How many decking boards do I need?
27 September 2026 · 4 min read · by Rodney Lee
Decking looks like a flooring job but it has one twist: the gaps. Every board is laid with a 5–6 mm drainage gap, so the boards you buy must cover more than the deck's surface area.
Board coverage
Take the stated board width and add the gap: a 145 mm board laid with a 5 mm gap covers 150 mm of deck per board. Divide the deck width by 0.15 m to get the number of board runs across, then multiply by the deck's length (boards usually run along the length) to get total metres of board.
A 4 m × 3 m deck: 3 m ÷ 0.15 = 20 runs × 4 m = 80 m of board. Add 10% for cuts and cross-laid boards around the edge, and you're buying roughly 90 m.
The frame underneath
- Joists at 400 mm centres (300 mm for diagonal boards or composite).
- A perimeter frame around the edge, plus noggins where boards butt end-to-end.
- Decking boards span joists, not the other way round — get the frame spacing right first.
Fixings and finishes
Two screws per joist crossing is the rule of thumb: on our 4 m × 3 m deck with 400 mm centres that's around 550–600 screws. Add weed membrane and gravel beneath, and posts or pedestals if the deck is raised.
Pressure-treated softwood is the standard choice; hardwood and composite cost more per metre but last longer with less maintenance. Whichever you pick, check the treated price per metre rather than per pack — packs hide the real unit cost.