Composite Decking Calculator
Calculate materials, quantities and costs for your composite decking project
Additional Components
Your Decking Requirements
Materials needed for your project
Deck Area
0m²
Decking Boards
0boards
Linear Metres
0m
Joist Runs
0runs
What These Results Mean
Material Breakdown
These calculations include your selected wastage allowance to account for cutting, fitting adjustments, and potential errors during installation. Always verify measurements on-site before ordering materials.
How to Plan Your Composite Decking Project
Planning a composite decking installation requires careful consideration of materials, measurements, and structural requirements. Composite decking offers superior durability compared to traditional timber, with minimal maintenance needs and excellent weather resistance making it ideal for British climates.
Measure Accurately
Measure your deck area multiple times to confirm dimensions. Account for any obstacles, slopes, or irregular shapes that may affect material requirements.
Choose Board Length
Select board lengths that minimise waste. Longer boards reduce joins but may be harder to handle. Consider access to your installation site.
Plan Subframe Layout
The subframe must provide adequate support. Joist spacing of 300mm is standard for composite decking, with closer spacing for heavier use areas.
Include Wastage
Always order extra material. A 10% wastage allowance covers cutting errors, board defects, and future repairs. Complex designs need 15%.
Board Spacing Requirements
Composite decking boards require a 5-6mm gap between each board to allow for thermal expansion and water drainage. This spacing prevents buckling during hot weather and allows moisture to escape, preventing mould growth. Hidden fixing systems automatically create the correct spacing, whilst visible screws require careful measurement.
Joist and Subframe Specifications
The subframe forms the structural foundation of your deck. For composite decking, joists should be spaced no more than 300-400mm apart, measured centre-to-centre. Closer spacing at 300mm provides better support and reduces board flex underfoot. Joists must be installed perpendicular to the decking board direction, with adequate ventilation underneath to prevent moisture accumulation.
Installation Direction
Decking boards can run parallel to your house or perpendicular, depending on your design preference and structural requirements. Running boards parallel often creates a visually elongated space, whilst perpendicular installation can make narrow areas appear wider. Diagonal installations at 45 degrees create visual interest but require closer joist spacing and generate more waste.
Material Selection Guide
Composite Decking Board Types
Composite decking comes in several grades, each suited to different applications. Standard composite boards contain recycled wood fibres and plastic, offering good durability at an economical price point. Premium capped composite features a protective polymer shell that enhances stain resistance and colour retention. The board profile affects both appearance and installation, with grooved edges designed for hidden fixing systems and square edges for visible screws.
Colour and Finish Options
Composite decking manufacturers offer extensive colour ranges from light greys to deep browns, with embossed wood-grain textures that replicate natural timber. Lighter colours reflect heat and stay cooler underfoot during summer, whilst darker tones create rich, contemporary aesthetics. Consider how colours will complement your property’s exterior and garden features. Most composites fade slightly during the first year before stabilising to their long-term colour.
Fixing Systems
Hidden fixing systems create a clean, screw-free surface appearance by securing boards from beneath or through grooved edges. These clips maintain consistent spacing automatically and allow individual boards to be removed if needed. Traditional face screwing remains viable for solid-edged boards and offers the simplest installation method. Stainless steel or coated fixings prevent rust staining and ensure long-term holding power in British weather conditions.
Frequently Asked Questions
How much composite decking do I need?
Calculate your deck area in square metres by multiplying length by width. Divide this by the coverage area of your chosen board size, then add 10% wastage. The calculator above automates this process and includes all necessary components.
What size joists do I need for composite decking?
Standard 47mm x 150mm treated timber joists suit most residential decking installations. For spans over 2.5m between support posts, upgrade to 47mm x 200mm joists. Aluminium composite joists offer enhanced durability but cost more than treated timber options.
Can I install composite decking directly onto concrete?
Yes, but proper preparation is vital. The concrete must have adequate drainage slope and be structurally sound. Install plastic pedestals or treated timber battens to create ventilation gaps beneath the decking. Never lay boards directly onto concrete as trapped moisture causes problems.
How long does composite decking last?
Quality composite decking typically lasts 25-30 years with minimal maintenance. Manufacturers usually provide 25-year warranties against structural defects, fading, and staining. Regular cleaning and prompt removal of debris helps maximise lifespan.
Do I need building regulations approval?
Decks under 300mm above ground level and not attached to buildings typically don’t require approval. Raised decks, those near boundaries, or attached structures may need planning permission and building control approval. Always check with your local authority before starting construction.
What is the difference between hollow and solid composite boards?
Hollow boards contain air channels that reduce weight and material costs whilst maintaining structural strength. Solid boards offer superior rigidity and screw-holding power but weigh more and cost approximately 20-30% more. Both types perform well when installed correctly with appropriate joist spacing.
How do I maintain composite decking?
Sweep regularly to remove debris and wash with warm soapy water twice yearly. Remove stains promptly with appropriate composite deck cleaners. Avoid pressure washers on high settings as they can damage the surface texture. Check fixings annually and tighten any loose screws.
Can composite decking be installed in winter?
Yes, but temperature affects installation. Composite boards contract in cold weather, so leave slightly larger expansion gaps when installing below 10°C. Avoid installation during frost or on frozen ground. The material becomes more brittle in cold conditions, so handle carefully to prevent damage.
Why include fascia boards?
Fascia boards create a finished edge that conceals the subframe and cut board ends. They provide a professional appearance whilst protecting the structural timber from weather exposure. Matching composite fascia maintains consistent aesthetics and eliminates the need to paint or maintain timber edging.
What foundations do I need?
Ground-level decks typically rest on concrete pads or paving slabs under each joist. Raised decks require concrete footings below frost line supporting timber or steel posts. The foundation must be level, stable, and prevent ground contact with timber components. Allow adequate ground clearance for ventilation.