Working out how many decking boards you need is straightforward once you break it down into a few simple steps. Measure your deck area, account for the actual coverage each board provides (including the gap between boards), then add a waste allowance for cuts and any defective timber. Get this right and you'll avoid costly over-ordering or frustrating mid-project trips back to the hardware store.
Step 1 – Measure Your Deck Area
Start by measuring the length and width of your deck in metres. Multiply these together to get your total area in square metres (m²).
For example: a deck 6 m long × 4 m wide = 24 m².
If your deck has an irregular shape — an L-shape or a section cut around a post — break it into rectangles, calculate each section separately, then add them together.
Step 2 – Understand Your Board Dimensions
Decking boards in New Zealand are commonly sold in these sizes:
| Nominal width | Actual width | Common thickness |
|---|---|---|
| 90 mm | ~85 mm | 45 mm |
| 140 mm | ~135 mm | 45 mm |
| 190 mm | ~185 mm | 45 mm |
Always use the actual (dressed) width, not the nominal size, in your calculations. Check the label or ask your supplier — Mitre 10, Bunnings, and PlaceMakers all stock a range of treated pine and hardwood decking.
Step 3 – Factor In the Gap Between Boards
Decking boards are never laid tight together. A 5–8 mm gap is standard in New Zealand to allow for drainage and timber movement as moisture levels change with our variable climate. This gap effectively reduces the coverage each board provides.
Effective cover per board = actual board width + gap
For example, a 140 mm board with a 6 mm gap has an effective cover of 146 mm (0.146 m).
Step 4 – Calculate the Number of Board Runs
Divide the deck width by the effective cover per board to find how many runs (rows) of boards you need:
Number of runs = deck width ÷ effective cover per board
Using our example: 4 m ÷ 0.146 m = 27.4 runs → round up to 28 runs
Always round up to the next whole number.
Step 5 – Work Out Board Lengths and Quantities
Decking boards are typically sold in standard lengths: 3.6 m, 4.2 m, 4.8 m, 5.4 m, and 6.0 m. Choose a length that suits your deck span to minimise joins and waste.
If your deck is 6 m long and you buy 6 m boards, each run needs one board. So 28 runs = 28 boards.
If your deck is 6 m long but you can only get 4.8 m boards, each run needs two boards with a join over a joist. In that case: 28 runs × 2 boards = 56 boards.
Step 6 – Add a Waste Allowance
Always add extra for:
- End cuts and angled cuts (especially on diagonal or picture-frame layouts)
- Defective boards — knots, splits, or bowing that make a board unusable
- Future repairs — a few spare boards stored in a dry spot are invaluable if a board needs replacing years later
Standard waste allowance:
- Straight lay: add 10%
- Diagonal or feature lay: add 15%
Multiply your board count by 1.10 or 1.15 and round up.
Example (straight lay): 28 boards × 1.10 = 30.8 → buy 31 boards
Quick Reference Formula
Here's the full process in one flow:
- Area (m²) = length × width
- Effective cover (m) = actual board width + gap
- Runs = deck width ÷ effective cover (round up)
- Boards per run = deck length ÷ board length (round up)
- Total boards = runs × boards per run
- Final order = total boards × 1.10 (or 1.15 for diagonal)
Timber Treatment and the NZ Building Code
In New Zealand, outdoor decking timber must be treated to the correct hazard class for ground contact and weather exposure. The MBIE Building Performance guidelines and NZS 3640 specify treatment requirements. For most above-ground decking, H3.2 treated timber is the minimum; posts in or near the ground typically require H4 or H5. Always confirm with your supplier or a qualified deck builder.
Decks over 1.5 m above ground or attached to the house may require a building consent — check with your local council before starting. Schedule 1 of the Building Act lists some exemptions, but rules vary by council and site.
When to Call a Professional
If your deck is large, elevated, or structurally complex, it's worth having a qualified deck builder check your plans and calculations. A professional will also ensure your framing, joist spacing, and fixings comply with NZS 3604 (Timber-framed buildings) and your council's requirements. You can find experienced local deck builders on tradedirectory.co.nz. If your deck involves any electrical work (outdoor lighting, power points), that must be carried out by a registered electrician — see the Electrical Workers Registration Board for more information.
Licensed Building Practitioner (LBP)
