Composite Decking Cost Calculator
Calculate your decking project requirements and costs instantly
Decking Dimensions
Quality & Pricing Tier
Substructure & Fixings
Finishing Accessories
How to Use This Calculator
This calculator helps you estimate the materials and costs required for your composite decking project. Follow these steps for accurate results:
Step 1: Measure Your Area
- Measure the length and width of your proposed decking area in metres
- For irregular shapes, break them into rectangles and calculate separately
- For total square metre calculations, enter the area as length and “1” as width
Step 2: Choose Board Specifications
- Select board length based on your deck dimensions to minimise waste
- Longer boards create fewer joins but may be harder to handle
- Choose board width according to your preferred aesthetic and budget
Step 3: Select Quality Tier
- Budget range offers good value for straightforward projects
- Standard range provides excellent durability and appearance
- Premium range features superior fade resistance and wood grain texture
- Luxury range includes advanced capping technology and 25+ year warranties
Step 4: Configure Substructure
- Aluminium joists are recommended for composite decking (won’t rot or warp)
- Closer joist spacing provides firmer feel underfoot
- Hidden fixings create a clean, professional finish without visible screws
- Weed membrane prevents growth underneath your deck
Step 5: Add Finishing Touches
- Fascia boards cover the deck framework for a polished appearance
- Edge trims provide neat transitions between different surfaces
- Step nosing offers safe, attractive stair edges
Composite Decking Material Breakdown
What You’ll Receive
Your composite decking project typically requires these components:
- Decking Boards: The visible walking surface, calculated with wastage allowance
- Joists/Bearers: Supporting framework, typically aluminium or timber
- Fixings: Hidden clips or screws to secure boards (approximately 20 clips per m²)
- Starter Clips: Special clips for the first and last boards
- Fascia Boards: Optional decorative edging to conceal framework
- End Caps: Finishing pieces for hollow board ends
Labour Costs
Professional installation in the UK typically ranges from £50-90 per m², depending on:
- Project complexity and site accessibility
- Ground preparation requirements
- Regional price variations
- Existing structure removal
Cost Factors Explained
Board Quality Differences
Composite decking prices vary based on several factors:
- Capping Technology: Premium boards feature protective polymer caps on all four sides, preventing moisture absorption and staining
- Wood Content: Higher quality products use finer wood particles for smoother texture
- UV Stabilisers: Better products resist fading and maintain colour longer
- Density: Denser boards feel more solid and resist scratches better
- Warranty Length: Premium products offer 20-25 year warranties versus 10-15 years for budget options
Hidden Costs to Consider
- Ground preparation and levelling (£15-30/m²)
- Concrete footings for posts (£5-15 per footing)
- Demolition of existing decking (£10-25/m²)
- Building regulations compliance (£200-400 for larger decks)
- Delivery charges (£50-150 depending on distance)
Composite vs Timber Decking Comparison
| Factor | Composite Decking | Timber Decking |
|---|---|---|
| Initial Cost (materials) | £35-95/m² | £15-45/m² |
| Lifespan | 25-30 years | 10-15 years |
| Annual Maintenance | Minimal – wash only | Sanding, staining, sealing |
| Maintenance Cost (yearly) | £0-20 | £150-300 |
| Fade Resistance | Excellent (minimal fading) | Poor (requires re-staining) |
| Splinter Risk | None | Moderate to high |
| Slip Resistance | Good (textured surface) | Variable (smooth when wet) |
| Eco-Friendliness | Recycled materials | Natural but requires treatment chemicals |
Long-Term Value Analysis
Over 15 years, a 20m² composite deck costs approximately £2,500-3,000 total (materials + minimal maintenance), whilst timber costs around £2,800-3,500 (materials + annual treatment + replacement boards). Composite decking typically pays for itself within 8-12 years through maintenance savings.
Planning Your Project
Do You Need Planning Permission?
In England and Wales, decking is considered permitted development if:
- The deck is no more than 300mm above ground level
- Total decking and outbuildings don’t exceed 50% of garden area
- Your property isn’t listed or in a conservation area
For raised decks over 300mm, you’ll need Building Regulations approval. Always check with your local authority before starting.
Best Installation Practices
- Slope: Create a 1:100 fall (10mm per metre) away from buildings for drainage
- Ventilation: Leave 5-8mm gaps between boards for expansion and water drainage
- Support: Place joists on solid foundations, not directly on soil
- Direction: Install boards perpendicular to joists for maximum strength
- Timing: Spring or autumn offers ideal installation conditions
Measuring Tips
- Always measure twice before ordering materials
- Account for obstacles like trees or garden features
- Consider leaving access panels for utilities underneath
- Measure perimeter accurately for fascia and trim calculations
- Take photographs with measurements for reference
Frequently Asked Questions
How much does composite decking cost per square metre in the UK?
Composite decking materials range from £25/m² for budget options to £95/m² for premium products. The average quality range costs £40-60/m². Including installation, expect total costs of £90-180/m² depending on complexity.
How many boards do I need for my deck?
Divide your deck area by the coverage of a single board. For example, a 3.6m board that’s 145mm wide covers 0.522m². A 20m² deck needs approximately 38 boards, plus 10% wastage = 42 boards total.
Can I install composite decking myself?
Yes, composite decking is DIY-friendly for those with moderate skills. You’ll need basic carpentry tools, a saw suitable for composite materials, and a drill. Most manufacturers provide detailed installation guides. Budget 2-3 days for a 20m² deck.
What wastage percentage should I add?
Add 5-10% for straightforward rectangular decks, 10-15% for decks with angles or multiple levels, and 15-20% for complex designs with curves or intricate patterns. This accounts for cuts, mistakes, and future repairs.
Should I use aluminium or timber joists?
Aluminium joists are recommended for composite decking. They won’t rot, warp, or require treatment. Whilst initially more expensive (£18-25/m vs £8-12/m for timber), they last longer and maintain a level surface. Timber joists can sag over time, causing composite boards to flex.
How long does composite decking last?
Quality composite decking lasts 25-30 years with minimal maintenance. Premium products with full capping can last even longer. In contrast, treated timber decking typically lasts 10-15 years before requiring replacement.
What’s the difference between hollow and solid composite boards?
Hollow boards are lighter, less expensive, and easier to install but may feel slightly less solid underfoot. Solid boards offer superior strength and a premium feel but cost 20-30% more and require more robust substructure support.
Do I need hidden fixings or can I use screws?
Hidden fixings create a cleaner appearance without visible screws and allow proper board expansion. They cost £8-15 per m² but significantly improve aesthetics. Face screwing is cheaper but can void warranties on premium boards and may cause cracking.
How do I maintain composite decking?
Sweep regularly to remove debris and wash with soapy water 2-3 times yearly. For stubborn stains, use a specialist composite decking cleaner. Avoid pressure washers above 1500 PSI as they can damage the surface. No sanding, staining, or sealing required.
Can composite decking be installed on existing timber frame?
Only if the existing frame is structurally sound, level, and properly treated. Inspect thoroughly for rot, insect damage, or sagging. Most manufacturers recommend fresh framing to maintain warranty coverage and optimal performance.
What joist spacing is required for composite decking?
Most composite boards require 350mm joist spacing (measured from centre to centre). Heavy-duty solid boards may span up to 400mm, whilst lighter hollow boards may need 300mm spacing. Always check manufacturer specifications.
Does composite decking get hot in summer?
Composite decking absorbs heat and can become warm in direct sunlight, similar to any dark surface. Lighter colours stay cooler. For south-facing decks in full sun, consider lighter shades or provide partial shade. It won’t get hot enough to cause burns but may be uncomfortable barefoot.
Common Calculation Mistakes
Mistake 1: Insufficient Wastage Allowance
Many people underestimate wastage, ordering exactly the calculated amount. This leaves no margin for cutting errors, damaged boards, or future repairs. Always add at least 10% wastage, more for complex layouts.
Mistake 2: Forgetting Fascia Coverage
Calculate fascia board requirements by measuring the entire perimeter, not just visible edges. Include any steps or level changes. Fascia boards are typically 2.9m or 3.6m long, so divide total perimeter by board length and round up.
Mistake 3: Wrong Joist Quantities
Joists run perpendicular to boards. For a 5m x 4m deck with boards running along the 5m length, you need joists spanning the 4m width at 350mm intervals = 12 joists, each 4m long. Don’t forget perimeter rim joists.
Mistake 4: Ignoring Weight Capacity
Composite boards weigh 8-12kg per metre. A 20m² deck with boards weighs approximately 250-350kg before adding people, furniture, or planters. Your substructure must support this plus live loads (typically 1.5-2.5 kN/m² for domestic decking).
Mistake 5: Not Accounting for Board Direction
Board orientation affects waste. Running boards along the longest dimension typically minimises cuts and waste. Plan board layout before ordering to optimise material usage.
Mistake 6: Mixing Imperial and Metric
UK suppliers use metric measurements (metres, millimetres) but some older properties have dimensions in feet. Always convert to metres before calculating. 1 foot = 0.3048 metres.
Cost-Saving Strategies
Timing Your Purchase
Composite decking prices often drop during winter months (November-February) when demand is lower. Many suppliers offer 10-20% off-season discounts. Order ahead for spring installation.
Choosing the Right Board Size
Longer boards reduce waste and joints but cost more per board. Calculate which length minimises cuts for your specific dimensions. Sometimes mixing board lengths optimises material usage.
DIY Installation
Professional installation adds £50-90/m² to costs. DIY installation can save £1,000-2,000 on a typical 20m² deck. Hire tools rather than buying them – a composite-specific saw blade costs £40 but hiring costs £15/day.
Bulk Ordering
Some suppliers offer discounts for orders over certain thresholds (typically 50m² or £2,000). If planning multiple projects or can coordinate with neighbours, bulk ordering saves 5-15%.
Standard Colours
Grey, brown, and teak shades are typically cheaper than specialty colours like black, charcoal, or dual-tone boards. Standard colours cost 10-20% less and have better availability.
Essential Accessories Only
Start with essential components (boards, joists, fixings) and add fascia or edge trims later if budget is tight. The deck remains functional without these finishing touches, which can be added anytime.