How Deck Board Width and Gap Change the Board Count

Two decks with the same footprint can need a different number of rows. Each row covers the width of one board, and every gap adds a little more reach, so the board width and the gap together decide how many rows fit across.

The gap count is one less than the board count because gaps sit between boards, not after the last one.

One deck, fifteen combinations

A 12 ft wide deck with a 16 ft run and 16 ft stock boards, so each row is one board and the boards to buy equal the rows. The widths and gaps are examples chosen to show the effect, with no extra allowance:

Reading across a row, a wider gap never needs more boards, and sometimes it needs fewer. Reading down a column, a wider board needs fewer rows. The change is not smooth: the count steps down only when the extra reach crosses into the next row.

Ignoring the gap can add a row

Trex’s count page divides the deck depth by the board width alone. For 12 ft and 5½ in boards that gives 27 boards. Trex’s spacing page gives 3/16 in as the recommended gap for its composite boards, and counting that gap brings the same deck to 26 boards. The simpler method leans toward buying one more, which can be a sensible habit, but it is not the same as the geometry.

Where the end edge matters

Take a deck 5 ft across with 5½ in boards and ½ in gaps. Dividing 60 in by the 6 in module says exactly 10 rows. But ten boards and nine gaps cover only 59.5 in, so a tenth row leaves a strip uncovered and the planner counts 11 boards.

Which gap to use

Not this page. The gap comes from the instructions for the decking you are installing, and products differ. Trex’s figure above is for Trex composite boards only. Enter your own board’s actual width and gap in the planner, and read the full purchasing workflow for the stock length and allowance.