Deck Calculator Australia — Boards, Joists, Posts & Footings
Deck dimensions
Run of decking boards
Span of joists
Decking boards
Typical: 3–6 mm for hardwood · 5–8 mm for treated pine (allows for swelling)
Choose a stock length ≥ your deck length where possible to avoid joins.
Joists & bearers
Posts & footings
Low-level decks: 0.3–0.6 m · Elevated decks: 0.6–2.4 m. Decks >1 m high require engineering sign-off under the NCC.
Minimum 300 mm; 450 mm typical for stable soils. Deeper for reactive or soft soils — confirm with your local council.
Deck materials estimate
Decking boards
Joists
Posts & footings
This deck calculator estimates the number of decking boards, joists, posts and concrete footings for a standard rectangular Australian deck. Board counts are calculated from deck width, board face width and the specified gap — the same method used by Australian hardware stores. Joist counts are based on centres measured along the length of the deck. Post and footing grid is derived from your bearer and post spacings. Footing volumes use the standard cylinder formula (π × r² × depth) and are converted to 20 kg pre-mix bag counts. All timber sizes align with standard Australian DAR and structural timber sections available from Bunnings, Reece, and trade timber merchants.
Frequently Asked Questions
What joist spacing should I use for a timber deck in Australia?
For treated pine decking boards (90×19 or 90×45), 450 mm joist centres is the standard residential spacing. Hardwood boards can span 450–600 mm depending on species and section. Composite decking typically requires 300–400 mm centres — always check the manufacturer's span tables. Joist size and span tables are covered in AS 1684 Residential Timber Framing.
How deep should deck footings be in Australia?
The National Construction Code (NCC) requires footings to reach stable ground, which typically means a minimum of 300 mm in good stable soil. In practice, most Australian residential decks use 450 mm deep footings. On reactive (expansive clay) soils, footings may need to be 600 mm or deeper — your local council or a geotechnical engineer can advise on the soil classification for your site.
Do I need a building permit for a deck in Australia?
In most Australian states, a deck over a certain height or area requires a building permit. Typical thresholds are decks more than 1 metre above ground level, or over 10 m² in area on a bushfire-prone block. Regulations vary by state and council — check with your local council before starting any deck construction. Decks over 1 m high also require engineering certification under the NCC.
What size footings does a residential deck need?
A 300 mm diameter × 450 mm deep concrete footing is the standard for most residential decks with post spacings of 1.8–2.4 m at typical residential loads. Larger post spacings, elevated decks, or poor-bearing soils require larger footings — an engineer should size these for you. Always use post stirrups set in the footing rather than embedding the timber post in concrete, which accelerates timber decay.
How much wastage should I allow for decking boards?
Allow 10% wastage for a simple rectangular deck with boards running parallel to the longest edge. Increase to 15% for diagonal board runs or where the deck shape has notched corners. Wastage includes end-cutting to length, any rejected boards with defects, and material at the perimeter where partial boards are required.
How to use this calculator
- Enter the deck length and width in metres. Length is the direction the boards run; width is the joist span. For an L-shaped deck, calculate each rectangle separately and add the results.
- Select the board face width and gap. The default 86 mm with 4 mm gap is correct for standard 90×19 treated pine with a 4 mm spacer.
- Choose the board stock length. Pick the nearest length above your deck length to avoid joins. A 4.8 m board suits a 4.5 m deck; a 6.0 m board suits spans up to 6.0 m.
- Enter joist spacing, bearer spacing, post height and footing details. Use the hint text to guide common Australian residential values.
- Click Calculate. The results show board count, lineal metres, joist count, post count, footing volume, and 20 kg bag count for the footings.
Worked example: A 5.4 m × 3.6 m deck with 86 mm boards at 4 mm gap, 4.8 m stock, 450 mm joist spacing, 1.8 m bearer spacing, 0.6 m post height, 300 mm dia × 450 mm deep footings with 10% wastage on boards. Board runs: ceil(3600 ÷ 90) = 40 runs. With 10% wastage: 44 runs × 2 boards per run (ceil 5.4 ÷ 4.8 = 2) = 88 boards. Joists: ceil(5.4 ÷ 0.45) + 1 = 13. Posts: 4 × 3 = 12. Footing volume: π × 0.15² × 0.45 × 12 = 0.38 m³ → 42 × 20 kg bags.
Understanding your results
The board count and lineal metres are the most important figures for your timber order. Present the lineal metres to your timber merchant — Australian merchants sell timber by the lineal metre. The board count assumes each board is one stock length; for longer decks where boards don't reach, adjust your board count manually and factor in the join location against your joist layout.
The footing concrete figures include a 10% wastage allowance automatically. Post hole footings always lose some concrete to over-excavated walls and variable hole shapes — this is built in.
Common mistakes: Confusing deck length (board direction) with deck width (joist span); not checking that the board stock length chosen clears the full deck length; and forgetting to add extra for end cuts and any notching around obstacles. The 10% wastage checkbox only applies to boards — concrete for footings always includes 10% waste regardless of this setting.
Building a timber deck in Australia
A timber deck is one of the most rewarding outdoor projects for Australian homes, and one that significantly adds to property value and liveability. Getting the structure right from the ground up ensures a deck that lasts 20–30 years with minimal maintenance.
Permits: In most Australian states, a deck more than 1 m above natural ground level requires a building permit and must be designed by a structural engineer. Decks under 1 m may still need a permit depending on size and local council rules. In bushfire prone areas (BAL ratings), decking material choices are restricted under the NCC — spotted gum, merbau, and other low-combustibility hardwoods are preferred over treated pine in BAL-12.5 and above.
Footings: Post footings must be in stable, undisturbed ground. In reactive soil areas (common across Victoria, South Australia, and parts of Queensland), footings should be deeper — 600–900 mm is common. Never set timber posts directly in concrete; use adjustable post stirrups set in the concrete footing to keep timber above ground contact and avoid decay at the base.
Framing: Use H3 or H4 treated pine for all ground-level or exposed framing. Bearers typically run parallel to the house; joists span between bearers. Use hot-dipped galvanised joist hangers at all bearer-joist connections — never toe-nail only. AS 1684 provides span tables for standard residential framing; always check joists and bearers against the relevant table for your load, spacing, and species.
Decking boards: Lay hardwood boards bark-side up to reduce cupping. Pre-drill nail holes near board ends to prevent splitting. Allow the correct gap from day one — treated pine boards are often supplied wet and will shrink; hardwood boards may be installed tighter. Secret-fix systems (hidden fasteners) give a cleaner look and eliminate tripping hazards from exposed screw heads.
Handrails and balustrades: Required where the deck is more than 1 m above ground. Under NCC Volume 2, balustrade height must be at least 1 m and openings must not allow passage of a 125 mm sphere. This is a safety-critical requirement.
Australian standards and references
- AS 1684.2:2010 — Residential Timber-Framed Construction: Part 2 (non-cyclonic areas) provides span tables for joists, bearers, and posts for standard residential decks.
- NCC Volume 2 (Building Code of Australia) — Part 3.9.2 sets minimum balustrade heights (1.0 m) and balustrade openings (125 mm sphere rule). Part 3.2 covers footing requirements.
- AS 1720.1:2010 — Timber Structures: Design Methods: referenced by engineers for structural timber design beyond standard span tables.
- AS 3959:2018 — Construction of Buildings in Bushfire Prone Areas: relevant to decking material selection in BAL zones.
- State Dividing Fences Acts — some deck construction adjacent to boundary fences also invokes fencing laws regarding boundary setbacks and shared costs.