The gap is part of the pitch
Decking boards are not laid touching. A gap of 5–8 mm between them lets water drain and gives the timber room to move, and that gap is part of the repeating unit.
A 140 mm board with a 6 mm gap occupies 146 mm of deck. Across a 3.6 m deck that is the difference between 25 boards and 26 — and ordering 25 leaves you with a visible shortfall at the last board that is very awkward to fix.
Set the gap deliberately rather than by eye. Consistent spacers during installation are worth the small effort; a deck with drifting gaps reads as amateur from across the garden.
| Timber | Typical gap | Reason |
|---|---|---|
| Pressure-treated softwood, wet | 3–5 mm | Will shrink as it dries |
| Pressure-treated softwood, dry | 5–6 mm | Will swell slightly when wet |
| Hardwood | 6–8 mm | Stable, gap is purely drainage |
| Composite | Per manufacturer | Thermal movement is significant |
Joist spacing is a structural decision
The calculator uses the joist spacing you give it, but the right figure comes from the board and the use. Standard softwood decking at 400 mm centres is the common domestic case. Boards laid diagonally need closer spacing, typically 300 mm, because the effective span between supports increases.
Composite decking is generally less stiff than timber and most manufacturers specify tighter centres — often 300 mm and sometimes less for diagonal layouts. That figure is in the installation guide and ignoring it produces a deck that feels bouncy underfoot.
Too wide a spacing does not fail dramatically. It sags slowly, and by the time it is obvious the deck is built.
Fixings and the things not counted
The screw count here assumes two per board at every joist crossing, which is standard practice. Use screws rated for exterior use and, if the timber is treated, ones compatible with the treatment — modern preservative treatments are corrosive to ordinary steel.
Not included: the frame timber itself, the posts and footings supporting it, the fascia boards, and any balustrade. Those are separate calculations, and the footings in particular are worth doing properly — a deck is a structure people stand on.
Joists and bearers are one decision, not two
A deck frame is two layers. Bearers — beams, in American usage — are the heavy timbers spanning between the posts that carry everything. Joists sit across them at right angles, and the boards sit on the joists. Change either spacing and the other needs rechecking, which is why they are worth working out together rather than one at a time.
Joist spacing is set by the boards: 400 mm centres for most softwood decking, 600 mm only where the board is thick enough to span it without bounce, and around 300 mm for a 45° layout, because a diagonal board crosses fewer joists over the same distance.
Bearer spacing is set by the joists and by the section you are buying. At 1.8 m apart a 150 × 47 mm joist is comfortable; push the bearers further and the joists must get deeper or you add another row of posts. The calculator gives both counts and the total length of each, and warns when the spacing you entered has gone past what an ordinary section will carry.
Common questions
What gap should I leave between decking boards?
5–6 mm for dry pressure-treated softwood, 3–5 mm if the timber is still wet since it will shrink, and follow the manufacturer for composite.
How far apart should deck joists be?
400 mm centres for standard softwood laid square. Reduce to around 300 mm for diagonal layouts or composite boards.
How many screws per board?
Two at every joist crossing is standard. The calculator multiplies boards by joists by two.