Deck Board Spacing Guide

1. Why spacing matters

Board spacing affects drainage, ventilation, expansion, and the finished look of a deck. Gaps let water drain and air move, which helps boards dry evenly and last longer. Spacing also changes how many rows fit across your deck, and therefore how many boards you buy.

2. Actual vs nominal width

Lumber is sold by nominal size, but boards are actually a bit narrower. A common decking board sold as 6 inches is usually 5.5 inches wide. Always estimate with the actual width so your row count is correct.

3. Choosing a gap

• Composite and dry hardwood: often installed with a small consistent gap, commonly around 1/8 inch.
• Wet pressure-treated lumber: may be butted tighter because it shrinks as it dries.
• End gaps: leave room where boards butt end to end, per the manufacturer.

Always follow the spacing your board manufacturer specifies for your climate and product.

4. How gaps change board count

Each row occupies the board width plus the gap. On a 12 ft (144 in) wide deck with 5.5 in boards:

Try your own numbers in the deck board calculator.