How to Count Deck Boards, Joists and Screws

A 4.2 by 3.0 metre deck with 145 mm boards. Three thousand divided by 145 is 20.7, so twenty-one boards. Order twenty-one, get to the last row, and discover you’re one board short — because you forgot the gaps.

Small omissions in decking arithmetic have a habit of appearing at the far end of the deck, on the last day, when the merchant is closed.

The number that matters is effective width

A board doesn’t occupy its own width on the deck. It occupies its width plus the gap beside it.

boards = deck width ÷ (board width + gap)

With 145 mm boards and a 5 mm gap, each board consumes 150 mm. Three metres of deck width needs 20 boards, and you’ll want one more for the trimmed edge — so 21, which happens to match the wrong calculation by coincidence at this size and won’t at others.

The gap itself isn’t arbitrary. Timber moves with moisture: boards installed wet will shrink, so they go down with a smaller gap. Kiln-dried boards installed dry will expand slightly, so they need a full gap. Composite boards move with temperature rather than moisture, and manufacturers specify end gaps that depend on the installation temperature — genuinely worth reading, because composite expansion along the length is significant.

Joist spacing is a safety question

Spacing follows the board’s span capability, and that comes from the manufacturer’s span table, not from habit. Around 400 mm centres suits most 25 mm timber boards. Many composite boards need 300 to 400 mm, and closer still for a diagonal layout, because a diagonal board spans further between joists than a perpendicular one.

Too wide and the deck feels bouncy — which is the polite symptom. The serious one is a board failing under load.

Why people get stuck here

• Nominal versus actual board width. A “150 mm” board is often 145 mm, and that 5 mm compounds across twenty boards.
• Gap forgotten. Which is the calculation error above.
• Span tables ignored. Joist spacing chosen from what the last deck used.
• Diagonal layouts. They increase both the span and the wastage, and both get overlooked.

Common mistakes to avoid

• Using the nominal board width instead of the actual measured one.
• Butting boards tight together because they look better on day one — they’ll cup and trap water.
• Screwing near board ends without pre-drilling, which splits timber.
• Ignoring the manufacturer’s end-gap figure for composite boards, which move noticeably along their length.
• Ordering exactly the calculated quantity with no allowance for cuts and defects.

How to do it with Deck Board Calculator

The Deck Board Calculator counts boards from effective width and estimates joists and fixings.

1. Enter the deck dimensions and the actual board width, not the nominal size.
2. Set the gap — smaller for wet timber that will shrink, full for kiln-dried.
3. Choose joist spacing according to your board’s span table.
4. Add 8 to 10% wastage, more for a diagonal or picture-framed layout.

Always check the specific product’s span table and fixing guidance — it’s a structural matter, not a preference. Other building calculators are in the tools directory.

Frequently asked questions

How big should the gap between boards be?

Around 5 mm for kiln-dried timber, smaller for wet timber that will shrink as it dries. Composite boards move with temperature, and the manufacturer specifies the gap based on installation temperature.

What joist spacing do I need?

It depends on board thickness and the product’s span table. 400 mm centres suits most 25 mm timber boards; many composites need 300 to 400 mm, and diagonal layouts need closer spacing than perpendicular ones.

Why include wastage?

Cuts, defective boards and edge trimming all consume material. Eight to ten percent is normal, and a complex shape needs more.

Final thought

Measure one board with a tape before ordering the rest. Nominal sizes and actual sizes differ, and that difference multiplied by twenty boards is what leaves you short at the last row.

Try the free Deck Board Calculator