JoistLedgerSpans, joists and footings straight from the code tables.

Decking for a 24 × 24 ft deck in 2 × 6 treated, herringbone layout

2 × 6 pressure-treated boards
125× 20 ft
52 rows across 24 ft — 1,498 linear ft including 20% waste
Plan — boards across the joists24 ft24 ft52rows5.625″ repeatactual width + gap125 boards · 2,600 screws
Boards to buy125× 20 ft
Linear feet1,248 ft plus 20% waste1,498ft
Rows of decking24 ft ÷ 5.625″ repeat52rows
Board repeat5.5″ actual width + 0.125″ gap — not the nominal size5.625in
Deck area24 × 24 ft576ft²
Joists crossedat 12″ o.c. over 24 ft25joists
Deck screwstwo per board-joist crossing — about 8 boxes of 3502,600screws
Layouttwo cuts on most boards and offcuts that fit nothingherringbone
Butt joints24 ft run exceeds the 20 ft board — stagger the joints and land every one on a joistYes
Deck width
24 ft
Deck length
24 ft
Board
2 × 6 treated
Layout
herringbone
Joist spacing
12 in o.c.

Nominal width is not actual width

This is where board counts go wrong. A 2 × 6 treated board measures its real width, and with the gap it needs the repeat is 5.625″ — which is what actually decides the row count.

24 ft of deck at that repeat is 52 rows, and 125 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

2,600 — two at every board-joist crossing, across 25 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 "2 ×" board measures 5.5″ across. With a 0.125″ gap the repeat is 5.625″, which is what decides the row count — 52 rows here. Working from the nominal size undercounts by close to a tenth.
  • Thicker and stiffer; spans further between joists. 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 24 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 — 2,600 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

Read more