Calculating the decking timber you need comes down to four steps: measure your deck area, choose your board profile, account for the gaps between boards, and add a waste factor. Get these right and you'll avoid costly over-ordering or frustrating mid-project trips back to the timber yard.
Step 1 – Measure Your Deck Area
Start with a simple length × width measurement in metres. For a rectangular deck this is straightforward — for example, a 4 m × 5 m deck gives you 20 m².
For L-shaped or irregular decks, break the area into rectangles, calculate each section separately, then add them together. Sketch it out on paper first; it saves mistakes.
Tip: Always measure the finished deck area (the surface you want covered), not the frame footprint.
Step 2 – Choose Your Decking Board Profile
In New Zealand, the most common decking board widths are 90 mm and 140 mm, with thicknesses of 45 mm for structural boards (confirm available profiles with your supplier). Narrower boards (90 mm) are popular for their classic look and are less prone to cupping; wider boards (140 mm) lay faster and can suit a more contemporary style.
Check that your chosen timber species is suitable for outdoor use. Common NZ options include:
- Treated pine (H3.2 or H4 treated, depending on ground contact) — widely available and cost-effective
- Kwila (Merbau) — a dense hardwood with a rich colour, though check sustainable sourcing
- Vitex — a durable Pacific hardwood
- Composite decking — timber/plastic blends; follow the manufacturer's own calculation guide as profiles vary significantly
Always confirm the treatment level required for your application with your supplier. For decks close to or in contact with the ground, BRANZ and the NZ Building Code specify minimum hazard class ratings.
Step 3 – Calculate the Number of Boards
This is where most people trip up. You can't simply divide your deck width by the board width — you must account for the gap between boards.
A standard gap in NZ is 5–6 mm (some builders use up to 8 mm for hardwoods that may swell). So the effective width each board occupies is:
Effective board width = board face width + gap
| Board face width | Gap | Effective width per board |
|---|---|---|
| 90 mm | 6 mm | 96 mm (0.096 m) |
| 140 mm | 6 mm | 146 mm (0.146 m) |
Calculate number of board runs
Divide the deck width by the effective board width:
Number of runs = Deck width (m) ÷ Effective board width (m)
Example: 4 m wide deck using 90 mm boards with 6 mm gaps: 4 ÷ 0.096 = 41.7 → round up to 42 runs
Calculate board lengths
If your boards run the full length of the deck (5 m in our example), you need 42 boards at 5 m each.
If your deck is longer than standard board lengths (typically 3.6 m, 4.2 m, 4.8 m, or 5.4 m), you'll need to plan joins over joists and may need extra boards.
Step 4 – Add a Waste Factor
Always order more than your net calculation. Waste occurs from:
- End cuts and trimming boards to length
- Defects in individual boards (knots, splits, warping)
- Angled cuts if your deck has a diagonal or herringbone pattern
As a general guide:
| Laying pattern | Recommended waste allowance |
|---|---|
| Straight (parallel to frame) | 10% |
| Diagonal (45°) | 15–20% |
| Herringbone or feature pattern | 20–25% |
Multiply your total board count by (1 + waste factor). For our example with straight laying: 42 × 1.10 = 46.2 → order 47 boards
Step 5 – Convert to Lineal Metres for Ordering
Timber yards often sell decking by the lineal metre (LM) rather than by board count. To convert:
Total lineal metres = Number of boards × Board length
47 boards × 5 m = 235 lineal metres
Take this figure to your supplier at Bunnings, Mitre 10, or PlaceMakers and they can help confirm the right product and quantity.
Don't Forget Fixings and Framing
Decking boards are only part of the material list. You'll also need:
- Joist framing timber (typically 190 × 45 mm or 140 × 45 mm H3.2 treated pine, at spacings specified by NZS 3604)
- Decking screws or hidden fasteners (stainless steel or hot-dipped galvanised for outdoor use)
- Post anchors, bearers, and hardware (all rated for outdoor/ground contact)
- Concrete for footings if required
Building Consent — Check Before You Build
In New Zealand, many decks require a building consent. Under Schedule 1 of the Building Act, some low decks may be exempt — but the rules depend on height above ground, proximity to boundaries, and other factors. Always check with your local council before starting work. You can find general guidance at MBIE Building Performance.
When to Call a Professional
While calculating timber quantities is a DIY-friendly task, the actual design and construction of a deck involves structural decisions — joist sizing, bearer spans, footing design — that must comply with NZS 3604 and the NZ Building Code. If your deck is elevated, large, or attached to the house, engaging a Licensed Building Practitioner (LBP) is strongly recommended and may be legally required for restricted building work. Find a qualified local builder or quantity surveyor on tradedirectory.co.nz.
A quantity surveyor can also prepare a full materials schedule and cost estimate so you know exactly what you're up for before you commit.
Site Safe
Licensed Building Practitioner (LBP)
ISO 9001 / 45001 / 14001
NZ Institute of Building (NZIOB)
