Wood-plastic composite decking, abbreviated WPC, is a manufactured decking board made from a blend of wood fibres or wood flour with a thermoplastic polymer matrix. Trex Company, founded in 1996 in Winchester, Virginia, pioneered the category by combining recycled polyethylene grocery bags and reclaimed sawdust into a decking product. Today Trex remains the largest WPC decking manufacturer globally, holding approximately 45% of the North American market, with major competitors including TimberTech, a division of Azek Building Products, and Fiberon.
Composition: Plastic Matrix and Wood Flour
WPC decking consists of two primary components. The polymer matrix is typically polyethylene, either high-density polyethylene reclaimed from post-consumer milk jugs and detergent bottles, or polypropylene, which offers greater stiffness and heat resistance at a higher cost. The wood component is wood flour or wood fibre, comprising 50 to 70% of the board by weight. This wood content gives the decking a more natural appearance and texture than pure plastic lumber, reduces thermal expansion, and lowers material cost.
The polyethylene or polypropylene encapsulates the wood particles, protecting them from moisture and fungal decay, while the wood stiffens the polymer and reduces the plastic-like appearance. Additives include mineral fillers such as talc or calcium carbonate for stiffness, coupling agents like maleated polyolefins to improve bonding between the hydrophobic plastic and hydrophilic wood, UV stabilisers, pigments, and antifungal biocides.
Capped vs Uncapped and Hollow vs Solid Profiles
First-generation WPC decking from the late 1990s and early 2000s was uncapped: the board had uniform composition throughout its cross-section. These products suffered from moisture absorption through exposed wood particles at the surface, leading to staining, mold growth, and surface degradation within 5 to 8 years. The industry's response was capped composite decking, introduced around 2010, where a coextruded outer shell of pure polymer, typically 1 to 2 millimetres thick, fully encapsulates the wood-filled core on all four sides. This shell is impervious to moisture and provides greatly improved stain and fade resistance. Capped boards now dominate the premium segment of the market and carry warranties of 25 to 50 years against staining and fading.
Board profiles are either solid or hollow. Solid boards have a continuous, rectangular cross-section and weigh more, requiring closer joist spacing. Hollow boards incorporate internal channels that reduce weight and material use while providing ventilation under the board. The hollow channels also accommodate hidden fastener clips that mount between boards, producing a fastener-free surface.
Thermal Expansion, Mold, and End-of-Life
The plastic content in WPC decking introduces significant thermal expansion, far greater than wood. Typical WPC boards expand and contract at a rate of approximately 0.04 to 0.06 millimetres per metre per degree Celsius, requiring end-to-end gaps of 3 to 6 millimetres depending on board length and expected temperature range. Butt joints must be installed on double joists with adequate gapping, or end-to-end connectors must be used. Ignoring expansion requirements leads to buckling on hot days.
Mold resistance varies significantly between capped and uncapped products. Capped boards resist surface mold effectively because the polymer shell provides no organic food source. Uncapped boards with exposed wood particles can support surface mold in damp, shaded conditions, though periodic cleaning with sodium hypochlorite solutions restores appearance. The antifungal additives compounded into the wood-plastic core slow internal decay but do not prevent surface mold entirely.
End-of-life remains an unresolved challenge. WPC decking is not accepted in municipal recycling programmes because the intimately mixed wood and plastic cannot be separated economically. Decommissioned boards are typically landfilled or, in some regions, incinerated for energy recovery. The Trex recycling programme accepts manufacturing scrap for reincorporation into production but does not accept post-consumer decking. Research into enzymatic separation and pyrolysis continues but has not reached commercial scale.