DIY projects
How much decking do I need? A 3.6 m × 4 m example
Count decking boards and joists for a real 3.6 m × 4 m garden deck, including board gaps, joist spacing and a wastage allowance.
Count rows of boards, not square metres
Decking is sold by the board, so work in rows. For an illustrative deck 3.6 m long and 4 m wide, with 3.6 m boards running the length, each row is one board. A 144 mm board with a 5 mm gap takes up 149 mm, so 4 m ÷ 0.149 m = 26.8 rows — round up to 27 boards.
Adding 10% for cuts, offcuts and the odd twisted board takes the order to 27 × 1.1 = 29.7, so 30 boards.
Joists underneath
Joists run at right angles to the boards. At 400 mm centres across the 3.6 m length you need 3.6 ÷ 0.4 = 9 spaces, so 10 joists, each spanning the 4 m width. Check the board manufacturer’s maximum joist spacing; some composite boards need closer centres.
The rest of the list
Screws, fixings, posts or bearers, weed membrane and edge fascia boards are extra items. The decking calculator estimates boards and joists from your own dimensions, and the materials planner gathers them into one shopping list. Estimates only. Check the manufacturer’s coverage rates.
