Decking Calculator

How This Is Calculated

Boards are assumed to run along the deck's length. The number of rows across the deck's width is the width divided by one board's width plus the gap, rounded up. Each row needs enough whole boards to cover the full length - a run longer than one stock board length needs a second board butt-joined over a joist - and your wastage allowance is added on top before rounding up to a whole board count.

Joists run across the width, spaced along the length at your chosen centres, with one extra joist added to cover both ends of the run. Screws assume the standard two fixings per board at every joist it crosses, multiplied across every row.

Common Questions

Why 400mm joist spacing?

400mm centres is the standard spacing used across most UK timber decking guides, including trade retailers like Screwfix, for typical residential decking boards. Composite boards or boards laid diagonally often need closer spacing, sometimes down to 300mm - check your specific board's technical datasheet, since span capability varies by material and thickness.

Why does the board gap matter?

A 5-7mm gap between boards lets timber expand and contract with moisture without buckling, and lets water drain through rather than pooling on the surface. Too tight a gap causes boards to bow or lift over time; too wide wastes material and looks untidy.

Why do I need more boards than the area suggests?

Boards come in fixed stock lengths, so a run longer than one board needs a second piece butt-joined over a joist, and that second piece is still a whole board even if only a short length of it is used. The wastage allowance on top covers cutting mistakes, damaged boards and offcuts too short to reuse elsewhere.

How many screws does each board actually need?

The standard is two screws per board at every joist it crosses, which stops the board twisting or cupping between fixings. A board crossing five joists needs ten screws, regardless of how long that particular board is.