Composite decking costs about £200–£300 per m² fitted in the West Midlands, against £120–£200 for treated softwood, so a typical 15m² deck costs roughly £1,200–£1,500 more in composite. It is worth it if you do not want to clean and treat the boards every year or two. If you are happy to maintain it, timber is better value.
The answer also depends on the frame underneath, how much shade the deck gets, and how long you plan to stay. Those points are covered below, with prices taken from the site's decking cost guide.
How much more does composite cost?
Prices are supplied and fitted, waste included, and indicative until a local specialist has seen the site.
| Deck size | Timber, ground level | Composite, ground level | Difference |
|---|---|---|---|
| Small, about 8m² | £960–£1,600 | £1,600–£2,400 | about £640–£800 |
| Medium, about 15m² | £1,800–£3,000 | £3,000–£4,500 | about £1,200–£1,500 |
| Large, about 28m² | £3,360–£5,600 | £5,600–£8,400 | about £2,240–£2,800 |
A raised deck adds about 20% to either material, because of the extra framing and posts. Taking up an old deck first adds £10–£20 per m², whichever boards you choose.
Part of the gap closes over time. A timber deck needs cleaning and treating every year or two, which costs either your weekends or a contractor's time. Composite still needs washing, but not staining or oiling.
How long does each last?
The Timber Decking and Cladding Association (TDCA), the industry's technical body, recommends that only timber capable of a minimum service life of 15 years is used for decking. It says British and European standards allow treated softwood to be specified for a 15, 30 or 60 year desired service life, and that 15 years is the industry default. That is a design target, not a guarantee: a deck in constant shade, sitting close to wet soil or never cleaned will fail sooner.
Composite boards do not rot in the way timber boards can, and many come with longer guarantees than timber. Guarantees vary a lot between products, though, and usually cover the boards only, not the frame or the fitting. Ask the specialist what the guarantee covers and for how long, and get it in writing.
Why the frame matters more than the boards
Most composite boards still sit on a timber frame of joists and bearers, so a composite deck is only as long-lived as the wood underneath it. Some are fitted on aluminium frames instead, which do not rot.
Since January 2020 the TDCA has recommended that deck joists, and any other timbers giving exterior structural support, are pressure-treated to Use Class 4 whether they are in ground contact or not. Use Class 4 is the treatment level for timber in contact with the ground. The TDCA also publishes guidance on building timber substructures for composite boards. When you compare quotes, ask:
- What treatment class the joists and posts are, and whether cut ends are re-treated.
- What joist spacing is used, since composite boards often need closer spacing than timber.
- What the frame sits on: concrete pads, posts in concrete or ground screws.
- Whether there is a weed membrane and drainage underneath.
A cheap frame under expensive boards is a common way for a composite deck to go wrong.
Does composite decking rot or go mouldy?
Composite is far more resistant to rot than timber, but it is not immune to everything. Algae, mildew and mould can grow on the surface of any deck in shade, and leaves left lying hold moisture against the boards. The difference is that on composite this is a cleaning job, while on timber, damp that is left too long can lead to rot in the boards or the frame.
Which is more slippery?
The TDCA says slippery decks are simply the result of surface deposits such as mildew, algae and fine mosses, which get worse in wet weather. That applies to both materials, so keeping the deck clean matters more than the board type. Timber in shade is the usual problem.
For a measurable comparison, ask whether the board has a published wet pendulum test value. The Health and Safety Executive classes a pendulum test value of 36 or more as low slip potential, 25 to 35 as moderate and 24 or under as high. Where figures are published, compare them like for like.
Does composite get hot or scratch?
Darker composite boards can get warm underfoot in full sun, and lighter colours stay cooler. If the deck faces south and you walk on it barefoot, ask for a sample and leave it outside on a sunny day.
Composite can also scuff where furniture is dragged, and like most plastics it expands and contracts with temperature, which is why fitting instructions set gaps at board ends and edges. Timber marks too, but can be sanded and re-treated. With either material, it is safest not to stand a fire pit or barbecue directly on the boards.
How should each be cleaned, and what should I avoid?
The TDCA's advice for timber is that sweeping with a stiff brush is usually enough in the early years. If you use a pressure washer, choose a lower pressure and keep the lance away from the surface, because too much pressure can damage the timber. A brush, mild soap and water on a dry day is the gentler alternative.
For composite, follow the board maker's care instructions. They differ between products, and many warn against harsh solvents, some cleaning chemicals and high-pressure washing. Using the wrong product can affect the guarantee, so check before you buy a cleaner.
Which should I choose?
- Choose timber if budget comes first, you like a natural look, you are happy to clean and treat it, or you may move within a few years.
- Choose composite if you want low upkeep, the deck is in a damp or shady spot, or you plan to stay long enough to recover the extra cost in time saved.
- Either way, put the money into a well-built frame first.
On a flat site, a patio can be a cheaper alternative to composite; see the patio cost guide. For how decks are priced and built, see the decking service page, and check the planning rules for raised decking if the deck will sit high.
Questions
Can composite boards go on a timber frame?
Yes, and most do. The frame should be treated to Use Class 4, as the TDCA recommends for deck joists, and the joists set at the spacing the board maker specifies, which is often closer than for timber boards.
Is hardwood decking better than softwood?
The TDCA notes that naturally durable hardwoods are usually denser than softwood, while softwood is used far more because it costs less and is easier to work. Smooth hardwood boards sit at the top of the timber price range.
Can I replace just the boards on an old timber deck?
Often, if the frame is sound. A specialist checks the joists and posts first, because soft or rotten framing under new boards will fail within a few years, and in that case a rebuild is the better buy.
Is it safe to pressure wash timber decking?
It can be, at a low pressure with the lance held well back. The TDCA warns that too much pressure can damage or bruise the timber surface, so a stiff brush and mild soap are the safer first step.