Decking for a 18 × 10 ft deck in 5/4 × 6 treated, diagonal layout

5/4 × 6 pressure-treated boards

45× 10 ft

39 rows across 18 ft — 448 linear ft including 15% waste

Boards to buy	45× 10 ft
Linear feet390 ft plus 15% waste	448ft
Rows of decking18 ft ÷ 5.625″ repeat	39rows
Board repeat5.5″ actual width + 0.125″ gap — not the nominal size	5.625in
Deck area18 × 10 ft	180ft²
Joists crossedat 24″ o.c. over 10 ft	6joists
Deck screwstwo per board-joist crossing — about 2 boxes of 350	468screws
Layoutat 45°, every board is cut at both ends	diagonal
Butt joints10 ft boards cover the 10 ft run in one piece	None needed

Deck width: 18 ft
Deck length: 10 ft
Board: 5/4 × 6 treated
Layout: diagonal
Joist spacing: 24 in o.c.

Nominal width is not actual width

This is where board counts go wrong. A 5/4 × 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.

18 ft of deck at that repeat is 39 rows, and 45 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

468 — two at every board-joist crossing, across 6 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 0.125″ gap the repeat is 5.625″, which is what decides the row count — 39 rows here. Working from the nominal size undercounts by close to a tenth.
Sold wet — it shrinks, so gap it tight at about ⅛″. 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 10 ft board covers the 10 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 — 468 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

Composite or Pressure-Treated? The 20-Year Arithmetic, Including the Part Nobody CountsComposite wins the twenty-year total only if you would actually have refinished the wood. Most people do not, which is why most wood decks look their age.
Deck Joist Cantilever: One Quarter of the Actual Span, Not the MaximumThis is the most misread line in the deck code, and it is misread in the direction that puts the beam too far in.
How Many Footings Does a Deck Need? Work Backwards From Tributary AreaNobody decides footing count directly. It falls out of how far the beam can span, and how much deck each post is holding up.
Cost to Build a Deck per Square Foot: Where the Money Actually GoesDecking boards are the part everyone shops for and about a quarter of the bill. The framing underneath is where the price is decided.