Geometrically complex building envelopes are typical in contemporary architecture. The conventional approach during their design is to provide a succession of layers and materials in their build-up, each one addressing a particular requirement (thermal, structural, water tightness etc.). This approach often becomes problematic and costly. Glass fibre reinforced polymer (GFRP) sandwich panels consisting of GFRP laminate face sheet and adhered to a polymer foam core can potentially provide an integrated, loadbearing and lightweight solution for geometrically complex building envelopes. One of the barriers to their uptake is the lack of data on their durability and long-term performance in service conditions. Facades are inevitably exposed to transient environmental conditions, involving several degrading weathering agents which in the field often exacerbate each other’s individual damage. A thorough understanding of the influence and synergistic effects of these agents is essential for an economic and safe design that accounts for the long service-life expected for façade panels. This thesis, firstly, provides a state-of-the-art review on FRP applications in the civil sector with focus on architectural projects. The literature review focusses on the long-term performance of FRP materials subjected to ageing parameters comparable to the natural weathering, which corresponds to the load regime the targeted application, a façade, will be subjected to under real life conditions. A comprehensive summary of the material degrading mechanisms resulting from the interaction of environmental and mechanical ageing factors is composed. Based on the outcome, an environmental ageing program is developed for the consecutive experimental investigation. The long-term durability of GFRP sandwich panels subjected to façade-like weathering are investigated under laboratory conditions. The GFRP specimens are subjected to a total of five weathering agents (1) abrasion, replicating debris and sand impact onto the outer skin of the building, (2) moisture only for the evaluation of rain/ high humidity impact, (3) moisture combined with temperature (freeze-thaw-cycles) as a realistic replication of temperature deltas specifically between day and night times, (4) sustained load mimicking the influence of adhesively attached cladding material or supporting structures, and, lastly, (5) fatigue load replicating the influence of wind load onto the building envelope causing component vibrations due to the nature of wind directionality and speed change. In the case of a façade system, UV radiation as a result of sun light exposure was deemed to have only a minor effect on the structural performance when system was to be protected by a UV prohibiting coating whilst being subjected to regular maintenance. Despite the initial hypothesis that material degradation would occur under the developed ageing regime, it was found that the investigated sandwich panel consisting of glass fibre reinforced laminates and a PU foam core were durable and more resistant towards the ageing parameters than anticipated. Degradation effects were within reason or negligible for most ageing conditions or ranged within the expected variation. Face sheet degradation that was measured in the laminates-only tests had no distinct effect on the overall performance of the sandwich panel when evaluating the sandwich panel configuration. This is attributed to the sandwich panels failure more which is not governed by the face sheet’s strength. The effects of thermal cycling were the only results that showed consistency, although the magnitude of degradation could be considered acceptable over the life span of a façade system.