Decking calculator
Timber decking needs a 5 to 6 mm gap between boards so water drains and the timber can move. Composite needs a similar gap for thermal expansion, and its manufacturer will specify it. Because the gap repeats across every board, the honest way to count is to add it to the board width before dividing — which is what the width field here asks for.
Deck board count is the deck area divided by the coverage of one board including its drainage gap. A 4 × 3 m deck with 150 mm boards laid at 155 mm centres over 3.6 m lengths needs 23 boards after a 10% cutting allowance.
How to work out decking
Board count is the easy half. The joists underneath are where a deck succeeds or fails: 400 mm centres for standard timber decking, dropping to 300 mm if the boards run diagonally, and often 300 mm for composite because it is less stiff than wood. Every joist needs support within its span, and the frame wants a slight fall away from the house so water runs off. Fixing at two screws per board per joist means a surprisingly large box of screws — a 12 m² deck is comfortably 400 fixings.
Questions
Five to six millimetres for timber, which allows drainage and movement. Composite makers specify their own gap and it is usually wider at the ends.