Deck Board Spacing Formula
Adjusted Gap: (Deck Width − (Boards × Board Width)) ÷ (Boards − 1)
Total Board Width: Boards × Board Width
Note: The adjusted gap is calculated to evenly distribute boards across the deck width. This ensures uniform spacing without partial board cuts.
Deck Board Spacing Calculator: Gap & Board Layout
Australians build over 200,000 new decks annually (HIA, 2024). Correct board spacing is essential for drainage, timber expansion, and a professional finish. This calculator determines the number of boards needed, optimal gap sizes, and starting positions.
Recommended Deck Board Gaps
| Timber Type | Gap (Dry) | Gap (Wet/Treated) | Min Drainage |
|---|---|---|---|
| Hardwood (Merbau, Spotted Gum) | 5–6mm | 3–4mm | 3mm |
| Treated Pine (H3/H4) | 4–5mm | 2–3mm | 3mm |
| Composite Decking | 5–6mm | 5–6mm | 5mm |
Formulas
Number of boards = (Deck Width + Gap) / (Board Width + Gap), rounded.
Adjusted gap = (Deck Width − (Boards × Board Width)) / (Boards − 1).
Frequently Asked Questions
What gap should I leave between deck boards?
For hardwood, 5–6mm between dry boards or 3–4mm if wet/green. Composite typically requires 5–6mm regardless.
How many deck boards do I need?
Divide deck width by (board width + gap). For 4m with 90mm boards and 5mm gaps: 4000/95 = ~42 boards. Add 5–10% wastage.
Should deck boards be installed crown up or down?
Crown side up for treated pine (sheds water). For hardwoods, either side works: many builders prefer the best face up.
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