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Decking calculator
Work out how many deck boards and joists a deck needs, allowing for the gaps between boards and for cuts.
Your details
Measure the deck and choose your board size.
In metres, along the boards.
In metres, across the boards.
In millimetres. 144mm is the common size.
In millimetres.
In millimetres, centre to centre.
A percentage.
Why the gap matters
Timber decking needs a gap of about five millimetres between boards so that water drains and the boards can move. That gap is part of the calculation: a 144 millimetre board with a five millimetre gap occupies 149 millimetres, so a three metre width takes twenty-one rows rather than twenty-one and a bit.
Joists
Four hundred millimetre centres suit most twenty-eight millimetre softwood boards. Composite boards and thinner timber usually want three hundred. Laying boards diagonally, which looks well, needs closer centres because the span between supports increases.