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.