Calculate boards and joists for your deck
Building a deck is one of the most popular garden improvement projects in the UK, adding usable outdoor living space and increasing property value. Accurate material estimation is essential -- decking boards and joists are bulky, heavy items that are expensive to over-order and inconvenient to collect as a top-up mid-project. The two main structural elements of a deck are the boards (the visible walking surface) and the joists (the supporting framework underneath). Boards run along the length of the deck, and the number needed depends on how many fit across the deck width. Joists run perpendicular to the boards at regular intervals, providing the structural support. Joist spacing is critical for both safety and longevity. Timber decking typically uses 400mm joist centres, while composite decking boards (which are more flexible) require closer spacing at 300mm centres. Using the wrong spacing can lead to boards sagging, bouncing, or even snapping under load.
To calculate decking materials: 1. Enter the deck length in metres. This is the direction the boards will run (typically the longer dimension). 2. Enter the deck width in metres. This is the dimension across which the boards are laid side by side. 3. Select the board width. The most common UK decking board width is 140mm. Narrower 90mm and 120mm boards create a more refined appearance, while 150mm boards provide faster coverage. 4. Select the joist spacing. Use 400mm for standard softwood timber decking, 300mm for composite decking boards, or 450mm for light-use areas with timber boards. Always follow the board manufacturer's recommendations. 5. Review the results. The calculator shows the number of boards needed, the number of joists, the total deck area, and the total linear metres of board required (useful when boards come in set lengths).
The decking calculation determines how many boards fit across the width and how many joists span the length: Boards needed = ceil(deck width / (board width in mm / 1000)) For a 3m wide deck with 140mm boards: ceil(3 / 0.14) = ceil(21.43) = 22 boards. Each board runs the full length of the deck. Total board length = boards x deck length. For 22 boards on a 4m deck: 22 x 4 = 88 linear metres. This helps when buying boards in standard lengths (typically 2.4m, 3.0m, 3.6m, or 4.8m). Joists = ceil(deck length / (joist spacing in mm / 1000)) + 1. The "+1" ensures there is a joist at both the start and end of the deck. For a 4m deck with 400mm spacing: ceil(4 / 0.4) + 1 = 10 + 1 = 11 joists. Joists typically run the width of the deck (3m in this example), so total joist length = joists x deck width = 11 x 3 = 33 linear metres.
Standard UK softwood decking boards are 28mm thick x 140mm wide and come in lengths of 2.4m, 3.0m, 3.6m, and 4.8m. Joists are typically 47mm x 100mm or 47mm x 150mm treated softwood. Leave a 5-8mm gap between boards for drainage and expansion. Composite boards expand more than timber, so check manufacturer guidance for gap sizes. All timber in contact with the ground or used structurally outdoors should be pressure-treated to Use Class 3 or 4. Decking screws should be stainless steel or specifically rated for treated timber to prevent corrosion.