Fabrics have been used in tensile surface structures for over 50 years. Their unique properties, such as their high flexibility and low self-weight, make that these materials can be used in very efficient and architecturally unique constructions. However, because these materials are constituted of woven fibres and different interacting layers, deriving their mechanical properties unambiguously has proven difficult. Current biaxial testing methodologies differ between institutes and much discussion exists as to how the obtained test results should be interpreted. To quantify the impact of the used testing protocols on the obtained results as well as the method for deriving the elastic constants from said results, we tested fabrics following the method described in MSAJ M-02-1995 (Membrane Association of Japan [1]), one of the only official biaxial testing guides for fabric materials currently existing. By then changing the provided standard protocol, the direct impact on the test results could be quantified. Following these tests, material parameters were derived using various methods to show the impact of the used methodology in this step. This paper describes the research conducted on a PVC coated polyester material (Sioen T2107), quantifying deviations in both the immediate test results and the derived material parameters. As a closing, the biaxial testing protocol is adapted to match the expected stresses in a prototype structure, after which the obtained material parameters are compared to the parameters obtained through the general testing protocols. The results obtained from this research show a large impact when the standard protocol is replaced by a biaxial test protocol which mimics the expected stresses in a structure. And although the differences in the obtained results through variations on the standard protocol were not as big, the method for deriving the material parameters proved to have an important impact on the value of the obtained elastic constants.