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Decking for a 18 × 12 ft deck in composite, herringbone layout

5½″ composite boards
46× 12 ft
38 rows across 18 ft — 547 linear ft including 20% waste
Plan — boards across the joists18 ft12 ft38rows5.75″ repeatactual width + gap46 boards · 532 screws
Boards to buy46× 12 ft
Linear feet456 ft plus 20% waste547ft
Rows of decking18 ft ÷ 5.75″ repeat38rows
Board repeat5.5″ actual width + 0.25″ gap — not the nominal size5.75in
Deck area18 × 12 ft216ft²
Joists crossedat 24″ o.c. over 12 ft7joists
Deck screwstwo per board-joist crossing — about 2 boxes of 350532screws
Layouttwo cuts on most boards and offcuts that fit nothingherringbone
Butt joints12 ft boards cover the 12 ft run in one pieceNone needed
Deck width
18 ft
Deck length
12 ft
Board
composite
Layout
herringbone
Joist spacing
24 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.

18 ft of deck at that repeat is 38 rows, and 46 boards once 20% waste for the herringbone 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

532 — two at every board-joist crossing, across 7 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 — 38 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.
  • Herringbone layout, 20% waste — two cuts on most boards and offcuts that fit nothing. Order the full allowance in one go; diagonal offcuts rarely fit anywhere else on the deck.
  • 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 — 532 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

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