Decking Materials Compared: Softwood, Hardwood, Composite and Aluminium — Costs and Durability

Quick Answer: UK decking falls into four categories — pressure-treated softwood (£15–£32/m² supply, 10–15 year life), tropical hardwood like balau or ipe (£55–£120/m², 25–40 years), wood-plastic composite (£35–£95/m², 20–30 years), and aluminium (£90–£170/m², 40+ years). All must be installed to BS 8417 use-class principles for ground contact, fixed at 16–20mm gap for drainage, and joists supported at 400–600mm centres depending on board thickness and span.

Summary

Choosing a decking material is the price-to-life trade-off compressed into one decision. A 4×4m deck in cheap pressure-treated softwood is £900–£1,400 in materials but needs replacement in 12 years. The same deck in a premium composite or aluminium is £2,500–£4,500 but lasts 25–40+ years and needs no annual oiling. Most homeowners undervalue the maintenance cost over a 20-year life, and most quotes don't make the comparison visible.

This article compares the four mainstream decking materials by upfront cost, life expectancy, slip resistance, fire performance, fixings required, and the ground-contact rules that determine whether the deck rots in 5 years or 25. It also covers the joist substructure, which is identical across material choices and represents 25–40% of the build cost — a material upgrade is rarely worth it if the substructure isn't designed to outlast the deck boards.

The biggest installation mistakes are at the ground contact: untreated softwood touching soil rots in 2–3 years; pressure-treated to Use Class 3 (UC3) instead of Use Class 4 (UC4) rots in 5–7 years on a deck post in concrete; and composite boards installed at 12mm gaps trap leaf debris and standing water that voids most manufacturer warranties. The right specification at the start is the difference between a 25-year deck and a 5-year warranty claim.

Key Facts

Quick Reference Table

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Material Supply £/m² Fitted £/m² Life (years) Maintenance
Pressure-treated pine (UC3) £15–£25 £55–£90 10–15 Annual oil/stain
Pressure-treated larch £25–£40 £75–£115 15–20 Bi-annual oil
Siberian larch £30–£45 £85–£125 20–25 Optional UV oil
Balau (Yellow Balau) £55–£85 £110–£170 25–35 Annual oil for colour
Ipe £85–£120 £150–£230 30–50 Optional
Cedar £40–£65 £100–£150 15–25 Annual oil
Composite (capped WPC) £45–£95 £110–£190 20–30 Wash only
Composite (uncapped) £35–£60 £95–£150 10–15 Wash + occasional
Aluminium decking £90–£170 £180–£280 40+ None
Recycled plastic boards £55–£90 £125–£195 30+ Wash only

Detailed Guidance

Pressure-treated softwood — the budget standard

Pressure-treated Scots pine and Nordic redwood is the default cheap deck — £15–£25/m² supply for a 28×125mm grooved board. The treatment (typically Tanalith E or equivalent copper-based preservative) drives preservative into the wood under pressure to give a UC3 or UC4 classification.

The pricing trap: most off-the-shelf "treated decking" at builders' merchants is UC3, not UC4. UC3 is fine for the deck boards themselves (not in ground contact) but cannot legally be used for posts in ground or for joists that touch soil. Specifying the wrong grade is the most common decking failure — joists rot at the bearings within 7 years.

Buy joists as UC4 specifically. Posts as UC4 with the cut ends end-grain treated on site (cuts expose untreated heartwood). All deck-to-ground contact requires UC4 or, better, a steel post anchor (Postsaver, GroundEasy, KryptoStub) that lifts the timber clear of moisture entirely.

Maintenance: annual oil/stain to slow UV greying and surface checking. Skip a year and the boards can grey but remain structurally fine; skip 5 years and you'll see rot on the corners.

Hardwood — the premium natural choice

Tropical hardwoods (balau, ipe, kapur, iroko) are dense, naturally durable timbers requiring no preservative treatment. Their density and natural oils give 25–50 year life with minimal maintenance. The trade-off is upfront cost (3–5× softwood) and the FSC/PEFC chain-of-custody question — buy only certified sources.

Specific points by species:

All hardwood deck boards need stainless A4 fixings — the tannins corrode mild steel and even A2 stainless on coastal/exposed sites. Pre-drill and counter-bore — direct screwing splits boards. Use hidden clip systems (Camo, Eurotec) for a fastener-free finish.

Composite — the low-maintenance compromise

Wood-plastic composite (WPC) boards are 50–60% wood fibre / 40–50% recycled HDPE polymer extruded with mineral fillers. Two grades:

Composite decking is heavier than timber (8–11 kg/m vs 5–7 for softwood) and expands more with temperature — board ends need 6mm minimum gap from any obstruction, and side gaps must respect manufacturer instructions. Mis-installed composite is the #1 warranty claim category.

Slip resistance varies hugely. Premium grooved capped boards meet BS 7976 PTV 36+ wet; budget smooth boards can drop to PTV 22 wet (slippery in winter). Always check the manufacturer's slip data — it's published on the technical datasheet, not the brochure.

Fire performance: BS EN 13501-1 Class B-s1,d0 for treated/capped products; lesser for uncapped. For decks within 1m of a building boundary, fire class can become a Building Regulations issue under Approved Document B for some configurations (rare in domestic gardens but common in HMO and apartment block external areas).

Aluminium decking — premium and permanent

Aluminium decking is extruded interlocking plank (typically anodised or powder-coated finish) supplied in lengths cut to size. Cost £90–£170/m² supply, but life expectancy is 40+ years with no maintenance beyond washing.

Use cases: rooftop terraces (light dead load — typically 8–10kg/m² vs 25–35 for timber), commercial settings, fire-sensitive locations (A1 reaction-to-fire class), and high-end residential where a warranty exceeding the homeowner's expected residency is genuinely valuable.

Surface temperature in direct sun is the watch-out — black anodised aluminium can reach 60°C+ on sunny days, uncomfortable for bare feet. Specify lighter colours or partial shade.

Joist substructure — the 25% you mustn't skimp

Substructure is identical across material choices: UC4 timber joists 47×100mm at 400mm centres for typical residential deck, supported by UC4 posts (or steel post saddles into concrete) and UC4 ledger to the house.

Component Typical Spec Notes
Posts 100×100mm UC4, post-saver sleeve Steel saddle on concrete pad preferred
Joists 47×100mm UC4 at 400mm centres 47×150mm if deck is composite or hardwood
Ledger 47×150mm UC4 with M10 coach screws into wall plate DPC felt between ledger and masonry
Beam (girder) 2× or 3× 47×150mm UC4 sandwich Carries joists at >2m post spacing
Joist hangers Galvanised or stainless Match material durability

Spacing matters: deck boards across joists at 400mm centres for 25mm boards; can stretch to 500mm for 32mm boards or premium hardwood; never over 600mm on a domestic deck.

Slip, falls and guarding

Approved Document K2 applies to deck guarding. A deck more than 600mm above the surrounding ground needs a guard — minimum 1,100mm high, 100mm sphere rule for infill (no opening allowing a 100mm sphere through).

Even on lower decks, slip resistance matters. Wet softwood without grooves drops to PTV 24–28 when wet — well below the BS 7976 36 threshold for "low slip risk". Specify grooved profiles, an annual treatment with a non-slip oil, or routinely clean off algae for any deck likely to be used in winter.

For homeowners — the 20-year cost compared

A 4×4m (16m²) deck, fully installed with substructure:

Material Year 0 cost 20-year maintenance 20-year total Replacement at year 20?
Pressure-treated pine £900–£1,400 £40 × 20 (oil) + 1 replace £2,500–£3,500 Yes, year 12–15
Larch £1,200–£1,800 £40 × 10 (less often) £1,800–£2,400 No
Balau hardwood £1,800–£2,700 £40 × 5 £2,000–£3,000 No
Capped composite £1,750–£3,000 £0 £1,750–£3,000 No
Aluminium £2,800–£4,500 £0 £2,800–£4,500 No

Composite and hardwood have similar 20-year costs to twice-replaced softwood. The cheapest deck is rarely the cheapest deck.

Frequently Asked Questions

Why does my decking rot at the joist bearings?

Almost always because either the joist is UC3 (treated for above-ground only) being used in ground contact, or the joist hanger is unflashed and water sits on the timber. Lift one bearing, check the joist mark or the supplier's delivery note. UC4 timber resists ground contact for 15+ years; UC3 in the same position fails in 5–7. Permanent fix: replace failing joists as UC4 with stainless hangers and DPC felt above the bearing.

Do I need planning permission for a deck?

Generally no, provided: deck height is below 300mm above natural ground; deck plus other extensions does not exceed 50% of the original garden area; the property is not in a Conservation Area or AONB. Above 300mm or in restricted designations, it may require planning permission — check with your local authority.

What's the best fixing system for hardwood decking?

Stainless steel A4 (marine grade) for coastal sites and tannin-rich timbers; A2 acceptable for inland use with non-tanniferous hardwoods. Pre-drill all hardwood. Hidden clip systems (Camo Marksman, Eurotec, Tigerclaw) eliminate visible fixings but add £8–£15/m² in labour and clip cost. For ipe specifically, side-grooved boards with biscuit-style hidden clips work better than face-screwing.

How wide should the gap between deck boards be?

Depends on material:

Gaps less than 4mm trap leaf debris that holds moisture against the boards — a slow path to rot for timber and warranty void for composite.

Can I install decking directly on grass or soil?

No. Direct ground contact rots even pressure-treated timber within 5–7 years. The minimum is concrete pad + steel post anchor + minimum 150mm air gap below the joists. For a permanent installation, dig pad foundations at 1,000–1,500mm centres on standard ground, set steel post anchors in concrete, and never let timber bridge the gap to soil.

Regulations & Standards