Decking for a 30 × 20 ft deck in cedar, herringbone layout

5/4 × 6 cedar boards

77× 20 ft

64 rows across 30 ft — 1,536 linear ft including 20% waste

Boards to buy	77× 20 ft
Linear feet1,280 ft plus 20% waste	1,536ft
Rows of decking30 ft ÷ 5.688″ repeat	64rows
Board repeat5.5″ actual width + 0.1875″ gap — not the nominal size	5.688in
Deck area30 × 20 ft	600ft²
Joists crossedat 16″ o.c. over 20 ft	16joists
Deck screwstwo per board-joist crossing — about 6 boxes of 350	2,048screws
Layouttwo cuts on most boards and offcuts that fit nothing	herringbone
Butt joints20 ft boards cover the 20 ft run in one piece	None needed

Deck width: 30 ft
Deck length: 20 ft
Board: cedar
Layout: herringbone
Joist spacing: 16 in o.c.

Nominal width is not actual width

This is where board counts go wrong. A cedar board measures its real width, and with the gap it needs the repeat is 5.688″ — which is what actually decides the row count.

30 ft of deck at that repeat is 64 rows, and 77 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

2,048 — two at every board-joist crossing, across 16 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 "5/4 ×" board measures 5.5″ across. With a 0.1875″ gap the repeat is 5.688″, which is what decides the row count — 64 rows here. Working from the nominal size undercounts by close to a tenth.
Sold dry, so it gaps wider than treated lumber. 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 20 ft board covers the 20 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 — 2,048 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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