Work out how many decking boards and joists a deck needs, using the actual face width of the board, the gap between boards and the joist spacing underneath.
Use the actual width of the board, not the nominal name. A 5/4×6 deck board is 5.5 inches wide and a 2×4 is 3.5 inches, using 6 or 4 in the arithmetic will under-count the rows.
Decking is laid as a series of parallel rows. The number of rows is set by the deck's width divided by the width of one board plus the gap between boards. Everything after that is a multiplication.
The one number people get wrong is the board's width. Lumber is named by its rough size, not its finished size. A 5/4×6 deck board is actually 5.5 inches wide, and a 2×4 is 3.5 inches. Using 6 or 4 in the arithmetic under-counts your rows and leaves you short of material.
The gap is not a rounding error either. Boards need a gap for drainage and for seasonal movement, ⅛ to 3/16 of an inch is typical. On a 12-foot-wide deck that gap repeatedly added across 25 rows changes the count by a row or more.
| Nominal size | Actual face width | Actual thickness | Typical use |
|---|---|---|---|
| 5/4×6 | 5.5 in | 1 in | Standard residential decking, spans 16 in joists |
| 2×6 | 5.5 in | 1.5 in | Heavier traffic, commercial decks |
| 2×4 | 3.5 in | 1.5 in | Railings, fascia, blocking |
A 5/4×6 and a 2×6 cover the same width; the difference is thickness and stiffness, not coverage.
| Spacing | When to use it |
|---|---|
| 16 in on centre | The residential standard for 5/4 wood decking and most composite boards |
| 12 in on centre | Stiffer feel, diagonal layouts, premium composites whose data sheet requires it |
| 24 in on centre | Only for full 2×6 or heavier solid wood laid perpendicular. Most composite brands forbid it and it voids the warranty |
Exceeding a decking manufacturer's maximum joist spacing voids the warranty and makes the deck feel springy. Diagonal and picture-frame layouts always need 12 or 16 inch spacing plus blocking between joists at the board ends.
| Layout | Waste | Why |
|---|---|---|
| Perpendicular (standard) | 5–10% | Straight trimming at the perimeter |
| Diagonal (45°) | 15% | Every board meeting the edge is angle-cut |
| Herringbone / chevron | 20%+ | Angled cuts on every row, plus blocking at each direction change |
| Picture-frame border | 15%+ | Mitred corners and a border that must be cut to length |
Hidden-fastener systems change the fastener count, not the board count, though they do require grooved boards or proprietary clips.
The row and board formulas follow the standard decking take-off method published by lumber suppliers and decking manufacturers: rows from the actual face width plus gap, boards from the row count and board length, and joists from the span divided by the on-centre spacing plus one. The 16 inch on-centre standard and the actual dimensions of nominal lumber come from the IRC and from the American Softwood Lumber Standard sizes. Always confirm joist sizing and spans against a span table for your specific decking and local code.
Divide the deck width in inches by the board's actual face width plus the gap to get the number of rows, then multiply by how many boards each row needs. A 12 ft wide deck with 5.5 in boards and a ⅛ in gap needs 26 rows; each row is one 16 ft board on a 16 ft deck.
About ⅛ to 3/16 of an inch. The gap provides drainage and lets the boards move with the seasons without cupping or crushing each other. Composite boards usually specify a gap in their installation guide, and some require clips that set it automatically.
16 inches on centre is the residential standard for 5/4 wood decking and most composite boards. Use 12 inches for a stiffer deck, for diagonal layouts, or where a composite manufacturer requires it. 24 inches is only acceptable for full 2× solid wood laid perpendicular.
Divide the deck length in inches by the joist spacing, round up, and add one for the far end. A 16 ft deck at 16 inch spacing needs ⌈192÷16⌉ + 1 = 13 joists, before adding doubled rim joists, the ledger and any blocking.
Actual. A 5/4×6 deck board measures 5.5 inches across its face, not 6. Using the nominal figure under-counts the rows, and on a wide deck that is a row or more of boards you will discover you are missing halfway through the job.