Deck Board Sizes and Spacing: Widths, Lengths and Gaps

Most deck boards are 5½ in (140 mm) wide. A "5/4 × 6" wood board is about 1 × 5½ in, a 2 × 6 is 1½ × 5½ in, and composite boards such as Trex are 0.94 × 5½ in. Leave a gap between boards: Trex requires at least 3/16 in (5 mm), and wood decks are commonly about 1/8 in once the boards are dry. In metric, 90 and 140 mm boards with 3–8 mm gaps are usual.

Rows = width ÷ (board width + gap). Screws = rows × joists × 2. Dashed lines are joists.

Deck board sizes

Lumber is sold by its nominal size, the size before drying and planing, so a board is always smaller than its name. Mataverde Decking's size chart gives the actual sizes below. Trex boards come in 12, 16 and 20 ft lengths (grooved boards for hidden fasteners) or 16 and 20 ft (square-edge). Australian hardwood decking runs from about 70 to 140 mm wide and 19 mm thick, according to Meyer Timber.

Common deck board sizes
Board	Actual size
5/4 × 6 wood	1 × 5½ in (25 × 140 mm)
2 × 6 wood	1½ × 5½ in (38 × 140 mm)
Trex 1 × 6 composite	0.94 × 5½ in
Trex Select composite	0.82 × 5½ in
Hardwood (Australia)	70–140 mm wide, 19 mm thick

Gap between deck boards

Recommended gaps between deck boards
Material	Gap	Source
Trex composite, side to side	3/16 in (5 mm) minimum, never over ½ in	Trex
Trex composite, board ends	1/8 in above 40 °F; 3/16 in below	Trex
Wood, US	About 1/8 in once the boards have dried	AWC deck guide
Timber, Australia	3–4 mm for 90 mm boards; 5–6 mm for 140 mm	Meyer Timber
Timber, UK	5–6 mm up to 90 mm wide; 7–8 mm wider	Ecochoice (TDCA advice)

Trex also asks for ¼–½ in (6–13 mm) where boards meet a wall, and 3/8 in (10 mm) between boards on docks and in heavily wooded areas. Meyer Timber suggests 10 mm at walls for drainage. Wood shrinks as it dries, so the gaps between boards that are still wet from treatment will widen as they dry. Ask your supplier whether yours are kiln-dried, and follow the board maker's instructions over any general figure.

How the gap changes the number of rows

With 5½ in boards, the gap changes the count by about one row on a typical deck. These rows assume the boards run along the deck and are laid side by side across its width; the last board is ripped to fit.

Rows of 5½ in deck boards across a deck
Deck width	1/8 in gap	3/16 in gap	¼ in gap
8 ft	18	17	17
10 ft	22	22	21
12 ft	26	26	26
16 ft	35	34	34
20 ft	43	43	42

Metric: rows of boards across a deck
Deck width	90 mm, 3 mm gap	140 mm, 5 mm gap	140 mm, 8 mm gap
3 m	33	21	21
3.6 m	39	25	25
4.8 m	52	34	33

Worked example: a composite deck 12 ft wide

Each board plus a 3/16 in gap takes 5.5 + 0.1875 = 5.6875 in.
12 ft is 144 in, plus one gap that the last board doesn't need: (144 + 0.1875) ÷ 5.6875 = 25.4, so 26 rows.
On a 16 ft wide deck, the same boards need 34 rows at 3/16 in but 35 at 1/8 in: the narrower gap adds a row.

For boards, joists and screws for your deck, use the deck board calculator, which takes any board width, gap and length.

Joist spacing

Trex specifies joists at 16 in (406 mm) centres for 1 in boards on a residential deck, and 24 in for 2 × 6 boards. In Australia, Meyer Timber says boards are usually laid on joists at 450 mm centres, which needs 19 mm hardwood boards. Diagonal boards need closer joists.

Common mistakes

Using the nominal width. A "6 inch" board is 5½ in. Planning with 6 in boards undercounts the rows.
No gap at walls and ends. Boards expand; butting them hard against a house wall traps water and can buckle composite boards.
Gaps too wide. Trex says never more than ½ in between boards; wide gaps catch heels and chair legs.