What a decking order is actually made of
Four separate counts, and they don't scale together. Boards go up with the area, joists go up with the length only, bearers go up with the width only, and screws go up with both at once. That's why guessing a deck off "so many square metres" lands wrong — you'll be close on boards and nowhere near on fixings.
The gap is not spare — it's cover
An 86mm board laid with a 4mm gap covers 90mm of deck. Over a 4m width that's 45 rows instead of 47 — and on a 6m long deck those two rows are 12 lineal metres of decking you didn't need to buy. On a big job the gap is worth real money, which is why every board maker publishes a cover width.
| Board | Gap | Cover | Lineal metres per m² |
|---|---|---|---|
| 66 × 19 hardwood | 4 mm | 70 mm | 14.3 |
| 86 × 19 hardwood | 4 mm | 90 mm | 11.1 |
| 90 × 22 treated pine | 4 mm | 94 mm | 10.6 |
| 136 × 19 hardwood | 5 mm | 141 mm | 7.1 |
| 140 × 22 treated pine | 5 mm | 145 mm | 6.9 |
How big should the gap be?
It depends on how wet the board is the day you screw it down, and nothing else much.
- Boards that arrive wet — a lot of merbau and unseasoned hardwood comes off the truck heavy. It is going to shrink. Lay it tight, around 3mm, and the gap will open on its own over the first summer.
- Kiln-dried boards stored under cover — these will take up moisture once they're outside and swell. Leave 4–5mm or you'll get boards buckling against each other.
- Wide boards move more. A 140mm board changes width roughly twice as much as a 70mm one for the same moisture change. Go bigger on the gap.
- Never zero. Water and leaf litter have to clear, or the deck rots from the top surface down. Three millimetres is a practical floor.
Sticker the pack on site for a week or two before you lay it, out of the rain and off the ground. Boards that acclimatise on the job move less afterwards than boards laid straight off the truck.
A worked example
A 5.0m × 3.0m deck, 86 × 19 hardwood at a 4mm gap, joists at 450 centres, bearers at 1800, boards bought in 5.4m lengths:
- Rows: 3000 ÷ 90 = 33.3, rounded up to 34 rows
- Decking: 34 × 5.0 = 170 m, plus 10% waste = 187 lineal metres — that's 35 lengths
- Joists: 5000 ÷ 450 = 11.1, rounded up to 12 bays, so 13 joists at 3.0m = 39 m
- Bearers: 3000 ÷ 1800 = 1.7, rounded up to 2, so 3 bearers at 5.0m = 15 m
- Crossings: 34 × 13 = 442, at two screws each = 884 screws — two boxes of 500
Joist and bearer spacing
Joist centres are set by the board, not by you. Most 19mm Australian hardwood decking is specified at a maximum of 450mm; some 22mm treated pine allows a bit more; composite and capped boards often want 300–400mm, and anything laid diagonally needs the centres tightened because the board is spanning further between supports. The number is on the board maker's spec sheet — use theirs.
Bearer centres are a different question entirely: that spacing is the joist's span, and it comes out of the AS 1684 span tables against the joist section, the species and grade, and whether the deck is close to the ground or up in the air. 1800mm is a common starting point for 140 × 45 treated pine joists on a low deck, but treat it as a placeholder until you've read the table.
Screws, and the ones that don't rust out
Two fixings per board at every joist is standard — it stops the board cupping and lifting at the edge. Use 65mm decking screws into 19mm board, pre-drill and countersink in hardwood so you don't split the ends, and keep fixings about 15mm in from the board edge.
Near the coast or over a pool, use grade 316 stainless. Inland, class 3 coated is usually enough. Cheap screws on an expensive deck is a false economy you get to look at every day — the rust bleeds down the face of the board within a season.
Before you build
A deck more than a metre above the ground generally needs a barrier around it, and often needs building approval — the threshold and the process vary by state and council. Check with your local council before the bearers go in, not after the boards do.
Deck priced up. Now send the quote.
Yamate turns the board metres, the subframe and your hours into a quote the customer accepts on their phone — then the invoice, then the payment. GST sorted, follow-ups chased.
Try Yamate free → Free to try · no card · sign in with a link, no password to rememberDecking FAQ
How many decking boards do I need?
Divide the deck width by the board width plus the gap to get the number of rows, then multiply by the deck length. A 6m × 4m deck in 86mm boards at 4mm gaps is 45 rows × 6m = 270 lineal metres before waste.
What spacing should decking joists be?
Most 19mm Australian hardwood decking is specified at a maximum of 450mm centres. Some 22mm softwood allows more, composite boards often want 300 to 400, and diagonal laying needs the centres tightened. Use the board maker's spec sheet.
How big a gap between decking boards?
Around 3mm for boards that arrive wet and will shrink, 4 to 5mm for kiln-dried boards that will swell once they're outside. Wider boards move more, so give them more. Never lay tight — water has to drain.
How many screws for a deck?
Two per board at every joist. A 6m × 4m deck at 450 joist centres works out around 1,350 screws — three boxes of 500. Buy the extra box; running out on a Sunday costs more than it saves.
How far apart should deck bearers be?
Bearer spacing is the joist span, and it comes from the AS 1684 span tables against your joist size, species, grade and deck height. 1800mm is a common figure for 140 × 45 treated pine on a low deck, but read the table for your build.
Do I need council approval for a deck?
Often, and the threshold varies by state and council — height above ground, size and whether it's attached to the house all matter. A deck more than a metre up also needs a barrier. Ask your council before you start.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. This tool counts boards, joists, bearers and fixings at the spacings you enter. It does not select joist or bearer sizes or spans, footing sizes, or bracing — those come from AS 1684, your board manufacturer's specification and your engineer. Australian Standards are published and sold by Standards Australia and are not reproduced here. Confirm anything structural with your certifier or building surveyor.