How to Use This Tool
Enter the deck, pick the material, and the tool sets a sensible fastening gap and predicts what it becomes once the boards reach service moisture.
The gap you fasten is not the gap you get
Pressure-treated decking is usually sold wet — often above 25 per cent moisture — because the preservative is forced in with water. In service it dries to around 12 per cent, and wood shrinks across the grain as it dries.
On a 5.5 in board, 2 to 3 per cent of shrinkage is 0.11 to 0.17 in. That is almost exactly the gap you want. So wet boards should be fastened tight against each other, and they will open up to the right spacing on their own.
5.5 in board, 2.5% shrinkage across the width = 0.138 in fastened at 0 -> 0.138 in gap about right fastened at 1/8 -> 0.263 in gap wide fastened at 1/4 -> 0.388 in gap too wide kiln-dried or composite: no drying shrinkage, so fasten at the gap you actually want.
Composite moves too, in a different direction
Composite and PVC boards barely change with moisture but expand and contract with temperature, mostly along their length. That is why manufacturers specify end gaps that vary with the installation temperature, and why butting composite tight against a wall on a cold day causes buckling in summer. Side gaps for composite are what you set and what you keep.
Board count, and why it rarely changes
Boards needed is the run divided by board width plus gap, rounded up. On a 12 ft span with 5.5 in boards, every gap from 1/8 to 1/4 in gives the same 26 boards, because the rounding absorbs the difference. The gap decision is about appearance and drainage, not quantity — which is another reason to set it by the moisture state rather than by what looks tidy at the yard.
Screws add up faster than people expect
Two screws per board per joist is the standard. A 12 x 16 ft deck at 16 in joist spacing has 13 joists and 26 boards, which is 676 screws. A 5 lb box holds roughly 350 of a typical 2.5 in deck screw, so that is two boxes, and running out on the last row is a specific kind of annoying.
Two things this does not size
Joists, beams and footings are structural and depend on span, species and load — check a span table and your local code. Picture-frame borders and diagonal layouts both add waste: diagonal decking needs joists at 12 in rather than 16, and every board gets two angled cuts.
