What Is Composite Decking Made Of? A Plain-English Guide to the Materials

Composite decking has become the most popular decking choice for UK homeowners in 2025 -but many buyers still are not entirely sure what it is actually made of, or why that matters when choosing between products. The manufacturing composition of a composite board directly determines how long it lasts, how well it resists moisture and UV, how much it expands and contracts with temperature, and how convincingly it replicates the look and feel of real timber.

In this guide from Paving and Slabs Ltd, we break down exactly what composite decking boards are made of, how they are manufactured, what the key differences between products are, and what you should look for when buying composite decking for a UK garden.

The Two Core Ingredients in Composite Decking

All composite decking boards are made from two primary materials combined together:

1. Wood Fibre (or Wood Flour)

The wood component in composite decking is typically recycled wood fibre or wood flour -a fine, dry wood powder produced from waste timber offcuts, sawmill by-products, and reclaimed wood material. Using recycled wood rather than virgin timber is one of the reasons composite decking is considered an environmentally responsible choice -it repurposes material that would otherwise go to landfill or be burned.

The wood fibre gives composite boards their natural appearance, surface texture, and thermal properties. It also contributes to the board's rigidity and structural strength. However, wood fibre is inherently absorbent -which is why the polymer component and the capping layer are so important in protecting it from the UK's persistent moisture.

2. Polymer (Plastic)

The polymer component is typically recycled polyethylene (PE) or polypropylene (PP) -the same plastics used in packaging, bottles, and consumer goods. Using recycled polymer further reduces the environmental footprint of composite manufacturing and gives the finished board its moisture resistance, UV stability, and structural flexibility.

The polymer binds the wood fibres together and, in the manufacturing process, partially encases them -creating a material that is denser, harder, and far more moisture-resistant than wood alone. The ratio of wood fibre to polymer varies significantly between manufacturers and product tiers, and this ratio is one of the most important performance determinants in any composite board.

Wood-to-Polymer Ratio -Why It Matters

The blend ratio of wood fibre to polymer significantly affects board performance:

• High wood fibre (60–80% wood): Cheaper to manufacture, feels more natural underfoot, but is more susceptible to moisture absorption, swelling, and surface mould growth. Common in entry-level and budget composite boards.
• Balanced blend (50/50): A good middle ground offering adequate moisture resistance and a natural feel. Used in most mid-range composite products.
• High polymer (60–70% polymer): Maximum moisture resistance and dimensional stability. Less susceptible to swelling or shrinkage with temperature and humidity changes. Used in premium composite boards designed for exposed or coastal UK installations.

For UK gardens -where rain, damp, and freeze-thaw cycles are the norm -a lower wood fibre content combined with full capping delivers the best long-term performance.

Additional Additives in Composite Decking

Beyond wood fibre and polymer, composite boards also contain a range of performance-enhancing additives:

• UV stabilisers: Protect the board surface from solar degradation and colour fading. Higher concentrations of UV stabilisers correlate with better long-term colour retention -one of the key quality differentiators between premium and budget boards.
• Biocides: Inhibit mould, mildew, and algae growth on the board surface. Premium boards have better biocide treatment than budget alternatives.
• Pigments: Provide the board's colour and surface tone. Through-coloured boards (where pigment runs through the entire board body) are more resistant to colour changes from scratching than surface-only coloured boards.
• Lubricants and processing aids: Used during manufacturing to ensure even blending and extrusion of the composite mixture.
• Fire retardants: Added in some commercial-grade composite products where fire performance standards are required.

How Composite Decking Is Manufactured -The Extrusion Process

The manufacturing process used for composite decking is called extrusion. The wood fibre and polymer are dried, blended with additives, then fed into an extruder -a machine that heats the mixture and forces it under pressure through a shaped die. The die determines the board's cross-sectional profile, including any hollow chambers, groove patterns, and surface texture.

As the extruded board emerges from the die, it is cooled, cut to length, and embossed with a wood-grain texture while still slightly warm. This embossing process is what gives composite boards their realistic timber appearance.

The capping layer (on capped boards) is applied during a co-extrusion process where the polymer shell and the composite core are extruded simultaneously -the cap bonds molecularly to the core rather than being glued on separately, which is critical for long-term adhesion.

Capped vs Uncapped Composite Decking -The Key Structural Difference

Understanding the difference between capped and uncapped composite boards is one of the most important buying decisions you will make:

Uncapped composite boards have a composite core that is exposed on at least some surfaces -typically the underside and sometimes the side edges. These surfaces can absorb moisture over time, leading to swelling, surface mould, and eventual degradation of the wood-fibre core. Uncapped boards are cheaper to buy but have a shorter lifespan, particularly in the damp UK climate.

Capped composite boards have the composite core completely enclosed in a protective polymer shell on all four sides. This cap is impermeable -it prevents moisture reaching the wood-fibre core entirely. Capped boards have significantly better moisture resistance, are more resistant to staining, easier to clean, and last considerably longer than uncapped alternatives. They are the recommended choice for any UK installation.

Hollow vs Solid Core Composite Boards

Most composite decking boards have a hollow-core profile -the board cross-section contains internal voids or chambers rather than being solid throughout. This reduces the weight and cost of material in each board while maintaining adequate structural stiffness for normal residential use.

Solid-core boards are denser and heavier, with better resistance to flexing under heavy loads. They are typically specified for commercial applications, high-traffic areas, or installations requiring closer joist spacing.

For domestic UK garden decking, hollow-core capped boards installed on correctly spaced joists (typically 400mm centres) perform excellently and represent the best value for money.

How Does Composite Decking Compare to Other Garden Surfaces?

Composite decking occupies a specific niche in outdoor surface design -it is the best material for raised platforms, balconies, and areas where a concrete sub-base for paving is not practical. For ground-level outdoor spaces, paved surfaces offer greater long-term durability:

• Porcelain paving slabs -non-porous, 40–50 year lifespan, zero maintenance. The toughest outdoor floor surface available.
• Indian sandstone paving -natural stone with 20–30 year lifespan, requires periodic sealing.
• Flagstones -large-format natural stone for paths and garden terraces.
• Granite paving and setts -virtually permanent, premium driveway and garden surface.

Many homeowners combine composite decking with paved areas -decking for elevated zones and patio slabs for the lower garden. For a full cost and value comparison: Composite Decking vs Patio Slabs -Which Is More Cost-Effective? and Break-Even Patio Options -Paving Slabs vs Gravel vs Decking.

Shop Composite Decking at Paving and Slabs Ltd

• Composite Decking Boards -premium fully capped composite, multiple colours, nationwide UK delivery
• Composite Fencing Panels -matching zero-maintenance boundary fencing
• Porcelain Paving Slabs -for ground-level patio areas alongside your deck
• Dark Paving Stones -charcoal and anthracite paving to complement dark composite decking
• Grey Paving Stones -mid-tone grey paving for a coordinated outdoor scheme

Frequently Asked Questions -Composite Decking Materials

Is composite decking waterproof?

Fully capped composite decking is highly water-resistant -the polymer shell prevents moisture reaching the wood-fibre core. It is not technically 100% waterproof as water can still enter through cut ends if these are left uncapped, which is why end caps and fascia boards should always be fitted at installation.

Does composite decking contain real wood?

Yes. All composite decking boards contain wood fibre -typically 40–70% of the board composition by weight depending on the product. This wood fibre is almost always sourced from recycled timber offcuts and sawmill waste rather than virgin timber.

Is composite decking recyclable?

Composite decking is technically recyclable but is not accepted by most standard recycling programmes due to the mixed-material composition. Some manufacturers operate take-back schemes for end-of-life boards. Check with your specific board supplier for guidance on responsible disposal.

Why do some composite boards feel hollow underfoot?

Hollow-core composite boards have internal chambers in their cross-section, which can produce a slight hollow sound underfoot -particularly when the joist spacing beneath is wide. This is normal and does not indicate a structural problem. Choosing a solid-core board, or reducing joist spacing to 300mm centres, eliminates this effect.