What gap should I leave between decking boards?

Standard practice is 5-8mm between timber decking boards. This allows for drainage, airflow, and seasonal expansion. Composite decking typically requires 5-6mm gaps. Always follow the manufacturer's recommendations for your specific product.

Do decking boards expand?

Yes. Timber decking expands and contracts with changes in moisture content. Boards can expand 1-3mm in width seasonally. Composite decking also expands with heat. Always allow for expansion in your spacing calculations.

Should I run decking boards lengthwise or widthwise?

Boards typically run perpendicular to the joists. Running them at 45 degrees (diagonal) creates a more interesting visual but requires approximately 15% more material and more cutting. The boards must be supported by joists at no more than 400mm centres.

WoodCalcs.uk

Deck Board Spacing Calculator — Optimal Board Gaps

Calculate optimal board spacing, number of boards, and total material for your deck.

Deck Length

Length of the deck

Deck Width

Width of the deck

Board Width

Expansion Gap

Gap between each board

Board OrientationPerpendicular boards run across the joists; diagonal (45°) looks striking but adds ~15% cutting waste. Joists must be at 400 mm centres or less for 28–32 mm boards (TDCA).

How We Calculate This

This calculator determines the number of decking boards needed and the optimal spacing between them.

The formula

Number of boards = ceil((Deck width + gap) / (Board width + gap))

The gap is the installed (edge-to-edge) expansion gap. Per the TDCA, this 5–8 mm gap already allows for seasonal movement — timber swells in width when it takes up moisture, which closes the gap, so the seasonal allowance is shown for reference but is not added on top. The final board carries no trailing gap, so the deck width plus one gap is divided by one board-plus-gap pitch.

For a diagonal (45°) layout the boards still run the full deck length, but a 15% waste factor is added to the total board length to cover the triangular off-cuts at each end.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.