Decking for a 12 × 32 ft deck in composite, diagonal layout
| Boards to buy | 60× 20 ft |
|---|---|
| Linear feet832 ft plus 15% waste | 957ft |
| Rows of decking12 ft ÷ 5.75″ repeat | 26rows |
| Board repeat5.5″ actual width + 0.25″ gap — not the nominal size | 5.75in |
| Deck area12 × 32 ft | 384ft² |
| Joists crossedat 24″ o.c. over 32 ft | 17joists |
| Deck screwstwo per board-joist crossing — about 3 boxes of 350 | 884screws |
| Layoutat 45°, every board is cut at both ends | diagonal |
| Butt joints32 ft run exceeds the 20 ft board — stagger the joints and land every one on a joist | Yes |
- Deck width
- 12 ft
- Deck length
- 32 ft
- Board
- composite
- Layout
- diagonal
- Joist spacing
- 24 in o.c.
Nominal width is not actual width
This is where board counts go wrong. A composite board measures its real width, and with the gap it needs the repeat is 5.75″ — which is what actually decides the row count.
12 ft of deck at that repeat is 26 rows, and 60 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
884 — two at every board-joist crossing, across 17 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 "5½″ composite" board measures 5.5″ across. With a 0.25″ gap the repeat is 5.75″, which is what decides the row count — 26 rows here. Working from the nominal size undercounts by close to a tenth.
- Expands with heat rather than shrinking — gap per the manufacturer, usually ¼″. 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.
- 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 — 884 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
- Decking for a 10 × 32 ft deck in composite, diagonal layout
- Decking for a 14 × 32 ft deck in composite, diagonal layout
- Decking for a 8 × 32 ft deck in composite, diagonal layout
- Decking for a 16 × 32 ft deck in composite, diagonal layout
- Decking for a 12 × 30 ft deck in composite, diagonal layout
- Decking for a 12 × 28 ft deck in composite, diagonal layout
- Decking for a 12 × 32 ft deck in 2 × 6 treated, diagonal layout
- Decking for a 12 × 32 ft deck in cedar, diagonal layout
- Decking for a 12 × 32 ft deck in 5/4 × 6 treated, diagonal layout
- Decking for a 12 × 32 ft deck in ipe, diagonal layout
- Decking for a 12 × 32 ft deck in composite, straight layout
- Decking for a 12 × 32 ft deck in composite, herringbone layout
- Decking for a 12 × 32 ft deck in composite, diagonal layout
- Decking for a 12 × 32 ft deck in composite, diagonal layout
Read more
- 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.