The short answer
Composite decking is made from a blend of wood fibres and plastic, usually recycled. The wood content is typically sawdust or reclaimed wood flour, and the plastic is commonly recycled polyethylene (HDPE) or polypropylene; the two are mixed with binders, pigments and additives, then heated and extruded or moulded into boards. The wood gives a natural look and grip; the plastic resists water, rot and insects and binds it together. Most quality boards are capped, with a hard polymer shell around the core that improves stain and fade resistance. The result sits between solid timber and pure plastic — more wood-like than PVC, but far more weather-resistant and lower-maintenance than real wood.
Composite decking is everywhere in UK gardens now, but many people aren't sure what it actually is. This page explains what goes into it and how that affects how it performs.
What's in it
- Wood contentRecycled wood fibre / sawdust
- Plastic contentRecycled HDPE or polypropylene
- Also includesBinders, pigments, UV additives
- Formed byExtrusion or moulding
- Outer shell (capped)Hard polymer cap
The basic recipe
Composite decking is, at heart, a wood-plastic composite (WPC) — a mixture of two main ingredients plus additives:
- Wood fibre — finely ground sawdust, wood flour or reclaimed wood, often a recycled by-product of timber processing. This gives the board its natural look, texture and some rigidity.
- Plastic — commonly recycled polyethylene (HDPE) or polypropylene. This binds the mix, resists water and rot, and adds durability.
- Additives — binding agents to fuse the wood and plastic, pigments for colour, UV stabilisers to limit fading, and sometimes mould inhibitors.
The proportions vary by brand, but a roughly even split of wood and plastic by weight is common. The balance affects the board's look, weight, strength and how wood-like it feels.
How it's made into boards
The ingredients are dried, mixed and heated until the plastic melts and coats the wood fibres, then the molten blend is forced through a die — a process called extrusion — to form a continuous board with the chosen profile, which is cut to length. Boards can be solid (a full cross-section) or hollow (with internal voids to save material and weight). A surface texture, often an embossed wood grain, is added during forming, and the board may be brushed or grooved for grip. Many boards are co-extruded with a cap: a separate hard polymer shell is formed around the core in the same process, bonded so it can't peel. Capping can cover the top and sides, or all four sides. The making process is what gives composite its consistency — no knots, no warping in the same way as timber.
Composition and what each part does
The table summarises the typical ingredients and their role.
| Component | Typical material | What it does |
|---|---|---|
| Wood fibre | Recycled sawdust / wood flour | Natural look, grip, rigidity |
| Plastic | Recycled HDPE / polypropylene | Water/rot resistance, binds the mix |
| Binders | Coupling agents | Fuse wood and plastic together |
| Pigments | Colour additives | Board colour |
| UV stabilisers | Additives | Limit fading in sun |
| Cap (if capped) | Polymer shell | Stain, fade and scratch resistance |
Typical composition; exact materials and proportions vary by manufacturer. Source: composite decking manufacturer specifications.
Why the make-up matters
What's in the board explains how it behaves. The plastic content is why composite doesn't rot, doesn't feed insects and barely absorbs water — the things that limit timber's life. The wood content is why it still looks and grips like wood rather than feeling like pure plastic, and why it has a more natural surface than PVC decking. The UV stabilisers and pigments are why quality boards hold their colour, and the cap is what resists stains and scratches. The flip side is also down to composition: the plastic makes boards expand and contract with temperature, so installation gaps matter, and a high plastic content can make a board warm in strong sun. Cheaper boards with poorer additives or no cap fade and stain more, which is why composition is worth checking when comparing products.
Why composition makes some boards better than others
Two boards that both call themselves composite can perform very differently, and the reasons trace straight back to what's in them. The quality and proportion of the plastic matter: virgin or well-processed recycled polymer binds the wood fibre more completely than poorly sorted recyclate, giving a denser, more stable board that absorbs less water. The fineness and drying of the wood fibre affect how well it bonds and how prone the board is to swelling. The additive package — UV stabilisers, pigments, mould inhibitors and coupling agents — decides how well the board holds colour and resists staining over years of British weather. And the cap, where present, is the single biggest quality differentiator, because a good polymer shell shields all of the above from sun, water and spills. Cheaper boards typically skimp on cap, additives or plastic quality, which is why they fade, stain and absorb more. When comparing products, the composition behind the price tells you more than the colour swatch does.
Composite versus solid wood and pure plastic
Understanding the make-up clarifies where composite sits. Against solid timber, it trades genuine grain and the ability to sand and refinish for far better resistance to rot, insects, splinters and fading, and much lower maintenance. Against pure PVC or plastic decking, the wood fibre gives composite a warmer, more natural look and feel, though PVC is lighter and even more rot-proof. It isn't solid wood and it isn't solid plastic — it's a deliberate middle ground that aims to combine wood's appearance with plastic's durability. Knowing that helps set expectations: composite looks wood-like and lasts well, but up close it reads as the manufactured product it is, and its behaviour in heat and its repair limits come straight from its plastic content. In short, composite is best understood not as a like-for-like replacement for either wood or plastic but as an engineered compromise designed for people who want most of timber's warmth with most of plastic's durability, and judging it on that basis — rather than expecting it to be indistinguishable from solid hardwood — is the surest way to be happy with the result.
Frequently asked questions
Is composite decking real wood?
Not entirely. Composite contains real wood fibre — usually recycled sawdust or wood flour — but it's blended with plastic and additives, so it isn't solid timber. That mix is what lets it look wood-like while resisting rot, insects, splinters and fading far better than real wood, at the cost of being unable to be sanded and refinished.
Is composite decking made from recycled materials?
Most composite decking uses a high proportion of recycled materials — recycled wood fibre such as sawdust, and recycled plastics like polyethylene — which is part of its environmental appeal. The exact recycled content varies by brand, so if it matters to you, ask the manufacturer for their specific figures.
What's the difference between capped and uncapped composite?
Uncapped composite is the bare wood-plastic core, while capped composite has an extra hard polymer shell formed around it during manufacture. The cap resists staining, fading and scratching much better, so capped boards keep their looks with less effort. Most quality composite sold today is capped, often on all four sides.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific garden. They are guidance, not a quotation.