The design process of membrane buildings and structural fabrics has to consider changes of the material properties due to long-term exposure to the environment. For the structural engineer the loss of tensile strength in the fabric, in the seams and in the mechanical fastenings is of major concern, further variations in tear strength and stiffness should be known. The degradation of the protective coating normally becomes apparent in the decrease of the optical properties, first. For basic synthetic materials the strength deterioration resulting from certain environmental impacts are known. But the prediction of properties for material compositions typically found in structural membranes are vague and a change of these properties during lifetime due to a time dependent environmental load spectrum containing UV radiation, condensate, rain, temperature and pollutants is not possible without experimental testing. Regarding long-term loading behaviour of structural membranes, creep-rupture or time-tofailure-tests are state of the art. A reevaluation of existing literature and new experimental results show that these tests do not cover loads with limited duration like snow. The objective of the present paper is to broaden the data basis for long-term behaviour of typical structural membranes for both environmental impacts and mechanical loading. Both aspects are investigated in contribution to the development of a new European design standard for structural membranes. PVC-coated polyester (PES) fabrics and PTFE-coated glass fiber fabrics are in the main focus of the standardization work as they cover the gross market for structural fabrics. In this paper, environmental impacts to the top coat, particular due to humidity are discussed for PES/PVC and glass/PTFE fabrics. Furthermore, effects of “long-term loading” but with limited duration are presented for glass/PTFE fabrics. Moreover, consequences on the linked strength reduction factors derived from the test results are analyzed and the implications on the design concept and test methods are discussed.