Garden Decking Prices: Composite vs Timber Costs

Q: How long does a softwood deck really last?

Properly built with C24 tanalised joists, stainless screws, DPC at post bases, and customer applying annual decking oil: 15–25 years. Built badly (untreated joists in contact with soil, mild-steel screws, no maintenance): 5–8 years. The difference is sub-frame quality and aftercare, not board grade.

Q: Why is composite decking so much more than timber?

Material cost. Composite boards are 3–5× the material cost of softwood. Once you factor in the no-maintenance lifetime cost (no annual oil, no sanding, no re-staining, no board replacement), the 25-year total ownership cost of composite is often lower than softwood. But the upfront install price is 2–3× higher.

Q: Can I lay decking over an existing concrete patio?

Yes — and it's cheaper because you skip the foundations. Lay a treated softwood batten sub-frame (47×75mm minimum) directly on the concrete on DPC pads, with 5mm gaps for drainage. Joists at 400mm centres. Check the concrete is level and falls away from house — if not, use packers.

Q: What's the difference between capped and uncapped composite?

Uncapped composite is just compressed wood-flour + polymer; absorbs moisture, fades, stains, and surface degrades within 5–8 years. Capped composite has a polymer shell bonded around the core; UV-stable, stain-resistant, slip-rated, 25-year warranty common. Always quote capped — the £10–£20/m² premium is worth it.

Summary

Decking is the highest-margin garden landscaping job available to a competent carpenter — and also the most commonly bodged. The visible part (the boards) is easy. The invisible part (the sub-frame, foundations, joist sizing, ground preparation, fixing detail) is what separates a deck that lasts 25 years from one that sags, twists, and rots within five.

The major pricing tier is set by material. Softwood is the volume material — UK tanalised pine or spruce, ~£15–£25/m² for the boards alone. Hardwood (oak, balau, ipe) is the premium look, ~£55–£120/m² for boards. Composite (Trex, Millboard, Hyperion, Ecodek) is the maintenance-free contemporary option, ~£55–£130/m² for boards. The sub-frame for any of these costs roughly the same — treated C24 joists at 47×150mm on concrete pad foundations is £30–£60/m² in materials and another 2–3 hours per m² in labour.

Where most cowboys lose money is treating the deck as a 1-day job for a £50 day-rate price. A 20m² deck done properly is 4–6 days for a 2-person crew including foundations, sub-frame, boards, balustrade, and clear-up. Underquoting this is endemic and the customer ends up with a bouncy, twisted, rapidly-rotting deck.

This guide covers material selection, sub-frame design, regulatory requirements (planning, Part K, Part A), and a worked pricing breakdown.

Key Facts

Softwood deck board (treated pine/spruce 28×145mm) — £4.50–£8.50/m or £15–£25/m²
Premium kiln-dried softwood (Siberian larch, Scandinavian redwood) — £7–£14/m or £22–£42/m²
Hardwood deck board (oak, balau, iroko) — £18–£32/m or £55–£100/m²
Premium hardwood (ipe, cumaru) — £25–£45/m or £75–£130/m²
Composite deck board (uncapped) — £20–£35/m or £45–£75/m²
Composite deck board (capped, premium — Trex Transcend, Millboard Enhanced Grain) — £30–£55/m or £75–£130/m²
Treated softwood joist (47×150mm C16/C24) — £6–£12/m
Treated softwood joist (47×195mm — for long spans) — £8–£16/m
Aluminium composite sub-frame system — £35–£65/m² (premium full system)
Concrete pad foundation (450×450×450mm) — £15–£30 each in materials (excluding labour)
Postcrete (rapid-set) — £7–£14 per 25kg bag
Joist hangers (galvanised, BS EN 845-1) — £2–£8 each
Decking screws (stainless 304/A2, 60–80mm) — £18–£35 per 250 box
Hidden clip composite fixings — £35–£65 per box (covers ~2–3m²)
Carpenter/landscaper day rate — £200–£320/day regional, £260–£400/day London
Specialist deck installer (composite/large) — £280–£450/day
Joist centres — Composite: 400mm max (most manufacturers); Timber: 400–450mm typical; reduces to 300mm on diagonal-laid composite
Span tables (decking joists, BS 5268-7 / BS EN 1995) — 47×150mm C24 joist, 400mm centres: max clear span ~2.4m for decking loads
Treated timber spec — Use Class 3 or 4 per BS 8417 (in ground or in contact with damp surfaces)
Planning permission threshold — Deck >300mm above ground level OR exceeding 50% of garden coverage = full planning per GPDO 2015 (England)
Building Regulations Part K — Guard-rail (balustrade) required where fall is >600mm; min 1.1m high in domestic; balusters <100mm gap
Building Regulations Part A — Structural; large or elevated decks may need Building Control sign-off
CDM 2015 — Single-dwelling domestic work over 30 days = client duties apply

Quick Reference Table

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Material	Boards (£/m² supply)	Sub-frame (£/m² supply)	Labour (£/m²)	Total installed (£/m²)	Lifespan
Treated softwood (standard)	£15–£25	£25–£40	£40–£75	£80–£140	12–20 yrs
Premium softwood (larch, redwood)	£25–£42	£25–£40	£45–£80	£100–£165	20–30 yrs
Hardwood (oak, balau, iroko)	£55–£100	£30–£50	£60–£110	£160–£260	25–40 yrs
Hardwood (ipe, cumaru — premium)	£75–£130	£30–£50	£65–£120	£190–£310	30–50 yrs
Composite uncapped	£45–£75	£35–£60	£60–£100	£150–£235	20–25 yrs
Composite capped (premium)	£75–£130	£35–£65	£65–£105	£180–£300	25–35 yrs
Aluminium-frame composite system	£75–£130	£55–£90	£65–£110	£200–£330	30–40 yrs

Job size	Material mix	Typical price (2026)
Small deck 10m²	Softwood	£900–£1,500
Small deck 10m²	Capped composite	£1,800–£3,200
Medium deck 20m²	Softwood	£1,700–£3,000
Medium deck 20m²	Hardwood	£3,400–£5,500
Medium deck 20m²	Capped composite	£3,800–£6,200
Large deck 35m² with balustrade + steps	Capped composite	£7,500–£12,000
Raised deck 25m² with steps + balustrade (over 600mm)	Hardwood	£6,000–£10,500

Detailed Guidance

What to ask the customer at quote

Use case — entertaining/BBQ (size for table + chairs), kids play, hot tub (separate structural design), garden seating area
Material preference — show samples; composite is much more expensive but no maintenance
Height above ground — drives sub-frame complexity, balustrade requirement, planning
Garden coverage — does the proposed deck exceed 50% of garden area? Planning issue under GPDO
Position — close to house (consider DPC level), in middle of garden (full sub-frame), against fence (joist hanger to neighbour structure not allowed)
Access — wheelbarrow/skip route, dig spoil disposal
Existing surface — grass (full sub-frame), concrete (pads not needed), patio (build over or strip)

Sub-frame design — the bit that matters

A bad sub-frame ruins any deck regardless of board cost. Key rules:

Joist sizing (per BS 5268-7 / BS EN 1995-1-1):

47×100mm C16 — max span ~1.5m at 400mm centres
47×150mm C16 — max span ~2.2m at 400mm centres
47×150mm C24 — max span ~2.4m at 400mm centres
47×195mm C24 — max span ~3.0m at 400mm centres
For composite boards, use 400mm centres maximum (most manufacturers)
For 45° diagonal-laid boards, reduce centres to 300mm

Foundations:

Concrete pads (450×450×450mm minimum) at 1.5–1.8m centres under bearers
Pad top should be above ground level (use slab/spacer to keep posts off wet earth)
Adjustable steel pedestals on concrete are an alternative on hard surfaces

Bearers and posts:

Treated softwood C24 posts 100×100mm minimum
DPC strip (5–10mm bituthene) between post base and concrete pad to prevent wicking
Posts bolted to galvanised post anchors (Simpson, Easyfix) — not buried in concrete

Joist hangers and connections:

BS EN 845-1 stainless or sherardised joist hangers (see joist hangers and sizing)
Sherardised square-twist nails (NOT round wire) per BS 1202 spec
Stainless screws for board fixings — ungalvanised mild steel will rust within 3 years and stain boards

Cross-bracing and noggins:

Mid-span noggins on any joist >1.8m to prevent twisting
Diagonal cross-bracing under raised decks for lateral stability

Composite vs timber — when to recommend each

Recommend softwood when:

Tight budget
Customer OK with annual oiling/staining
Lower deck (<300mm) with simple geometry
Short-term ownership (renting out / planning to move)

Recommend hardwood when:

Long-term own-home installation
High-end aesthetic important
Customer wants natural silver-grey weathering (oak, balau) or rich brown maintained (ipe)
Budget mid-range

Recommend capped composite when:

Customer hates maintenance — no oiling, no sanding, no staining
Family with kids/pets (splinter-free, no rough surface)
Pool/hot tub deck (water-resistant, slip-rated boards available)
20+ year ownership planned
Budget for premium

Don't recommend uncapped composite — fading, staining, scratching issues; capped is worth the £10–£20/m² premium.

Balustrade — Part K requirements

Building Regulations Part K (Protection from falling, collision and impact) applies to decks. Trigger thresholds:

Fall height >600mm = balustrade required
Domestic balustrade height: 1.1m above deck surface
Baluster gap: max 100mm (no sphere of 100mm should pass through)
Loading: 0.74 kN/m horizontal line load on top rail
Step on/off (e.g. to garden) does not need balustrade if fall is <600mm

Material options:

Treated softwood balustrade — £35–£85/m (cheapest, matches softwood deck)
Composite balustrade — £85–£180/m
Stainless or aluminium wire/cable balustrade — £120–£280/m
Glass balustrade (frameless) — £280–£550/m (premium)

Steps and access

Steps to and from the deck price separately:

Treated softwood step — £55–£140 per linear metre of riser
Composite step — £85–£180 per linear metre
Step rise should be 150–175mm consistent (per Part K)
Step going (depth) 280–340mm typical for decking

Planning permission and Building Regulations

Planning permission required when:

Deck >300mm above natural ground level (GPDO 2015 Class E)
Deck + other outbuildings cover >50% of garden
Property in conservation area, AONB, national park, or listed (Article 4 Direction)
Deck within 2m of boundary >2.5m high

Building Regulations approval required when:

Structural deck (e.g. cantilever, attached to dwelling, hot tub support)
Deck forms means of access from above-ground floor
Raised deck where guard-rail under Part K applies (most installers don't apply for this routinely; technically required for >600mm fall)

Always advise customer to check with Planning department. Quote should state "subject to planning permission / Building Regs approval if required."

Drainage and ground preparation

Strip turf and 100mm of topsoil from deck footprint
Lay weed membrane (terram or geotextile) under the deck — prevents weeds growing through
Slope ground 1:80 away from house for surface water drainage
Allow airflow under deck — minimum 100mm gap between underside of joists and ground; otherwise condensation rots underside of boards
Vent the deck perimeter — air bricks or ventilation grilles in skirt at 1–2m centres

Common pitfalls

Joists not treated to Use Class 3/4 — rot from below within 5 years
Mild steel screws instead of stainless — rust stains down boards within 18 months
No DPC between post and concrete pad — water wicks up post, rots base, deck collapses in 8–12 years
Composite boards at 450mm joist centres — sag/flex within 12 months; manufacturer warranty voided
Decking butt-jointed without expansion gap — boards push and warp in summer heat
Quote omits balustrade — customer discovers Part K cost £600–£1,500 after job priced
No air gap under deck — black mould, rot, smell from rotting timber
Single board over a span — joints unsupported; always join over a joist

Pricing breakdown for a typical job

A 20m² capped composite deck, ground-level (no balustrade), 4 steps to lawn, single side return:

Item	Cost
Capped composite boards (Trex Enhance or similar) 20m² × £45/m²	£900
Treated softwood C24 joists (47×150mm) 50m × £8/m	£400
Treated softwood C24 bearers (75×150mm) 12m × £14/m	£168
Concrete pads (12 × £20)	£240
Postcrete (10 × £10)	£100
Steel post anchors (Simpson) 8 × £12	£96
Joist hangers + fasteners	£85
Hidden clip system (composite)	£150
Stainless screws (sub-frame fixing)	£40
Composite fascia trim 14m × £18/m	£252
Composite steps (4 × £130)	£520
Weed membrane + vents	£40
Sundries (DPC, brackets, sealant)	£35
Skip 6-yard (excavation spoil)	£290
Labour: 2 carpenters × 4 days @ £260/day	£2,080
Travel & overhead	£120
Subtotal	£5,516
Markup at 22%	£1,213
Total to customer	£6,729

Round to £6,700. Per-m² = £335 — at top end of composite range due to steps + fascia. Compare against £150/m² × 20m² = £3,000 — that's a softwood equivalent. The customer chooses the spec.

Frequently Asked Questions

Do I need planning permission for a deck under 300mm?

In most cases no — under GPDO 2015 it's permitted development if (a) <300mm above ground level, (b) doesn't exceed 50% of the garden coverage when added to other outbuildings/extensions, (c) not in a conservation area / Article 4 / listed building. Always check with local Planning before quoting to protect the customer (and yourself).

How long does a softwood deck really last?