Decking for a 24 × 8 ft deck in composite, straight layout

5½″ composite boards

54× 8 ft

51 rows across 24 ft — 428 linear ft including 5% waste

Boards to buy	54× 8 ft
Linear feet408 ft plus 5% waste	428ft
Rows of decking24 ft ÷ 5.75″ repeat	51rows
Board repeat5.5″ actual width + 0.25″ gap — not the nominal size	5.75in
Deck area24 × 8 ft	192ft²
Joists crossedat 12″ o.c. over 8 ft	9joists
Deck screwstwo per board-joist crossing — about 3 boxes of 350	918screws
Layoutboards run perpendicular to the joists — least waste	straight
Butt joints8 ft boards cover the 8 ft run in one piece	None needed

Deck width: 24 ft
Deck length: 8 ft
Board: composite
Layout: straight
Joist spacing: 12 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.

24 ft of deck at that repeat is 51 rows, and 54 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

918 — two at every board-joist crossing, across 9 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 — 51 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 8 ft board covers the 8 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 — 918 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

Deck Screws per Square Foot: The Count, the Length, and the Coating That MattersRoughly 350 screws per hundred square feet at standard spacing — and the coating rating matters more to the deck's life than the count does.
Cost to Replace Deck Boards Only: When Resurfacing Beats RebuildingResurfacing saves roughly half the cost of a rebuild, provided the frame underneath has more life left than the boards you are about to screw to it.
Hidden Deck Fasteners or Face Screws: Cost, Speed, and the Repair ProblemHidden fasteners cost roughly three times as much and look better. The question that decides it is what happens when one board in the middle needs to come out.
Deck Board Gap: Why Wet Lumber and Composite Need Opposite NumbersThere is no single correct gap. There is a correct gap for a board that is going to shrink and a different one for a board that is going to grow.