Deck boards· 2 min read
A 4 × 3 m Deck: How Many Boards, Joists and Screws
A worked example of a 4 × 3 m (13 × 10 ft) deck with 145 mm boards: rows, joists, screws — and what changes with a wider gap or joist spacing.
A 4 × 3 m deck is 12 m² (about 129 sq ft). Area is only the start, though — boards are bought by the piece, joists by length and screws by the box. Let’s work through a real example.
The brief
- Deck length along the boards 4,000 mm, width across them 3,000 mm.
- Deck boards 145 mm wide with a 5 mm gap (5–10 mm is typical; for composite boards the clip often sets it).
- Joists at 400 mm centre to centre.
- Boards sold in 4,000 and 5,000 mm, 2 screws where each board crosses a joist (standard for face screwing; hidden clips follow the manufacturer).
The result
| Item | Quantity |
|---|---|
| Board rows | 20 |
| Boards | 20 × 4,000 mm |
| Joists | 11 × 3,000 mm |
| Screws | 440 |
20 boards at 145 mm plus 19 gaps at 5 mm make exactly 2,995 mm — the last 5 mm is just one more gap, so no board has to be ripped. The deck is as long as a board, so there are no joints either.
Small change, big difference
Widen the gap to 7 mm and 20 full boards no longer fit: the last row has to be ripped to 112 mm. A strip that narrow looks odd at the edge. Keep the 5 mm gap, adjust the deck width, or rip the two outer rows evenly.
Joist spacing drives the screw count: at 500 mm instead of 400 mm you would need 9 joists and 360 screws instead of 440. Never pick the spacing to save money, though — always take it from the board manufacturer’s installation guide. Composite boards often need closer joists than wood — typically 30–40 cm (16 or 12 in), while for wood guides allow up to about 20 × the board thickness. Diagonal laying always needs closer joists.
Deck boardsOpen this example in the deck calculatorThe calculator counts materials; it does not do structural design. Joist size and spacing, supports and fixings must follow the manufacturer’s instructions and your local building rules.