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Decking for a 30 × 28 ft deck in composite, herringbone layout

5½″ composite boards
152× 20 ft
63 rows across 30 ft — 2,117 linear ft including 20% waste
Plan — boards across the joists30 ft28 ft63rows5.75″ repeatactual width + gap152 boards · 3,654 screws
Boards to buy152× 20 ft
Linear feet1,764 ft plus 20% waste2,117ft
Rows of decking30 ft ÷ 5.75″ repeat63rows
Board repeat5.5″ actual width + 0.25″ gap — not the nominal size5.75in
Deck area30 × 28 ft840ft²
Joists crossedat 12″ o.c. over 28 ft29joists
Deck screwstwo per board-joist crossing — about 11 boxes of 3503,654screws
Layouttwo cuts on most boards and offcuts that fit nothingherringbone
Butt joints28 ft run exceeds the 20 ft board — stagger the joints and land every one on a joistYes
Deck width
30 ft
Deck length
28 ft
Board
composite
Layout
herringbone
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.

30 ft of deck at that repeat is 63 rows, and 152 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

3,654 — two at every board-joist crossing, across 29 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 — 63 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.
  • The 28 ft run is longer than a 20 ft board, so there will be butt joints. Stagger them so no two adjacent rows join over the same joist, and double the joist under every joint — a butt joint on a single joist gives each board about ¾″ of bearing.
  • Fasten with two screws at every board-joist crossing — 3,654 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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