400mm centre-to-centre is the UK standard for domestic decks with boards laid square to the joists. Drop to 300mm for commercial use, heavy furniture, or boards laid diagonally (diagonal boarding increases the effective span).
| Deck type | Joist centres | Why |
|---|---|---|
| Domestic deck, timber boards, square laid | 400mm | UK standard |
| Diagonal boards (45°) | 300-350mm | Span increases ~42% |
| Composite boards | 300-350mm (check manufacturer) | Warranty requirement |
| Commercial / heavy use | 300mm | Building control expectation |
| Balcony on pedestals | 400mm with 47x50mm joists | Pedestal points as supports |
Yes. Most composite manufacturers require 300-350mm centres because composite flexes more than timber — always check the board's spec sheet; invalidating the warranty is expensive.
47x150mm C16 treated timber at 400mm centres spans up to about 3m between supports. For longer spans or ground-level decks on pedestals, the planner calculates the exact schedule.
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