Work out how many decking screws you need and how far apart to space them, based on your deck size, board width and joist centres.
Decking screw quantity comes down to two things: how many boards cross how many joists, and how many screws you fix at each crossing. This tool counts the board/joist crossings across your whole deck, multiplies by the screws you fix per joist, and adds a 10% buffer for dropped and stripped screws.
For timber decking the UK standard joist spacing is 400 mm centres. Composite decking is heavier and more flexible, so it usually needs closer joists at 300–350 mm centres — always check the board manufacturer's guidance.
The industry standard is 2 screws per joist for boards around 144 mm (6 inches) wide. Wider boards (deck boards over ~150 mm) or boards that are cupping may need 3–4 screws per joist to hold them flat.
Along each board, decking screws are spaced at the joist centres — so on a UK timber deck with joists at 400 mm centres you fix screws every 400 mm along the board. There is no separate "screw spacing" to set: the joists dictate it. Across the board you fix 2 screws (one near each edge, about 20 mm in), or 3–4 on wider boards.
Count how many joists the board crosses, then multiply by the screws you fix per joist. A 3.6 m board crossing joists at 400 mm centres crosses about 10 joists, so at 2 screws per joist that is roughly 20 screws per board. This calculator works it out for your whole deck automatically.
The UK standard for timber decking is 400 mm joist centres. Composite decking is heavier and more flexible, so it usually needs closer centres of 300–350 mm — check the manufacturer’s instructions for your specific boards.
As a rough guide, timber decking with 144 mm boards at 400 mm joist centres and 2 screws per joist works out at roughly 35 screws per square metre. Wider joist spacing uses fewer; composite decking at closer centres uses more. Enter your deck size above for an exact figure.
For standard 25 mm thick deck boards, use 63 mm (2.5 inch) decking screws so about two-thirds of the screw bites into the joist. Thicker boards need longer screws. Always use stainless steel or exterior-coated screws outdoors to resist rust.
Yes. Composite boards flex more than timber and are heavier, so they need closer joist spacing — typically 300–350 mm centres rather than the 400 mm used for timber, and often as little as 250 mm for diagonal or stair-nosing details. Set your joist spacing above to match your board maker’s guidance.
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