Cold-formed steel Z-sections are widely used as roof purlins in industrial steel buildings. Due to their thin-walled nature, these members are highly susceptible to local, distortional, and lateral–torsional buckling, which often governs their ultimate load-carrying capacity. The flexural strength of Z-sections primarily depends on the mechanical properties of the material and the geometric characteristics of the cross-section. This study experimentally investigates the influence of web stiffeners on the biaxial flexural behavior of cold-formed Z-sections. Three specimens—two without stiffeners and one with an intermediate longitudinal web stiffener—were tested under simply supported conditions to evaluate the effect of stiffening on buckling behavior. The purlins were tested individually, without lateral bracing or roof sheeting, and were loaded in a sloped configuration to simulate actual roof conditions. A concentrated load was applied at midspan, aligned with the shear center of each section. The experimental results revealed that the stiffened specimen exhibited greater ductility and delayed local buckling compared with the unstiffened specimens. The addition of the web stiffener changed the failure mode from local buckling at midspan to lateral–torsional buckling, thereby improving overall stability and load-carrying capacity. It is concluded that introducing a web stiffener effectively enhances the flexural performance and buckling resistance of cold-formed Z-sections by providing additional stiffness and delaying local instability.