How the calculator counts
boards (lineal m) = rows × deck length × (1 + allowance)
joists = deck length ÷ joist spacing, rounded up, + 1 · fixings = rows × joists × 2
For the 4.8 × 3.6 m example, each 86 mm board plus its 3.5 mm gap covers 89.5 mm. So 3,600 mm needs 41 rows, the last one ripped to width. The rows are 4.8 m long, so 41 × 4.8 = 196.8 m, or 216.5 m with 10 %. At 500 mm spacing, 4.8 m needs 11 joists, each 3.6 m long.
Most suppliers sell decking by the lineal metre, in random or set lengths. If yours sells set lengths, the table under the result shows how many boards each length takes and the offcut left on every row. Where the deck length matches a stock length, there is almost no waste.
How far apart the joists can be
The Australian standard for timber-framed construction, AS 1684, links decking thickness and grade to the maximum joist spacing. The timber industry's deck design guide, published by WoodSolutions, reproduces the figures:
| Decking | Thickness | Joists at most |
|---|---|---|
| Hardwood, standard or medium-feature grade | 19 mm | 500 mm |
| Cypress, grade 1 | 19 mm / 21 mm | 400 mm / 450 mm |
| Treated softwood (pine), standard grade | 19 mm / 22 mm | 400 mm / 450 mm |
These are spacings for boards laid square across the joists. Laid at 45°, a board spans further between joists, so the joists must come closer. The guide's example is 350 mm for 19 mm hardwood. The calculator warns you if the spacing you enter is wider than the standard allows for the chosen board.
Gaps between boards
Boards shrink and swell with the weather, so they are laid with a gap. It also lets water and leaves drain through. The deck design guide recommends these gaps at installation, and notes that 3–4 mm works best over the long term:
| Board | Cover width | Seasoned | Unseasoned |
|---|---|---|---|
| Hardwood | 86–90 mm | 3–4 mm | 2–3 mm |
| Hardwood | 91–140 mm | 4–5 mm | 2–3 mm |
| Softwood | 70–90 mm | 3–4 mm | – |
| Softwood | 91–120 mm | 4–5 mm | – |
Fixings
- Two at every joist. The guide requires each board to be fixed at each joist with at least two nails, finished flush with the surface and staggered, so they don't split the board along one line. Screws follow the same pattern, to the screw maker's specification.
- Keep off the edges. Stay at least 12 mm from the edges and ends of boards and pre-drill, which stops hardwood splitting.
- The right metal. Use hot-dip galvanised or stainless steel. Electroplated fasteners break down early. Near pools and the coast, the guide calls for more durable fixings.
- Ends on a joist. Every board end needs a joist under it to fix to. Stagger the joins so they don't line up across the deck.
Per square metre, 86 mm boards with 3.5 mm gaps take about 11.2 lineal metres. With joists at 500 mm they need about 45 fixings.
Next to the house
Keep decking boards 10 mm clear of the house wall so water can drain. Where the deck meets the house, AS 3660.1 requires a 25 mm gap between the timber and the building so termite inspection is possible. Decks fixed to the house also have their own requirements in the National Construction Code. That, together with the height of the deck, is why the calculator stops at the joists: bearers, stumps and footings belong in the plans. Councils set their own approval rules for decks, so check before you build.
Decking questions
How many lineal metres of decking per square metre?
1,000 ÷ (board width + gap). For 86 mm boards with 3.5 mm gaps that is about 11.2 m per m². For 140 mm boards with 4.5 mm gaps it is about 6.9 m per m². Add about 10 % for offcuts.
How far apart should deck joists be?
No more than 500 mm for 19 mm standard-grade hardwood decking. For treated pine, the limit is 450 mm with 22 mm boards and 400 mm with 19 mm boards, as set by AS 1684. For boards laid diagonally, bring the joists closer, to 350 mm for 19 mm hardwood at 45°.
What gap should I leave between decking boards?
For seasoned boards: 3–4 mm for boards up to 90 mm wide and 4–5 mm for wider boards. For unseasoned hardwood: 2–3 mm, because it shrinks as it dries. About 3–4 mm is ideal over the long term.
How many screws do I need for a deck?
Two for every board at every joist. A 4.8 × 3.6 m deck in 86 mm boards with joists at 500 mm has 41 rows and 11 joists, so 41 × 11 × 2 = 902. That is about 45 per square metre.
Sources and method
- Forest and Wood Products Australia, WoodSolutions Technical Design Guide 21: Domestic Timber Deck Design (2025). Covers Table 2 (maximum joist spacing, from AS 1684), Table 4 (gaps at installation), fixing at every joist with at least two nails, edge distances, fastener durability, and clearances to the house.
- Standards Australia, AS 1684 Residential timber-framed construction, and AS 3660.1 Termite management, as quoted in the guide above.
Written by DueRiva · Last reviewed 24 September 2026 · Report an error