Decking Calculator

How decking quantities are worked out

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.

Worked example: a 4 m × 3 m deck in 140 mm boards

The deck is 12 m². With a 5 mm gap between boards, each 140 mm board covers 0.145 m of width. Running the boards along the 4 m length means the deck needs 3 ÷ 0.145 ≈ 21 rows, each 4 m long — so about 84 linear metres of board. Sold in 3.6 m lengths, that is 24 boards, plus 10% for cuts around posts and edges.

Joists, noggins and supports

Use 100 × 47 mm treated timber joists at 400 mm centres — that span suits 28 mm timber decking with no bounce. For a 4 m width you need 4 ÷ 0.4 + 1 = 11 joists, each 3 m long. Noggins between joists every 1.2 m stop twisting. Where the deck sits low, use adjustable pedestals or concrete pads rather than burying timber in soil.

Ventilation, membrane and fixings

Keep at least 50 mm of clear airflow under the boards and lay a weed membrane over the ground beneath — damp, still air is what rots a subframe. Fixings depend on the board: stainless steel screws for treated timber (roughly 20–25 per m²), and composite boards need screws rated for composite so the heads countersink cleanly. Joist hangers are needed wherever a joist meets a ledger board on a wall.

Keeping it in good order

Timber decking benefits from a clean and a re-oil every 12–18 months; untreated boards grey within a year and split where water sits. Check for soft spots at ground contact points each spring — that is where rot starts. Composite needs no oiling, but it does need the gaps kept clear of leaf litter.