Composite Decking Calculator UK
Calculate the exact number of composite decking boards, joists, and clips needed for your UK garden project. Includes wastage allowance and instant material estimation.
Material Estimation
Enter your deck dimensions and material specifications
Add 10% for standard decks, or 15% for complex shapes and angles.
Material Estimation
Boards, joists, and clips required for your project
Enter your deck dimensions and material specs above, then click Calculate Materials.
Standard Composite Decking Specifications
Typical sizes and spacing guidelines for composite decking installations in the UK. Always check your specific manufacturer’s guidelines.
| Material | Typical Size | Spacing / Notes |
|---|---|---|
| Composite Board | 140mm x 3600mm | 5-8mm expansion gap between boards |
| Composite Joist | 40mm x 60mm x 3600mm | 400mm centres (UK standard) |
| Hidden Fastener Clips | N/A | 1 clip per board/joist intersection |
| Post / Pedestal | 100mm x 100mm | 1.2m – 1.5m spacing for support |
Composite Decking FAQ
Everything you need to know about planning and installing composite decking in the UK climate.
To calculate composite decking, measure the total area of your deck in square metres (length x width). Divide the deck width by the board width plus the expansion gap to find the number of boards across. Divide the deck length by the board length to find the number of board lengths. Multiply these together and add a 10% wastage allowance for cuts and mistakes.
In the UK, the standard joist spacing (centres) for composite decking is 400mm. This ensures the boards are adequately supported and prevents sagging or bouncing, especially important for composite materials which can be more flexible than traditional timber.
Yes, it is highly recommended to add a 10% wastage allowance for standard rectangular decks. If your deck has complex shapes, angles, or multiple cutouts, you should increase this to 15%. This accounts for off-cuts, mistakes, and future repairs.
For hidden fastener systems, you typically need one clip at every intersection where a board meets a joist. Calculate the number of joist lines and multiply by the total number of boards. It’s always wise to buy an extra 5% of clips in case of breakage during installation.
Yes, composite decking expands and contracts with temperature changes. In the UK climate, you must leave a 5mm to 8mm expansion gap between boards, and a 10mm to 15mm gap around the perimeter and against any solid structures to allow for this movement.