What Is Composite Decking Made Of? | Beyond The Wood Grain

If you’ve been shopping for a new deck, you’ve likely seen boards that look like wood but promise to skip the annual staining. That promise comes from what’s inside. Composite decking is not solid timber; it’s a manufactured product that combines natural fibers with thermoplastics to create a board that resists moisture, rot, and insects far better than traditional lumber.

The exact recipe varies by brand, which matters more than you might think. Understanding the core ingredients — and where products like capped composite and PVC differ — is the difference between buying the right deck and paying for the wrong one.

The Core Ingredients: Wood And Plastic

Standard composite decking, often called wood-plastic composite (WPC), is made from two primary components: wood flour or sawdust and thermoplastic polymers. The wood fiber gives the board its natural appearance and stiffness, while the plastic provides moisture resistance and durability.

The plastic portion is typically one of these thermoplastics:

• Polyethylene (PE) — the most common choice, often sourced from recycled film or post-consumer plastics.
• Polypropylene (PP) — a stiffer polymer used in some premium lines.
• PVC — a fully synthetic polymer used in a separate category of decking (more on that below).

Beyond the two main ingredients, manufacturers add a small percentage of performance additives: pigments for color, UV stabilizers to fight fading, coupling agents to bond the wood and plastic, and lubricants to help the mixture flow through the extruder. Some brands also substitute other plant fibers — like rice hulls or bamboo — for wood flour.

How Composite Decking Is Made

The manufacturing process is straightforward at a high level. Manufacturers mix the wood fibers and plastic pellets with the additive package, then feed the blend into an extruder. The extruder heats the mixture and forces it through a die that shapes the molten material into the board’s profile. The board cools, is cut to length, and receives any surface finishing.

The result is a dense, uniform board with no knots, no grain direction, and no natural weaknesses. That uniformity is why composite resists splintering and cupping better than wood.

Capped Composite vs. PVC: Two Very Different Products

Not all “composite” boards are the same, and the distinction matters at the register. Here’s how the main types compare:

TimberTech’s composite product pages confirm that their standard line pairs real wood fibers with high-performance polymers, while their Advanced PVC line is zero organic material and uses a blend of virgin and recycled polymers. That’s the clearest illustration of the split: one is wood-plus-plastic, the other is plastic only.

The capped composite category deserves special attention. These boards feature a protective outer polymer shell bonded to the WPC core. The cap is engineered to block moisture absorption and resist stains, fading, and scratching — the three biggest complaints about first-generation composites. The trade-off is that the cap’s integrity matters; a deep scratch that penetrates the shell exposes the core to the elements.

What’s Actually In The Big Brands

If you’re comparing the two largest U.S. brands, the ingredient lists differ in a way that affects eco-credentials and price. Trex, the market leader, states its decking is made in the U.S.A. using up to 95% recycled plastic film and reclaimed sawdust. That’s a closed-loop story: the plastic comes from things like shrink wrap and grocery bags, and the wood fiber from industrial scrap.

TimberTech, by contrast, positions its composite line around performance rather than recycled content. Its standard composite blends real wood fibers with polymers, but the company doesn’t promote a high recycled percentage the way Trex does. Neither approach is wrong — it’s a question of what you value in a deck.

One caution: “composite” is an umbrella term in the industry. Some retailers group PVC decking into the composite category, but PVC contains no wood fiber at all. Checking the fine print on the product label is the only reliable way to know what you’re buying. If you’re at the decision stage, our tested roundup of the best composite decking boards compares the top options on composition, durability, and price.