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.
Decking calculator — frequently asked questions
How many decking boards do I need per square metre?
It depends on the board width and gap size. Standard 90 mm Merbau boards with a 5 mm gap cover about 10.5 linear metres per square metre. The calculator works out board count and linear metres based on your deck dimensions and the board profile you select.
What is the best timber for decking in Queensland?
Merbau is the most popular hardwood decking in Australia — durable, termite-resistant and handles Queensland's humidity well. Spotted Gum and Blackbutt are excellent native alternatives. Treated pine (H3 or H4 rated) is a cheaper option but needs more regular maintenance. Composite decking is increasingly popular for its low maintenance requirements in coastal Queensland environments.
What gap should I leave between decking boards?
5–6 mm is standard for hardwood decking in Australia. This allows for drainage and some movement without creating a trip hazard. Timber expands and contracts with moisture — in Queensland's humid climate, boards may close up slightly during the wet season and open in the dry.
How many screws do I need for decking?
Two screws per board end per joist is standard — that means two screws at every point where a board crosses a joist. The calculator estimates screw count based on your joist spacing and deck area. Buy stainless steel or hot-dipped galvanised screws — standard screws rust and stain the timber in Queensland's conditions.
Do I need council approval for a deck in Queensland?
In Queensland, decks under 1 m above natural ground level and under 10 m² generally don't need a building approval. Decks over 1 m high, over 10 m², or attached to the house typically require a building approval and potentially a building certifier. Check with your local council or a licensed builder before starting.