How this decking calculator works
Three steps under the hood:
- Coverage factor: your deck area is multiplied by a coverage factor to account for the gaps between boards. For 90 mm boards with 5 mm gaps, the coverage factor is 1.10 — you need 10% more board material than your deck area. For 140 mm boards with 5 mm gaps, it's 1.08.
- Linear metres: the covered area (in m²) is divided by the board width to get total linear metres of decking needed.
- Board count: total linear metres divided by your chosen board length, rounded up, then a waste margin added on top.
Example: a 6 × 4 m deck (24 m²) using 140 mm boards at 4.8 m length with 15% waste needs approximately 40 boards. That's 24 × 1.08 = 25.9 m² of board area ÷ 0.14 m wide = 185 linear metres × 1.15 waste ÷ 4.8 m per board = 44 boards.
Australian decking board sizes
- 90 × 19 mm — traditional narrow board. Classic Australian look, suits heritage homes. Needs more boards and more screws. More cutting on complex shapes.
- 120 × 19 mm — mid-width option. Less common but a good middle ground between traditional and contemporary.
- 140 × 19 mm — the most popular width in AU right now. Contemporary look, fewer boards to lay, fewer screws, faster installation. The default choice for new decks.
Standard board lengths: 2.4 m, 3.0 m, 3.6 m, 4.2 m, 4.8 m, 5.4 m. Always choose the longest board that avoids a join in your main deck run — visible joins reduce the finished look and create potential water entry points.
Choosing a decking timber
- Merbau — the traditional Australian hardwood choice. Rich reddish-brown colour, Class 1 durability above ground (40+ year lifespan), naturally termite resistant. Requires regular oiling (annually for the first 3 years, then every 2-3 years). More expensive than pine.
- Treated Pine (H3/H4) — the budget option. H3 for above-ground use, H4 for in-ground contact. Needs oiling or staining. Much cheaper than hardwood, widely available at Bunnings and timber yards. Good for lower-level or covered decks.
- Spotted Gum — premium Australian hardwood, grey-brown tones. Class 1 durability, very hard and durable. Popular in QLD and NSW. Beautiful grain, striking appearance.
- Composite / PVC — low maintenance, no oiling required, consistent colour. Higher upfront cost but lower long-term maintenance. Brands: Futurewood, CleverDeck, Ekodeck. Board dimensions vary slightly from timber — check your specific brand's coverage figures.
Gap spacing and screws
Standard gap spacing for AU decking: 5–6 mm for 90 mm boards, 5 mm for 140 mm boards. Gaps allow water drainage and air circulation — too tight and the deck holds moisture, too wide looks sloppy and catches heels.
Screws: use 65 mm × 10g decking screws for 19 mm boards into H4 treated pine joists, or 75 mm × 10g for thicker boards or hardwood joists. Stainless steel screws are mandatory for coastal areas and recommended everywhere — galvanised screws corrode within 5–7 years in most AU climates.
Two screws per board per joist is standard — one either side of the board centreline. Spacing joists at 450 mm centres (not 600 mm) significantly reduces board flex underfoot, especially for 140 mm × 19 mm boards.
Do you need council approval for a deck in Australia?
In most Australian states, a freestanding deck under 1 m high and under a certain floor area (varies by council — typically 10–25 m²) is exempt from building approval. Attached decks, elevated decks over 1 m, or decks over the size threshold usually require a building permit.
In Queensland: decks under 10 m² attached to a house, or under 25 m² freestanding and under 1 m high, are generally exempt. Check the Queensland Development Code and your local council before building.