Sun Deck Calculator

Each decking board occupies a module width on the deck surface — the board’s face width plus the gap between boards. A 95mm board with a 5mm gap occupies a 100mm (0.1m) module. Dividing the total deck area by this module width gives the running metres of board required. For a 12 m² deck, 12 ÷ 0.1 = 120 running metres. Add the waste factor (typically 10%) and divide by the standard board length to get the number of full-length boards to buy. Diagonal (45°) boarding adds approximately 15% more material because boards must be cut at angles at every edge, producing more offcut waste than a straight layout.

Joists are the structural timbers that support the decking boards. The standard joist spacing for a domestic deck using 32mm softwood boards is 400mm centres — this prevents bounce and deflection underfoot. Wider 600mm spacing is sometimes used for thicker or composite boards. This calculator uses a simplified grid model — in practice, the actual joist layout depends on the specific deck design, the direction of the decking boards relative to the joists, and any intermediate beams. The quantities shown are a reliable planning estimate; final quantities should be confirmed against a detailed framing plan.

Deck posts bear the load of the entire deck structure and must be founded below the frost line or on a solid concrete pad. A typical domestic garden deck uses 100 × 100mm timber posts at 2.0m maximum centres, set in fast-setting postcrete at least 450mm deep. The post count is calculated from a grid across each deck area. Corner posts, end posts, and intermediate posts along each side and across the interior are all included in the estimate. For raised decks over 300mm above ground, consult a structural engineer — loading requirements increase significantly with height. Pair with the Concrete Calculator for footing volumes or the Paint Coverage Calculator for staining and sealing.