Garden SurveyorHow to work out decking materials
Start with your deck size, then work out boards from the laid width of each board plus the gap. Joists are set out at 400 mm centres for standard softwood boards, or 300 mm for composite or diagonal layouts. The calculator above sizes boards, joists, posts, screws and clips together and adds a waste allowance so your order is complete in one go.
Decking spec guide (UK)
| Item | Guide |
|---|---|
| Joist spacing — standard boards | 400 mm centres |
| Joist spacing — composite / diagonal | 300 mm centres |
| Gap between boards | 5–6 mm |
| Board waste allowance | +10% for cuts |
Always check the board manufacturer's maximum joist span — composite boards usually need closer joist centres than softwood.
Building a raised deck on a base?
Size the sub-frame base and footings, then build a full materials take-off.
Frequently asked questions
- How many deck boards do I need?
- Divide your deck area by the coverage of one board (board length × the laid width including the gap). Add about 10% for cuts and waste. The calculator above does this and rounds up to whole boards so you order enough to finish.
- What spacing should decking joists be at?
- For standard softwood deck boards, space joists at 400 mm centres. For thinner or composite boards, or for diagonal board patterns, reduce to 300 mm centres for a firm, flex-free deck. Always check the board manufacturer's recommended span.
- What gap should I leave between deck boards?
- Leave a 5–6 mm gap between boards for drainage and to allow timber to move with moisture. Use a consistent spacer as you fix each board so the gaps stay even across the deck.
- How many screws do I need for decking?
- Use two screws per board at every joist crossing. The calculator estimates total screws from your board and joist layout, with a small surplus so you don't run short mid-job.
- Do I need posts and a sub-frame under a deck?
- Yes — a deck sits on a joist sub-frame supported by posts or a solid base. Ground-level decks can sit on a compacted, levelled base, while raised decks need posts set on concrete footings. Plan post spacing to suit the joist span.