Decks are the outdoor project with the biggest material surprise: the board count depends as much on gap and width as on square footage. Nail the counting and the build is half-planned.
Area in, boards out
Total deck area with waste, divided by the coverage of each board, gives the board count. Because boards have a width and a gap, the number of rows is set by width-plus-gap — a detail that quietly changes the total.
Joists: the structure under the boards
Joists are spaced along the deck's length and sized for the load above. Timber decks carry their boards at 40 cm spacing; composite boards are stricter. Getting spacing right prevents bounce and sag for the deck's whole life.
The waste allowance earns its keep
Every cut — at ends, around posts, along curved edges — discards part of a board. Ten percent is a sensible buffer, and it's cheaper than a second delivery for a few boards mid-build.
Frequently asked questions
Divide the deck area (with waste) by the area of one board (length × width). A 4×3 m deck with 3 m × 14 cm boards needs about 30 boards including 10% waste.
Usually 40 cm (16 in) center-to-center for timber, and often tighter for composite decking — check the manufacturer's specification.
Yes — a 3–6 mm gap lets water drain and air circulate, preventing rot and cupping. Most calculators assume a small gap in the count.