Deck Calculator

What this means

A deck's material need is its area divided by how much decking each board covers. Boards are laid across the width with a small drainage gap, so the number of rows is the width divided by one board-plus-gap unit. Joists run underneath, perpendicular to the boards, spaced per your joist spacing. We add a waste allowance for cuts at the ends, around posts and for knots or damaged boards, then round up to whole boards.

Recommendations

• •Space joists at 40 cm (16 in) or closer for composite decking — its manufacturers specify a stricter spacing than timber.
• •Add a 3–6 mm gap between boards so water drains and the deck dries out.
• •Buy 10% extra — the last few boards are the ones that always get miscut.

Watch out

• •Composite and timber decks have different joist-spacing requirements — follow the decking brand's spec.
• •Structural loads depend on joist size and span; this is a material estimate, not engineering.