Composite vs timber decking: how the quantities differ
Last reviewed 1 July 2026
Short answer
The board count is broadly similar, but almost everything else changes. Composite typically needs joists at 300–400 mm centres instead of 400–500 mm, which can mean 30% more frame timber. It uses clips instead of screws, needs larger end gaps, and weighs roughly twice as much per square metre — which matters for delivery and for the frame beneath it.
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Joist centres are the big one
Composite boards are less stiff than timber of the same thickness and they creep under sustained load. Manufacturers respond by specifying closer joist centres, commonly 300–400 mm for boards laid square and as little as 250 mm for boards laid at 45°.
Going from 400 mm to 300 mm centres on a 4 m run takes you from 11 joists to 14 — a 27% increase in frame timber, plus the extra hangers or fixings that go with it. On a large deck that difference outweighs several boards.
| Timber at 400 mm centres | Composite at 300 mm centres | |
|---|---|---|
| Joists | 11 × 3 m = 33 m | 14 × 3 m = 42 m |
| Board rows (144 mm + 5 mm) | 21 | 21 |
| Fixings | 462 screws (2 per crossing) | 294 clips (1 per crossing) |
| Approximate board weight | About 130 kg | About 250 kg |
Always use the span table for the specific board. These figures illustrate the shape of the difference, not a specification.
Fixings
Face-fixed timber decking takes two screws per board at every joist. Composite systems mostly use hidden clips that sit in a groove in the board edge, one clip per board per joist — half the count, but each clip costs several times more than a screw.
Clips also set the side gap for you, which removes one source of error. Check whether the system needs starter clips at the first board and finishing trim at the last, because those are separate items that are easy to leave off an order.
Weight and delivery
Solid composite runs around 1,100–1,250 kg/m³ against roughly 550 kg/m³ for treated softwood. A 12 m² deck in 25 mm solid composite is about 250 kg of boards; the same deck in 28 mm softwood is about 130 kg.
Hollow-core composite closes much of that gap, at around 40% less than solid, but it is less robust at exposed edges and usually needs end caps.
The practical consequence is at the kerb. Composite arrives on a pallet, and a pallet needs somewhere flat to be set down and a way to move it.
What does not change
- The row calculation. Rows are still span divided by board module, rounded up.
- The bearer and post layout, which is driven by the joist span, not the board material.
- The need for ventilation beneath the deck. Composite still cups if it can only dry on one face.
- The waste allowance. If anything composite needs slightly less, because boards arrive straight and undamaged more often than timber does.
Important: Do not reuse a timber-spec frame for composite
Safety notice: Material planning only