and 168 Nm in a compression test, while the mortise and tenon joint achieved 344 Nm in a tensile test and 325 Nm in a compression test. However, a significant difference was not revealed between the compression and tension tests for both types of joints. The differences for both types of joints were explained via the different sizes of the bonded surfaces, which was higher for the tested triple tenon and mortise joint by 29%.