This is the second most common question we get after roofing sheet counts, and it is easier than it looks. There are only two measurements and one decision.
Step 1: the area
Length times width, in metres. If the deck is not a rectangle, split it into rectangles and add them up. Do not deduct small openings — a post penetration or a tree cut-out does not save you a board, because the board still has to be cut around it.
Step 2: which way do the boards run?
Decide this before you count anything, because it changes the answer.
Boards should run across the joists, and on a deck that drains one way, along the fall rather than against it. Beyond that it is a look: running the boards the long way makes a deck feel longer, running them across makes it feel wider.
It also decides your waste. Every decking board we stock is 2.9m long, so any run longer than that needs a joint — and a joint needs a joist doubled up underneath to land on. A 4.2m deck run the long way means a joint and an offcut on every single run. Turn the boards through 90° and the same deck may lay in whole boards with no joint at all.
Step 3: board coverage, not board width
This is the step people get wrong, in exactly the same way they get roofing cover width wrong.
A board is not laid tight against its neighbour. Composite needs a gap between boards for drainage and for movement, and on a clipped system the clip sets that gap for you. So the coverage of a board is its width plus the gap, not its width.
Our decking boards are 2900 × 140 × 25mm. Add the gap between boards and one board covers roughly 0.41m². There is one length, so the sum is the same whichever board you choose.
| Board | Size | Rough coverage each |
|---|---|---|
| 2.9m Select Capped / Signature Woodgrain | 2900 × 140 × 25mm | ~0.41m² |
| 2.9m Select Capped Edge Board | 2900 × 142 × 23mm | Edge and fascia use |
Deck area divided by board coverage, rounded up. That is your board count.
Step 4: the edge
A cut composite board shows its core. On a hollow-core board that means visible voids along every exposed edge, which looks unfinished no matter how good the deck is.
The edge board is what closes it — a square-edged, capped board fixed to the perimeter to hide the ends and the fascia of the frame. Measure the exposed perimeter, divide by 2.9m, round up.
Step 5: the spare
One board per twenty, minimum one. Boards get dropped, cut short and occasionally arrive marked. A spare board sitting in the garage costs the price of a board. A missing board halfway through a build costs a day and a delivery.
Keep the spare out of direct sunlight so that if it is ever needed it still matches.
A worked example
A 5m × 4m deck, boards running the 5m way.
- Area: 20m²
- Boards: 20 ÷ 0.41 = 49 boards
- Exposed perimeter: three sides visible, 5 + 4 + 5 = 14m ÷ 2.9 = 5 edge boards
- Spare: 49 ÷ 20 = 3 boards
Fifty-two boards and five edge boards, and the only thing left to price is the frame.
The frame, and why we have not given you a joist spacing
Joist centres for composite depend on the board, the direction the boards run relative to the joists, and what the deck is carrying — and running boards diagonally tightens the centres again. We would rather tell you the right number for the boards you have chosen than publish one figure that is wrong for half the decks it gets used on.
Send us the deck size, the height off the ground and which board you are looking at, and we will give you the centres, the board count and the edge with it.
