Decking Calculator — Boards & Cost

Decking board quantity is calculated by dividing the deck area by the coverage of each board. Standard softwood decking boards are 2.4 m, 3.0 m, 3.6 m or 4.8 m long and 120–145 mm wide (face width). Each board covers approximately 0.29–0.70 m² depending on size. A 5–10% waste allowance covers cuts and offcuts. To put figures on it, a 4m × 3m deck is 12 m², or 13.2 m² with 10% waste. With 145mm-wide boards and 3-5mm gaps between them, each m² needs about 7 linear metres of board, so 93 linear metres in total, which is 26 boards at the standard 3.6m length as a minimum. Buy 2-3 extra boards, because a replacement later will need the same batch for the colour to match.

Substructure requirements include joists (typically 47 × 150 mm at 400 mm centres, supported on post-and-beam or concrete pads) and screws (approximately 20–25 stainless-steel screws per m²). The calculator also estimates joist hangers, post brackets and any concrete needed for footings. Enter the deck dimensions and it shows boards, joists, screws and an estimated cost. For treated softwood decking, expect £20–£40 per m² for materials. Composite decking is £40–£80 per m² but lasts longer and requires less maintenance. Any raised deck above 300 mm may need Building Regulations approval.

The board material sets the lifespan as much as the price. Pressure-treated softwood (pine or spruce) lasts 10-15 years and needs oiling every year. Hardwood such as oak, ipe or balau costs £60-£140/m², has a premium look, lasts 25-50 years and needs minimal maintenance. Composite boards from brands such as Trex, Millboard and Ecoscape carry 25-30 year guarantees and do not rot, splinter or fade. PVC at £100-£200/m² is lightweight and needs almost no upkeep, but looks plasticky. Whichever you choose, the full project adds £20-£50/m² for joists, bearers, screws and ground anchors on top of the boards.

The joist structure is the hidden cost and the hidden skill. Joists span the deck at 400mm centres, and 100×47mm (4×2) C16 grade treated softwood is fine where supports are no more than 1.8m apart. Longer spans need the deeper 150×47mm section. Bearers, the beams running perpendicular under the joists, sit on concrete piers or posts, with a damp-proof membrane between concrete and timber. Leave at least 150mm of ground clearance so air can circulate and the timber does not rot. Joists connect to bearers with joist hangers or notched-and-bolted joints, and the foundations, whether concrete pads or post anchors, go in at roughly 1.5m centres.

Planning permission is not needed for most garden decks, but there are conditions. The deck must be under 300mm above ground level, must not sit in front of the house, and together with other extensions and sheds must not cover more than 50% of the garden. Above 300mm it counts as a raised platform and needs planning permission, and conservation areas have tighter rules, so check with the local council. Building Regulations do not usually apply to ground-level decking, but they do if the deck is attached to the house and more than 600mm above ground, because it is then structural. Keeping the deck at least 1m from the neighbour's fence avoids disputes about overlooking.

A few details at installation stage save trouble later. Slope the deck slightly, about 10mm per metre, away from the house so rain drains off. Leave 3-5mm gaps between boards for expansion, pre-drill the screw holes and keep them at least 25mm from board ends to avoid splitting. Use stainless steel or coated decking screws, since ordinary ones rust. Softwood needs an annual clean, a light sanding of any splinters and a fresh coat of oil, while composite only needs a pressure-wash once or twice a year. Clear moss every year on any deck, because wet timber gets dangerously slippery in autumn and winter.