Composite Decking Calculator — Boards, Clips & Coverage
Calculate composite decking boards, hidden clips and any accessory profiles needed for your deck. Composite deck boards have different standard widths and lengths than timber — use this calculator to get accurate quantities for popular brands and profiles.
How the composite decking calculator works
- 1Effective board coverage = board width (mm) + clip gap (typically 6–8 mm for composite)
- 2Board count = deck area ÷ (effective width × board length)
- 3Apply 10% waste for straight lay, 15% for diagonal (composite cuts easily with a saw)
- 4Hidden clip count = (board count × board length ÷ joist spacing) rounded up
- 5Starter/fascia clips different from field clips — check manufacturer spec
- 6End caps and corner trims ordered separately by linear metre
Worked example
Deck: 5 m × 4 m = 20 m². Composite board 145 mm wide, 7 mm clip gap, 4 m board length. Joists at 400 mm centres.
Effective coverage: 152 mm. Rows: 4000 ÷ 152 = 27. Total boards: 27 rows × (5 m ÷ 4 m length) = 34 boards + 10% waste = 38 boards. Clips: ~380 (38 boards × 4 m ÷ 0.4 m joist spacing).
Frequently asked questions
What width are composite decking boards?
Most composite boards are 140–150 mm wide (140, 145 or 150 mm are common standard widths). Some premium profiles are 200 mm. Width affects the number of rows and the number of clips required — always use the actual board width for your calculation.
Do composite decking boards expand?
Yes — composite boards expand and contract with temperature, more than timber. Always follow the manufacturer's gap specifications precisely (typically 5–8 mm between board ends, 6–8 mm between boards). Under-gapping leads to buckling in summer heat.
Can composite decking be cut with a normal saw?
Yes — composite decks cut easily with a standard circular saw or mitre saw with a fine-tooth blade. A 60-tooth blade produces the cleanest cut. Seal cut ends with end grain sealant to prevent moisture ingress, especially on hollow-core boards.
Do I need special fixings for composite decking?
Most composite manufacturers supply their own hidden clip systems — these are designed to allow for thermal movement and should not be substituted with generic clips. Using the wrong clips can void the warranty and cause board movement or damage.
Does composite decking get slippery when wet?
Quality composite decking with a wood-grain embossed or brushed surface has good slip resistance when wet (typically R11–R13 slip rating). Smooth composite surfaces can be slippery — check the manufacturer's slip resistance classification before purchasing for exposed decks.