Decking for a 12 × 12 ft deck in composite, straight layout

5½″ composite boards

28× 12 ft

26 rows across 12 ft — 328 linear ft including 5% waste

Boards to buy	28× 12 ft
Linear feet312 ft plus 5% waste	328ft
Rows of decking12 ft ÷ 5.75″ repeat	26rows
Board repeat5.5″ actual width + 0.25″ gap — not the nominal size	5.75in
Deck area12 × 12 ft	144ft²
Joists crossedat 16″ o.c. over 12 ft	10joists
Deck screwstwo per board-joist crossing — about 2 boxes of 350	520screws
Layoutboards run perpendicular to the joists — least waste	straight
Butt joints12 ft boards cover the 12 ft run in one piece	None needed

Deck width: 12 ft
Deck length: 12 ft
Board: composite
Layout: straight
Joist spacing: 16 in o.c.

Nominal width is not actual width

This is where board counts go wrong. A composite board measures its real width, and with the gap it needs the repeat is 5.75″ — which is what actually decides the row count.

12 ft of deck at that repeat is 26 rows, and 28 boards once 5% waste for the straight layout is added.

Working from the nominal size undercounts by close to a tenth.

The gap is not optional in either direction

Treated lumber is sold wet and shrinks. Composite expands with heat. Gap too tight on composite and it buckles in July; gap too wide on wet treated lumber and by autumn you have gaps that swallow a phone.

Screws

520 — two at every board-joist crossing, across 10 joists. One screw per crossing lets the board cup and lift at the corner, and that is the single most common reason a deck looks tired after two summers.

Read the end grain

Lay boards bark-side up. A board cups away from its bark side, so bark-up sheds water off the crown rather than holding it in a trough.

Butt joints

Where the run exceeds a standard board length, stagger the joints so no two adjacent rows meet over the same joist — and double the joist under every joint. A butt joint on a single joist gives each board about ¾ inch of bearing, which is not enough for either board.

Notes

**Nominal width is not actual width.** A "5½″ composite" board measures 5.5″ across. With a 0.25″ gap the repeat is 5.75″, which is what decides the row count — 26 rows here. Working from the nominal size undercounts by close to a tenth.
Expands with heat rather than shrinking — gap per the manufacturer, usually ¼″. The gap is not optional in either direction: too tight on composite and it buckles in July; too wide on wet treated lumber and you get gaps you can drop a phone through by autumn.
Straight layout, 5% waste — boards run perpendicular to the joists — least waste. This is the least wasteful way to lay a deck, and the fastest.
A 12 ft board covers the 12 ft run in one piece. Worth paying for: an unbroken board is stronger, drains better and looks like a deck rather than a repair.
Fasten with two screws at every board-joist crossing — 520 screws here. One screw lets the board cup and lift at the corner, and it is the single most common reason a deck looks tired after two summers.
Lay the boards bark-side up if you can read the end grain. A board cups away from the bark side, so bark-up sheds water off the crown instead of holding it in a trough.

Nearby sizes

Composite or Pressure-Treated? The 20-Year Arithmetic, Including the Part Nobody CountsComposite wins the twenty-year total only if you would actually have refinished the wood. Most people do not, which is why most wood decks look their age.
Deck Joist Cantilever: One Quarter of the Actual Span, Not the MaximumThis is the most misread line in the deck code, and it is misread in the direction that puts the beam too far in.
How Many Footings Does a Deck Need? Work Backwards From Tributary AreaNobody decides footing count directly. It falls out of how far the beam can span, and how much deck each post is holding up.
Cost to Build a Deck per Square Foot: Where the Money Actually GoesDecking boards are the part everyone shops for and about a quarter of the bill. The framing underneath is where the price is decided.