In recent years wood-polymer composites appears at the European market more frequently, mainly in decking applications as an alternative for durable tropical hardwood decking. This study focuses on extruded decking products based on polypropylene (PP), polyethylene (PE) or polyvinylchloride (PVC) and wood flour. All products are commercially available on the Belgian market. Initially WPC’s were said to be resistant for biological degradation as the wood particles should be encapsulated by the polymer, but over the years several cases of fungal decay of WPC have been reported. Therefore the moisture behaviour of the different WPC decking materials was first assessed by various moistening methods to determine if the moisture content can reach levels that initiate fungal growth. Concerning this, WPC shows better results than the traditional wood composites like particle board, MDF or OSB, but clearly absorbs sufficient moisture to be critical if sorption time is long enough. Therefore, when biological durability is tested, an adequate standard, more specific for WPC products, is needed in stead of just copying the standards designed for wooden panel products. For WPC materials a moistening pre-treatment is needed prior to the proper fungal test to include moistening time as a critical factor for assessing biological durability of these materials. Furthermore placing the products in a fungal control unit that evaluates the susceptibility for airborne surface stains revealed different results and showed that fungi grew earlier and faster on weathered samples. Concluding, in spite of the different composition of the tested products, no product was significantly better or worse than the other concerning the moisture behaviour. Furthermore only a few products were to a small extent covered by moulds in the fungal control unit
