Decking boards and joists for a level garden deck — board and joist counts.
Divide deck area by the board width to get total linear metres, add a 10% waste allowance, then divide by the board length. Joists run across the deck and are spaced 400mm apart — divide the deck length by the spacing and add one.
400mm centres is standard for decking boards; 600mm is the absolute maximum and can feel bouncy.
Common UK decking boards are 120mm or 140mm wide and come in 3.6m or 4.8m lengths.
Yes — leave a 5–6mm gap between boards for drainage and expansion.