Tips for Building
The count comes from joist spacing and board width, not from area alone. A 12 m² deck in 145 mm boards laid with a 5 mm gap is 20 rows of boards; at 500 mm joist centres over 4 m, that is 9 lines of joists, and 360 screws at two per board per joist crossed. Spacing and gap are not guesswork: they come from the board maker's instructions and the code or guidance that applies where you build.
The assumptions behind this calculation
- Boards: the number of rows is the width divided by the board width plus the gap, rounded up to the next row; the last row is ripped down. The linear metres are turned into boards of the length you buy, then your waste allowance is added. The calculation assumes offcuts are reused at row ends; a broken bond or a pattern uses more.
- Joists: one line at every spacing along the length, plus one at the end, each as long as the deck is wide. Doubled joists under butt joints, trimmers and the edge frame are added by hand.
- Screws: two screws per board per joist crossed, plus two at every butt joint, where two boards share the same joist. The total is rounded up to the hundred, the usual box. The stainless grade and screw length are in the board instructions.
- Not counted: pedestals or ground screws, whose number depends on the support spacing under each joist, joist tape, weed membrane and gravel.
What this calculator does not replace
It gives quantities, not a design. Joist centres, joist section, support spacing and the gap between boards depend on the thickness and species of the boards, their moisture content on the day and the laying direction: the board maker's instructions and the rules that apply where you build give them, and only those count. The values in the fields are examples to replace with yours.
The use class of the timber, set by EN 335 in Europe, says what may stay in contact with the ground or standing water: a joist is chosen in the class its exposure calls for, and the supplier states the class of each product. A raised deck, one on posts or one fixed to the house is a structure and falls outside this calculation.
The steps of the job, in order
Frequently asked questions
What joist spacing should I use?
The one in the board instructions, for that thickness and species. There is no single figure: a thin board or a composite board needs closer joists than a thick hardwood one. The calculator takes the spacing you enter, and the joist and screw counts follow.
How many screws per square metre of deck?
The figure is worked out, not guessed: two screws per board per joist crossed. The narrower the boards and the closer the joists, the more you need. With 145 mm boards, a 5 mm gap and 500 mm centres it comes to about 27 screws per m²; with 90 mm boards at 400 mm centres, about 53.
Do I need a gap between deck boards?
Yes, and its size is in the instructions: it depends on the species and on the moisture content of the timber when laid. Wet timber shrinks, dry timber swells. Enter that gap in the calculator: it changes the number of rows, so the number of boards and screws.