Decking for a 18 × 20 ft deck in cedar, diagonal layout

5/4 × 6 cedar boards

44× 20 ft

38 rows across 18 ft — 874 linear ft including 15% waste

Boards to buy	44× 20 ft
Linear feet760 ft plus 15% waste	874ft
Rows of decking18 ft ÷ 5.688″ repeat	38rows
Board repeat5.5″ actual width + 0.1875″ gap — not the nominal size	5.688in
Deck area18 × 20 ft	360ft²
Joists crossedat 24″ o.c. over 20 ft	11joists
Deck screwstwo per board-joist crossing — about 3 boxes of 350	836screws
Layoutat 45°, every board is cut at both ends	diagonal
Butt joints20 ft boards cover the 20 ft run in one piece	None needed

Deck width: 18 ft
Deck length: 20 ft
Board: cedar
Layout: diagonal
Joist spacing: 24 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.

18 ft of deck at that repeat is 38 rows, and 44 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

836 — two at every board-joist crossing, across 11 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 — 38 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.
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.
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 — 836 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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