A Simple Look at the Materials Behind Composite Decking

Composite decking is everywhere in UK gardens right now - and if you're considering it for your own outdoor space, one of the first questions you should ask is: what is it actually made of?

The answer matters more than you might think. The specific materials and construction method used in composite decking determine how it performs in the real world - how well it resists UV fading, how it handles the UK's freeze-thaw winters, whether it stains, warps, or degrades, and ultimately how long it lasts.

At Paving and Slabs Ltd, we believe informed buyers make better decisions. This guide strips back the marketing language and explains exactly what composite decking is made of, how the different types compare, and what the material composition means for your purchase decision.

The Basic Ingredients

All composite decking contains some combination of two primary ingredients:

1. Wood Fibre

The "wood" component of composite decking is typically wood flour or wood chips - essentially sawdust and wood processing waste from the timber manufacturing industry. This wood content is what gives composite decking its warm, wood-like appearance and (in most products) a surface that can be embossed or textured to mimic natural timber grain.

Wood fibre content in composite decking typically ranges from 50% to 70% by weight, though some products vary significantly from this range. Higher wood content generally means a more natural look but can also mean higher moisture absorption - which is why the plastic component is critical.

2. Recycled Plastic

The plastic binder in composite decking is what makes it fundamentally different from timber. It surrounds the wood fibres, binding them together and - critically - blocking moisture from penetrating the board.

The plastic used is typically:

• High-density polyethylene (HDPE) - from recycled plastic bottles and packaging; flexible, moisture-resistant
• Polyvinyl chloride (PVC) - higher rigidity, excellent moisture resistance; used in cap layers and some full-PVC composites
• Polypropylene (PP) - less common; used in some specific formulations

The plastic content in composite decking typically ranges from 30% to 50% by weight. More plastic generally means better moisture resistance but can affect appearance and surface texture.

3. Additives

Beyond wood fibre and plastic, composite boards contain a range of additives:

• UV stabilisers - absorb or reflect UV radiation to slow colour degradation
• Pigments - provide colour; quality UV-stable pigments are significantly more fade-resistant than standard
• Bonding agents - improve adhesion between wood fibre and plastic matrix
• Biocides - inhibit mould and fungal growth
• Fire retardants - included in some commercial-grade products
• Anti-slip additives - surface texture agents in some grooved profiles

The quality and quantity of these additives, particularly UV stabilisers and biocides, significantly affects long-term performance. Unfortunately, additive specifications are rarely published in consumer marketing materials - this is worth asking manufacturers or suppliers about directly.

Types of Composite Decking Construction

Understanding the construction type is as important as understanding the ingredients - because the same materials assembled differently produce very different products.

Uncapped Composite Decking

The oldest and most basic construction type. The wood-plastic composite mix is extruded directly into the board profile - what you see on the surface is the same material as the core.

Advantages:

• Typically lower cost
• Homogeneous material throughout - no risk of delamination between layers

Disadvantages:

• More porous surface - higher water absorption rate
• More susceptible to staining
• UV fading more visible at the surface
• Can show surface fibre raise over time (wood fibres swell and become slightly rough after prolonged moisture exposure)

Uncapped composite is adequate for many domestic applications in sheltered, low-maintenance situations - but its limitations mean it has largely been superseded by capped products in the premium market.

Capped Composite Decking

The current mainstream standard for quality composite decking. A co-extrusion process bonds a protective polymer cap (usually PVC or PE-based) to the outside of the composite core during manufacturing.

The cap layer is typically 1–2mm thick and wraps around some or all of the board surfaces:

• 3-sided cap - cap on the face and two long edges; board bottom is uncapped
• 4-sided cap - cap on all four sides including the board back; best moisture protection

Advantages:

• Very low water absorption (often <1%)
• Excellent stain resistance - contamination sits on the cap surface rather than penetrating
• Superior UV resistance - the cap layer contains concentrated UV stabilisers
• No surface fibre raise
• Longer warranties typically offered

Disadvantages:

• Higher cost than uncapped
• If the cap is damaged, the exposed core is more vulnerable than the board surface
• Some early-generation capped products experienced cap delamination - less common in modern formulations

For most UK homeowners, 4-sided capped composite decking represents the best long-term value - the moisture protection advantages in the UK climate are substantial. This is reflected in our composite decking range, where we favour capped products for their performance credentials.

Full-PVC (Cellular PVC) Decking

Some "composite" products contain no wood fibre at all - they're entirely PVC-based. These offer excellent moisture resistance and zero risk of biological decay but have a noticeably different appearance and underfoot feel compared to wood-composite products.

True full-PVC decking is less common in the UK residential market but is popular in coastal environments or applications where even a small amount of moisture-related movement is unacceptable.

How to Read Material Specifications

When comparing composite decking products, these are the specifications that matter most:

Water Absorption Rate

What it means: The percentage of weight gained when a board is submerged in water for a specified period (typically 24 hours).

Why it matters: Lower absorption means better frost resistance, less dimensional movement with moisture change, better stain resistance, and longer lifespan.

What to look for: <1% for capped products; <3% for uncapped products in UK conditions.

Janka Hardness

What it means: A measure of the force required to embed a steel ball into the board surface - effectively a hardness rating.

Why it matters: Harder boards resist scratching and surface indentation better. For composite decking, Janka values are less commonly published than for timber, but are a useful comparison point.

See our guide on how to remove scratches from composite decking for practical guidance on surface care regardless of hardness rating.

Thermal Expansion Coefficient

What it means: How much the board expands and contracts with temperature change, expressed as mm per metre per degree Celsius.

Why it matters: UK gardens experience a wide temperature range - from below 0°C in winter to 35°C+ in summer. High thermal expansion means wider gaps needed at installation. See our 10 tips on installing composite decking for correct gap allowances.

Slip Resistance (R-Value or Pendulum Test Result)

What it means: A standardised measure of wet surface grip.

Why it matters: UK decks are wet for much of the year. Inadequate slip resistance is a genuine safety risk. For more on managing slip safely, see our guide on stopping composite decking feeling slippery.

What Recycled Content Means in Practice

Most composite decking manufacturers claim environmental credentials through their use of recycled materials - both the wood fibre (sawmill waste) and the plastic (recycled HDPE from packaging). This is genuine: composite decking does divert significant amounts of waste material from landfill.

However, "recycled content" claims vary in what they measure. A useful question when evaluating products is: what percentage by weight is post-consumer recycled material? Some manufacturers count all wood fibre as "recycled" even when it comes from fresh timber processing waste rather than post-consumer waste.

The environmental case for composite decking is also strengthened by its longevity - a product that lasts 30–40+ years represents a better environmental outcome than a cheaper product replaced every 10–15 years, regardless of material composition.

Material Quality and Price: The Connection

Price in composite decking is directly connected to:

• Quality and proportion of UV stabilisers
• Presence and quality of the cap layer
• Pigment quality
• Manufacturing precision (board straightness, dimensional consistency)
• Warranty terms offered

Budget composite products at the lower end of the market may use lower-quality pigments, minimal UV stabilisers, or a thinner or lower-quality cap. The results show in performance over 5–10 years: more rapid fading, surface degradation, and colour inconsistency.

This doesn't mean the most expensive product is always the right choice - but it does mean that extremely cheap composite decking should be evaluated carefully against the claims made for it.

Browse our full composite decking range for quality products with transparent specifications. Our range also includes composite fencing for a coordinated garden boundary solution.

Summary

Composite decking is made primarily from wood fibre and recycled plastic, with UV stabilisers, pigments, biocides, and bonding agents making up the balance. The construction type - capped vs uncapped - is the most important quality differentiator: capped composite boards offer significantly better moisture resistance, stain resistance, and UV performance.

When comparing products, focus on water absorption rate, UV stabiliser quality, cap layer specification, and warranty terms. These specifications tell you far more about long-term performance than colour ranges or board profiles.

For expert guidance on choosing the right composite decking for your project, contact the Paving and Slabs Ltd team - we're here to help you make the right decision for your garden and your budget.