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Decking for a 22 × 30 ft deck in composite, straight layout

5½″ composite boards
97× 20 ft
46 rows across 22 ft — 1,449 linear ft including 5% waste
Plan — boards across the joists22 ft30 ft46rows5.75″ repeatactual width + gap97 boards · 2,852 screws
Boards to buy97× 20 ft
Linear feet1,380 ft plus 5% waste1,449ft
Rows of decking22 ft ÷ 5.75″ repeat46rows
Board repeat5.5″ actual width + 0.25″ gap — not the nominal size5.75in
Deck area22 × 30 ft660ft²
Joists crossedat 12″ o.c. over 30 ft31joists
Deck screwstwo per board-joist crossing — about 9 boxes of 3502,852screws
Layoutboards run perpendicular to the joists — least wastestraight
Butt joints30 ft run exceeds the 20 ft board — stagger the joints and land every one on a joistYes
Deck width
22 ft
Deck length
30 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.

22 ft of deck at that repeat is 46 rows, and 97 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

2,852 — two at every board-joist crossing, across 31 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 — 46 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.
  • 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 — 2,852 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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