How it is worked out
Each row takes up one board plus one gap. The deck width divided by that gives the number of rows, rounded up. Rows × deck length is the lineal metres.
Joists run across the boards. The count is the deck length ÷ joist spacing, plus one for the far end. Two Australian decking guides give 450 mm centres as the maximum for standard 19 mm boards.
The fixing count is two per board at every joist, the pattern those same guides describe. It does not allow for doubled joists where boards butt end to end.
Worked example
The numbers this page opens with: deck length (the way the boards run) 4.8 m, deck width (across the boards) 3 m, board width 90 mm, gap between boards 3 mm, board length you are buying 4.8 m, joist spacing 450 mm, extra for offcuts 10 %.
The answer: deck area 14.40 m²; rows of boards 33 rows; lineal metres, exact 158.4 m; lineal metres with 10% extra 174.2 m; boards at 4.8 m 37 boards; joists 11 joists; fixings, two per board per joist 726 fixings.
Questions
Does this cover the frame?
It counts the joists so you know how many there are, but it does not size bearers, posts or joists. Those depend on spans and loads and belong to your plans.
Should I order by lineal metre or by board?
Suppliers sell both ways. Take both figures. If your deck is longer than the boards you can get, allow more extra, because every join lands on a joist and makes offcuts.
Where the figures come from
- Pentarch Forestry decking installation guide (gaps, 450 mm joist centres)
- Dahlsens decking installation guide (spacers, joist span, fixings)