Timber vs Composite Decking Comparison

Summary

Timber and composite are the two dominant decking material choices for UK domestic gardens and light commercial outdoor space, and — much like the equivalent choice in fencing — the decision rarely comes down to a simple like-for-like material swap. Timber decking is the traditional default: cheaper to buy, familiar to install, repairable board-by-board, and available in a range of species from budget pressure-treated softwood through to premium hardwoods and modified timbers. Composite (wood-plastic composite, WPC) decking has grown from a niche premium product into a genuinely mainstream choice over the last decade or so, on the strength of a single core pitch: no annual oiling, no re-staining, and a consistent factory finish that resists splintering, warping and fading far better than timber.

The right recommendation depends heavily on how long the customer plans to stay in the property, how much maintenance effort they're realistically willing to put in, and their budget ceiling at the point of installation. A customer planning to sell within a few years gets little practical benefit from composite's 25-year-plus service life. A customer settled long-term, or one who has explicitly said they're tired of annual decking maintenance, is a strong composite candidate even though the upfront material cost is typically 1.5–2.5 times higher than a comparable timber spec.

This guide covers material specification for both options, structural considerations that apply regardless of decking material (joist spans, foundations, planning thresholds), the cost and lifecycle comparison, installation and fixing differences, and how to run the value conversation with a customer who's fixated on the upfront quote. Structural framing (joists, posts, foundations) is largely shared between the two options and is covered in more depth in decking and decking permits — this article focuses on the board-material decision itself.

Key Facts

• Timber decking board thickness — minimum 25mm; 28–32mm preferred for spans over 400mm between joists, per general decking construction guidance
• Timber species range — pressure-treated softwood (budget), Siberian larch (natural Class 3 durability, attractive grain), Western red cedar (light, stable, Class 2–3), Accoya (acetylated softwood, Class 1 durability), and hardwoods such as balau/ipe for premium projects
• Timber preservative treatment — BS 8417 governs Use Class selection; UC3b for above-ground exposed boards/joists, UC4 for ground-contact posts and sleeper timbers
• Composite board construction — wood-plastic composite (WPC): typically 50–70% recycled wood fibre bonded with recycled or virgin HDPE, extruded into a hollow or solid board profile
• Capped vs uncapped composite — capped composite has a co-extruded protective outer layer resisting UV fade, staining, and mould; uncapped composite behaves closer to a slightly better-than-timber material — always specify capped for a genuinely low-maintenance sell
• Composite fixing systems — hidden clip systems (co-extruded groove + screwed clip) or top-down screw with colour-matched plug; see composite decking fixing methods for the fixing-method comparison in detail
• Joist centres — 400mm maximum standard for most timber deck boards; composite typically requires closer centres (300–400mm, product-specific) due to lower stiffness across a span — always check the manufacturer's span table rather than assuming timber spacing transfers
• Drainage gap — 3mm minimum between timber boards (5mm in wetter regions); composite typically specifies 5–8mm, manufacturer-specific, to allow for greater thermal expansion
• Governing standard, composite board material — BS EN 15534 (composites made from cellulose-based materials and thermoplastics); no equivalent finished-product British Standard exists for composite decking the way BS 8417 covers timber treatment
• Planning threshold (both materials, identical) — decking over 300mm above natural ground level requires planning permission; measured from ground to top of deck board surface, under the Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class E
• Building Regulations Part K guarding (both materials, identical) — required wherever a deck edge is more than 600mm above adjacent ground; 1,100mm minimum guarding height, maximum 100mm baluster gap
• Part A structure (both materials, identical) — raised decks over 600mm, decks carrying significant additional loading (hot tub, planters with soil), or decks structurally connected to the house may require Building Control involvement
• Thermal expansion — composite expands and contracts significantly more than timber across seasonal temperature swings; manufacturers specify expansion gaps at board ends and post/wall junctions that must be followed exactly, or boards can buckle
• Ground clearance — both materials need minimum clearance from soil (typically 150mm+ air gap beneath joists) to control moisture and, for timber, rot risk; composite doesn't rot but still stains and algaes at ground contact
• Cutting and waste — composite offcuts and dust are a plastic-wood composite, general construction waste, and must not be burned; treated timber offcuts also must not be burned due to preservative content
• Warranty ranges — timber deck boards typically carry no meaningful manufacturer warranty beyond the preservative treatment guarantee; composite boards commonly carry 10–25 year structural/fade warranties from the manufacturer, conditional on correct installation per their fixing system

Quick Reference Table

Detailed Guidance

Cost and lifecycle — running the real comparison

Timber wins on upfront material cost every time — typically by a factor of 1.5–2.5x against a comparable composite spec, and considerably more against premium hardwood. Composite's case rests almost entirely on total cost of ownership: no annual oiling (a genuine time cost for the customer, not just a cash one), no re-treatment labour every 2–3 years if the customer pays a tradesperson to do it, and a longer base service life before any board replacement is needed. Run the comparison over a realistic 20–25 year ownership horizon for an undecided customer: softwood timber's repeated treatment cost and earlier replacement (potentially two full deck replacements in that period against one for composite) narrows the upfront price gap considerably, and can flip the total-cost comparison in composite's favour, particularly for customers who would otherwise pay a contractor for annual maintenance rather than doing it themselves.

Hardwood decking sits in an interesting middle position — a higher upfront cost than softwood, a naturally long service life without pressure treatment, but still requiring periodic oiling to maintain appearance (though it will survive untreated for longer than softwood, just greying in colour). For customers who want the natural timber look without softwood's shorter lifespan, and who don't mind the higher upfront cost, hardwood is worth quoting alongside both softwood and composite.

Structural framework — shared between both materials

Regardless of which board material is chosen, the substructure follows the same rules: posts (minimum 100×100mm for decks up to 3m height, increasing for taller structures or where a pergola loads the posts), joists at appropriate centres for the board span, and foundations set to adequate depth with concrete. See decking for the full joist and post specification, and decking joist spacing calculator for span-specific joist centre calculations by board thickness and material. The board material decision does not change the foundation or planning-permission requirements — a common customer misconception is that composite decking, being a "modern" product, is somehow exempt from planning thresholds. It is not; the 300mm PD threshold and 600mm Part K guarding trigger apply identically to both materials.

Installation differences — timber

Timber decking is built up on site in a familiar sequence: substructure (posts, beams, joists) first, then individual boards fixed directly across the joists, typically top-down with stainless steel or coated decking screws, working from one edge across the deck with consistent 3–5mm gaps maintained with spacers. Boards are cut to length on site as needed, and any damaged or split board can be individually unscrewed and replaced without disturbing the rest of the deck — a genuine advantage for long-term repairability.

Installation differences — composite

Composite decking substructure is built identically to timber, but board fixing is system-specific rather than a generic screw-down. Hidden-clip systems slide the board's grooved edge into a bracket screwed to the joist, giving a clean, fixing-free top surface but requiring careful attention to the manufacturer's specified expansion gap at every board end and post junction — get this wrong and boards can buckle or gap unevenly as temperatures swing through the seasons. Top-down screw systems with colour-matched plugs are faster to fit and easier to repair a single board later, at the cost of (slightly) visible fixing points. See composite decking fixing methods for the full comparison of the two fixing approaches and when each is preferable.

Ground clearance, drainage and DPC — largely shared rules

Timber boards in continuous ground contact or poor airflow rot significantly faster — inadequate under-deck ventilation is one of the most common causes of premature timber deck failure. Composite doesn't rot, but continuous soil contact still traps moisture against the board underside and encourages algae and green staining on the lowest courses — a cosmetic complaint, not a structural one, but still worth designing out with proper clearance. Both materials need the same minimum air gap beneath the joists (typically 150mm, ideally 300mm for inspection access) and a gravel layer to suppress weed growth and improve drainage beneath the deck. Where a deck meets the house wall, the deck surface must sit at least 150mm below the building's damp-proof course regardless of board material — bridging the DPC with decking (or planting, or a build-up of debris) is a common and entirely avoidable cause of damp ingress into the house.

Having the cost conversation with a price-focused customer

Composite loses the upfront price comparison against timber every time, so don't try to win that argument directly — reframe around total cost of ownership and how much ongoing maintenance the customer is actually willing to do. For a customer planning to stay in the property 10+ years, or one who has been explicit that they don't want an annual oiling task, the total-cost case for composite is genuinely strong and worth quoting proactively rather than only when asked. For a customer on a tight budget, planning to sell within a few years, or specifically wanting the natural warmth and re-stainable finish of real timber, softwood or hardwood decking remains the more sensible recommendation — don't oversell composite where it doesn't fit the customer's actual timeline and preferences.

Frequently Asked Questions

Does composite decking get as hot underfoot as some people claim?

Darker composite boards can get noticeably warmer in direct summer sun than lighter timber, particularly capped boards with a dark co-extruded finish — this is a genuine, commonly reported customer complaint, not a myth. Lighter composite colourways and boards with a textured (rather than smooth) surface finish tend to perform better here. Worth mentioning proactively to customers choosing a dark composite colour for a south-facing, sun-exposed deck.

Is composite decking as slip-resistant as timber when wet?

Broadly comparable when both are specified with an appropriate textured or grooved surface finish; smooth uncapped composite and smooth planed softwood can both become genuinely slippery when wet or covered in algae. Specify a textured/grooved profile for either material on decks with significant rain exposure or shade, and clean regularly to prevent algae buildup regardless of material.

Can composite decking be installed directly over an existing timber deck?

Not generally recommended without a proper structural assessment of the existing substructure — joist centres, condition, and level need to meet the composite manufacturer's requirements independently of what worked for the original timber boards. In most cases it's more reliable to strip back to the substructure, inspect and repair as needed, and then install to the composite manufacturer's specification from that point.

How does the labour cost compare between the two materials?

Broadly similar for the substructure stage — identical foundation, post, and joist work regardless of board material. Board-fitting labour is sometimes marginally higher on composite hidden-clip systems, where each board needs individual alignment into a channel, versus a continuous screw-down run on timber. Budget a similar or slightly higher labour allowance for composite of an equivalent deck size.

Which option is better for a raised deck that needs Part K guarding?

Neither material has a compliance advantage for the guarding requirement itself — Part K's 1,100mm height and 100mm baluster gap rules apply identically regardless of decking material. Aluminium or composite balustrade systems are commonly paired with composite decking for a consistent, low-maintenance overall finish, while timber post-and-rail guarding is the natural pairing for a timber deck, but there's no regulatory reason either combination couldn't be mixed if the customer prefers.

Regulations & Standards

• Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class E — Permitted Development rights and the 300mm height threshold for decking, identical for both materials

• Building Act 1984 and Building Regulations 2010 — statutory framework underlying Part A and Part K requirements

• Approved Document A: Structure — structural adequacy requirements for raised decks

• Approved Document K: Protection from falling — guarding requirements; 1,100mm minimum height, 100mm maximum baluster gap

• BS 8417 — preservation of wood — code of practice; governs Use Class selection (UC3b, UC4) for timber decking components

• BS EN 15534 — composites made from cellulose-based materials and thermoplastics (wood-polymer composites); the material standard most commonly referenced for composite decking boards

• BS EN 350 — natural durability of solid wood; relevant to hardwood and softwood species durability classification

• Planning Portal — Decking — official PD rules summary for decking

• Approved Document K — guarding height and baluster gap requirements

• TRADA — Timber Decking Guide — timber species and specification guidance

• Wood Protection Association — Use Classes and timber treatment standards

• TDCA — Timber Decking and Cladding Association — trade body technical guidance covering both timber and composite decking

• decking — Decking construction: joist sizing, spans, and Building Regs thresholds

• decking permits — Full planning and Building Regulations breakdown for decking height thresholds

• composite decking fixing methods — Detailed comparison of hidden-clip vs top-down composite fixing systems

• decking installation pricing guide — Softwood, hardwood, composite and aluminium sub-frame pricing by project size

• garden decking pricing guide — Per-m² pricing breakdown including Part K balustrade costs

• decking joist spacing calculator — Joist centre calculator by board thickness and material

• timber vs composite fencing comparison — Equivalent material comparison for fencing, distinct span/fixing rules