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Decking for a 10 × 30 ft deck in composite, diagonal layout

5½″ composite boards
49× 20 ft
21 rows across 10 ft — 725 linear ft including 15% waste
Plan — boards across the joists10 ft30 ft21rows5.75″ repeatactual width + gap49 boards · 966 screws
Boards to buy49× 20 ft
Linear feet630 ft plus 15% waste725ft
Rows of decking10 ft ÷ 5.75″ repeat21rows
Board repeat5.5″ actual width + 0.25″ gap — not the nominal size5.75in
Deck area10 × 30 ft300ft²
Joists crossedat 16″ o.c. over 30 ft23joists
Deck screwstwo per board-joist crossing — about 3 boxes of 350966screws
Layoutat 45°, every board is cut at both endsdiagonal
Butt joints30 ft run exceeds the 20 ft board — stagger the joints and land every one on a joistYes
Deck width
10 ft
Deck length
30 ft
Board
composite
Layout
diagonal
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.

10 ft of deck at that repeat is 21 rows, and 49 boards once 15% waste for the diagonal 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

966 — two at every board-joist crossing, across 23 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 — 21 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.
  • Diagonal layout, 15% waste — at 45°, every board is cut at both ends. Order the full allowance in one go; diagonal offcuts rarely fit anywhere else on the deck.
  • The 30 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 — 966 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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