Looking at composite decking's structure
Premium quality composite boards have a protective outer layer, commonly referred to as a cap. This cap is usually made from plastic and resin and provides a durable and moisture-resistant shield. It helps to prevent water from seeping into the board's core and protects it from stains, fading and mould growth.
However, the cap is not entirely impervious to water and may still be susceptible to moisture absorption over time. The quality of the cap can affect how water resistant the board is, so it pays to source your materials from a trusted, reputable, high-quality supplier.