Decking for a 20 × 32 ft deck in cedar, straight layout

5/4 × 6 cedar boards

91× 20 ft

43 rows across 20 ft — 1,445 linear ft including 5% waste

Boards to buy	91× 20 ft
Linear feet1,376 ft plus 5% waste	1,445ft
Rows of decking20 ft ÷ 5.688″ repeat	43rows
Board repeat5.5″ actual width + 0.1875″ gap — not the nominal size	5.688in
Deck area20 × 32 ft	640ft²
Joists crossedat 12″ o.c. over 32 ft	33joists
Deck screwstwo per board-joist crossing — about 9 boxes of 350	2,838screws
Layoutboards run perpendicular to the joists — least waste	straight
Butt joints32 ft run exceeds the 20 ft board — stagger the joints and land every one on a joist	Yes

Deck width: 20 ft
Deck length: 32 ft
Board: cedar
Layout: straight
Joist spacing: 12 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.

20 ft of deck at that repeat is 43 rows, and 91 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

2,838 — two at every board-joist crossing, across 33 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 — 43 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.
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 32 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 — 2,838 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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