Decking guide
How much decking do I need?
To work out how many deck boards to buy, count the rows of boards across the deck, work out how many boards each row needs, then add an allowance for waste. The gap between boards matters, because it changes how many rows fit.
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Open the deck plannerShape your deck, preview the board layout and estimate the boards and perimeter materials.
Step 1: coverage width of one board
Coverage width = actual board width + the gap between boards. Use the real width, not the nominal size. A nominal 6 in board is about 5½ in wide, and a typical 140 mm board is 140 mm. Composite makers often specify gaps around ⅛ to 3/16 in (3 to 5 mm), and timber usually needs a little more, so check the manufacturer's instructions.
Step 2: number of rows
Rows = the deck dimension that the boards fill (their stacking direction) ÷ coverage width, rounded up. Boards run perpendicular to the joists, so choose the direction first.
Worked example (metric)
- Deck: 3.6 m × 4.8 m. Boards run along the 4.8 m side.
- Board: 140 mm wide with a 5 mm gap, so 145 mm coverage.
- Rows: 3,600 ÷ 145 = 24.8, so 25 rows.
- Using 4.8 m boards, each row needs one board, so 25 boards.
- Add 10% for cuts and defects: 25 × 1.10 = 27.5, so order 28 boards.
Worked example (imperial)
A 12 ft × 16 ft deck with 16 ft boards, 5½ in boards and a ⅛ in gap has 5⅝ in of coverage per board. Rows: 144 in ÷ 5.625 = 25.6, so 26 rows and 26 boards. With a 10% allowance, order 29 boards.
Waste allowance
| Layout | Typical allowance |
|---|---|
| Straight, boards the full length of the deck | About 10% |
| Boards need joining over a joist | 10% to 15%, depending on how well lengths suit the deck |
| Diagonal or picture-frame layout | 15% to 20% |
Board lengths and joins
Boards are commonly sold in fixed lengths, so choose lengths that match your deck to avoid joins and offcuts. Where one board is not long enough, join the ends over the middle of a joist, staggering the joins from row to row. A mix of lengths planned around your seams often wastes less than ordering a single length.
How many fixings do you need?
Count one clip or fastener position wherever a board crosses a joist. A deck with 13 joists and 26 rows has 13 × 26 = 338 crossings. That means 338 hidden clips, or about 676 screws if you face-screw with two per crossing. Order a few percent extra.
Frequently asked questions
How do I calculate how many deck boards I need?
Divide the deck dimension the boards stack across by the board's coverage width (board width plus gap) and round up to get the rows. Multiply by the boards needed per row, then add 10% or more for waste.
How much extra decking should I buy?
Around 10% for a straight layout and 15% to 20% for diagonal or picture-frame layouts, where more boards are cut at angles.
What gap should I leave between deck boards?
It depends on the material and the manufacturer. Composite boards are often gapped around 1/8 to 3/16 inch (3 to 5 mm), and timber usually a little wider. Follow the installation instructions for your boards.
Which way should deck boards run?
Boards run perpendicular to the joists. Running them along the longer side of the deck usually needs fewer joins and wastes less.
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