Decking Calculator — Boards & Cost
Calculate decking boards, joists and screws needed. Get material cost estimates.
By Konstantin Iakovlev · Founder, Calks.uk
Last updated: · Verified against HMRC and GOV.UK 2026/27 rates
Boards
26
Joists
11
Screws
208
Area
12.0 m²
Board Cost
£208.00
Disclaimer
This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.
How It Works
Decking board quantity is calculated by dividing the deck area by the coverage of each board. Standard softwood decking boards are 2.4 m, 3.0 m, 3.6 m or 4.8 m long and 120–145 mm wide (face width). Each board covers approximately 0.29–0.70 m² depending on size. A 5–10% waste allowance covers cuts and offcuts.
Substructure requirements include joists (typically 47 × 150 mm at 400 mm centres, supported on post-and-beam or concrete pads) and screws (approximately 20–25 stainless-steel screws per m²). The calculator also estimates joist hangers, post brackets and any concrete needed for footings.
Enter the deck dimensions. The calculator shows boards, joists, screws and an estimated cost. For treated softwood decking, expect £20–£40 per m² for materials. Composite decking is £40–£80 per m² but lasts longer and requires less maintenance. Any raised deck above 300 mm may need Building Regulations approval.
How much decking do you need? Coverage formula: total area = length × width. Add 10% waste for cutting and offcuts. Sample 4m × 3m deck = 12 m² + 10% = 13.2 m². With 145mm-wide boards: 1m² needs ~7 linear metres of board (after 3-5mm gaps). For 13.2 m²: 93 linear metres of board. Standard board length 3.6m = 26 boards minimum. Always buy 2-3 extra boards for replacements (a single damaged board needs the same batch for colour match).
Decking material types and costs UK 2026. Pressure-treated softwood (pine/spruce): £15-£30/m² + screws. Lifespan 10-15 years, requires annual oiling. Hardwood (oak, ipe, balau): £60-£140/m². Premium look, 25-50 year life, minimal maintenance. Composite (Trex, Millboard, Ecoscape): £80-£180/m². 25-30 year guarantee, no rot, no splinters, fade-resistant. PVC: £100-£200/m², lightweight, ultra-low maintenance but plasticky look. Total project cost adds £20-£50/m² in joists, bearers, screws and ground anchors.
Joist structure — the hidden cost and skill. Joists span the deck — 100×47mm (4×2) C16 grade treated softwood at 400mm centres for residential decking. Span: max 1.8m between supports for 47mm joists. Longer spans need deeper joists (150×47mm). Bearers (beams running perpendicular under joists) on concrete piers/posts. Ground clearance: minimum 150mm to allow airflow and prevent rot. Damp-proof membrane between concrete and timber. Joist hangers or notched-and-bolted connections.
Planning permission for decking. Permitted development if: deck height under 300mm above ground level; combined with other extensions/sheds, doesn't cover more than 50% of garden; not in front of house. Above 300mm height: planning permission required (raised platforms). Conservation area: tighter rules — check local council. Building Regulations: usually not required for ground-level decking but apply if attached to house and over 600mm above ground (structural). Boundary distance: typically 1m+ from neighbour's fence to avoid disputes and overlooking.
Decking installation and maintenance. Foundation: concrete pads or post anchors at ~1.5m centres. Slope deck slightly (10mm per metre) away from house for drainage. Board gaps: 3-5mm to allow expansion. Screw into pre-drilled holes (avoid splitting at board ends — 25mm from end minimum). Use stainless steel or coated decking screws (rust-proof). Annual maintenance softwood: clean, sand light splinters, re-oil. Composite: pressure-wash once or twice yearly only. Anti-slip: clean moss annually — wet timber gets dangerously slippery in autumn/winter.
Example: Deck 4 m × 3 m, 145 mm softwood boards, 3.6 m lengths
- Area: 12 m²
- Boards (145 mm wide, 3 m deck width): 3,000 ÷ 145 ≈ 21 boards
- Add 10% waste: 23 boards at 4 m length
- Joists (400 mm centres, 4 m span): 3,000 ÷ 400 + 1 = 9 joists at 4 m
- Screws: 12 × 22 ≈ 264 screws (2 boxes of 200)
Frequently Asked Questions
- How long does a UK decked area last?
- Pressure-treated softwood: 10-15 years with annual oiling and cleaning; less if untreated or constantly wet. Hardwood (oak, ipe, balau): 25-40 years with annual maintenance, can reach 50+ with care. Composite decking (Trex, Millboard, Ecoscape): 25-30 year manufacturer warranties typical; lifespan 30-40 years. Adverse factors: shaded north-facing decks rot 2× faster than south-facing; algae and moss accelerate decay. Best practice: 2-3% slope for drainage, 150mm+ ground clearance, annual clean and re-oil.
- Decking vs patio — cost and longevity compared.
- Decking installed: £80-£180/m² (softwood to composite). Paved patio installed: £80-£200/m² (concrete to natural stone). Decking advantages: warmer underfoot, doesn't crack, easier on irregular ground, faster installation. Patio advantages: longer lasting (40-100+ years), no maintenance (washed annually only), no slip-when-wet issues. Best for British climate: paved patio for ground level + small composite deck for raised section. Resale value: stone patios usually add more property value than softwood decks.
- Do I need planning permission for decking?
- Permitted development if: deck/platform under 300mm above ground level; combined with extensions doesn't cover over 50% of garden curtilage; not in front of house; doesn't overlook neighbours. Above 300mm: requires planning permission (counted as 'raised platform'). Building Regulations: usually not required for ground-level. Apply if attached to house and over 600mm above ground. Listed buildings: any external work needs listed building consent. Conservation areas: tighter rules — check local council. Always check before building — retrospective planning is more expensive than getting permission upfront.
- Composite vs timber — which is better?
- Cost: composite 2-3× upfront cost (£80-£180/m² vs £30-£60/m² timber); composite cheaper over 20+ years (no annual oiling). Performance: composite doesn't rot, splinter, warp, or fade — but can get hot in direct sun (50°C+ on dark colours). Timber: warmer, more 'natural' aesthetic; biodegradable. Slip resistance: most composites have textured/grooved surface — better than wet timber. Environmental: timber (FSC-certified) lower carbon footprint; composite often made from recycled plastic but harder to recycle at end of life.