Decking timbers in Australian construction: species, sizes, fixings, BAL ratings

TL;DR

Australian decking, on Class 1a houses through to Class 2 apartment balconies and Class 3-9 commercial deck surfaces, is dominated by four hardwood species: Spotted Gum (the volume default for new decks), Blackbutt (lighter colour, similar performance), Jarrah (WA-supplied premium), and Merbau (imported, ethical concerns but historically cheap). All four are Class 1 or 2 durability under AS 5604:2022, meaning untreated heartwood lasts decades exposed without preservative treatment. The two specification calls: section size (90 × 19 is the volume default; 130 × 19 and 140 × 19 read wider; 70 × 19 reads narrow and traditional) and fixing method (face-fix screws through pre-drilled timber, or hidden clip systems like Camo, Eclipse, or Klevaclip). On bushfire-prone sites under AS 3959:2018, the species and density determine the BAL rating: Spotted Gum, Blackbutt, Ironbark, Jarrah, and Merbau all meet BAL-29 with correct installation; Ironbark reaches BAL-40 (verified 2026-05-13, Meyer Timber bushfire fact sheet). Composite decking (ModWood, Trex, Ekodeck) is a separate product class outside the scope of this article and sits under its own data sheets.

What it is

Decking timber is the visible top layer of a deck system: the board the user walks on. Underneath sits a structural sub-frame of joists and bearers (typically hardwood or H3-treated softwood) anchored to footings. The decking boards span across joists at right angles and carry the foot traffic load.

The decking timber spec is mostly an appearance, durability, and maintenance call:

Appearance: colour, grain character, knot density, board width
Durability: natural decay resistance per AS 5604, drives maintenance cycle
Maintenance: oil reapplication interval, sanding frequency, lifespan before replacement
Cost: per-square-metre installed cost, sensitive to species rarity and region

Structural capacity is rarely the gating factor because decking spans typical 400 to 500 mm joist centres, well within any seasoned hardwood’s capacity.

Common species

Species	Typical AS 5604 durability (above-ground)	Colour	Density (kg/m3, dry)	Where used
Spotted Gum (Corymbia maculata)	Class 1	Pale to mid brown with darker streaks	~950	The volume default; widely supplied east-coast
Blackbutt (Eucalyptus pilularis)	Class 1	Pale honey to mid brown	~900	Similar to Spotted Gum, slightly lighter colour
Jarrah (Eucalyptus marginata)	Class 1 (or 2 in some grades)	Deep red-brown	~830	WA-supplied; premium colour
Merbau / Kwila (Intsia bijuga)	Class 1	Dark red-brown	~800	Imported (Indonesia, PNG, Malaysia); supply ethics scrutiny; CITES II listed
Grey Ironbark (Eucalyptus paniculata)	Class 1	Grey-brown	~1100	Premium high-density; harder on tools, harder underfoot
Tallowwood (Eucalyptus microcorys)	Class 1	Yellow-olive to red-brown	~990	Niche on east coast
River Red Gum (Eucalyptus camaldulensis)	Class 1	Red to dark red	~900	Less common in retail but available

The durability rating applies to untreated heartwood only. Sapwood of any species is materially less durable (typically Class 4) and should be excluded from the order. Reputable suppliers grade out sapwood, but lower-cost timber merchants may include sapwood-flagged boards in standard packs.

Note on Merbau supply ethics: Merbau is CITES Appendix II listed; some supply chains are linked to illegal logging in Indonesia and PNG. Confirm Chain of Custody (FSC, PEFC, or Indonesian SVLK) certification before ordering. Many Australian builders have moved away from Merbau on supply-ethics grounds in favour of Spotted Gum or Blackbutt.

Standard sections

Section (mm)	Style	Where used
70 × 19 KD	Narrow, traditional	Heritage and federation matching, period restoration
86 × 19 KD	Slightly narrow	Less common, some imperial-conversion stock
90 × 19 KD	Volume default	The standard Australian decking section; widest stock range
130 × 19 KD	Mid-width	Contemporary; premium look; less common stock
140 × 19 KD	Wide	Premium contemporary; long-board feature; greater movement
90 × 22 KD	Standard width, thicker	Higher-traffic or commercial; some species supplied this way
130 × 25 KD	Wide, thick	Premium feature, deck with widely-spaced joists
19 mm thick (any width)	Standard	Default deck board thickness
22 mm thick	Premium / wider span	Joist centres up to 600 mm, heavy-traffic decks
25 mm thick	Premium feature	Architectural decks, wider boards, less deflection underfoot

All section sizes assume kiln-dried (KD) hardwood. Unseasoned (green) hardwood decking is rare in modern construction and not recommended: shrinkage opens gaps and pulls fixings.

Fixing methods

The fixing call drives both appearance and long-term performance.

Face-fix screws (visible fixing)

The traditional method: drive a stainless steel or hot-dip galvanised decking screw down through the board face into the joist. Two screws per board per joist crossing, evenly spaced from the board edges.

Detail	Spec
Screw type	Stainless 316 grade for coastal exposure; hot-dip galvanised for inland; bugle-head or Torx-drive
Screw size	10g × 50 mm typical for 19 mm board into 90 mm joist
Pre-drilling	Required in all dense hardwoods (Class 1 and 2). Pre-drill the board only; joist takes the screw thread directly.
Counter-sinking	The bugle head sets flush with the face; over-driven screws split or crush the board around the head
Plugging	Optional: a 12 mm timber plug cap can hide the screw head for a flush finish

Visible screw heads are part of the deck’s character; many Australian builders consider the visible screw pattern a feature rather than a flaw.

Hidden clip systems

Clip systems hide the fixing by anchoring each board from the side or end-grain. Common Australian products:

Camo Hidden Fastening: drives a screw at an angle through the side of the board into the joist; no visible top fixing
Eclipse decking clip: a stainless steel clip that sits in a groove machined into the board edge
Klevaclip: similar, with a polymer clip and stainless screw
Starborn Pro-Plug: a hybrid that uses a face screw concealed with a colour-matched timber plug

Hidden clip systems require pre-grooved boards (most major suppliers offer them as a stock option) and increase the install time by roughly 30 to 50%. The result is a clean board face with no visible fixings.

Spacing and ventilation

Decking boards are installed with a controlled gap between boards to allow ventilation, drainage, and seasonal movement.

6 mm (typical KD hardwood, dry climate): board separation that allows summer expansion without pressing
8 to 10 mm (humid climate, coastal, or wider boards): more separation for stronger movement and faster drying
Spacer technique: use a wedge or screw shank of the right diameter to set the gap consistently as each board is laid

Boards laid flush without a gap will press against each other in summer, lifting the deck or splitting the boards. The 6 to 10 mm gap is mandatory.

Bushfire Attack Level (BAL) ratings

Under AS 3959:2018, decking on bushfire-prone sites must use timber species meeting the BAL rating of the property. The BAL is determined by the site’s exposure to vegetation type, slope, and distance.

BAL rating	Timber species permitted for decking	Notes
BAL-12.5	Any seasoned hardwood at minimum density per AS 3959; Spotted Gum, Blackbutt, Jarrah, Merbau, Ironbark all qualify	Density-based; check AS 3959 Appendix F
BAL-19	High-density hardwoods only: Spotted Gum, Blackbutt, Ironbark, Jarrah, Merbau	Confirm with supplier and bushfire consultant
BAL-29	Same high-density hardwoods; Spotted Gum, Blackbutt, Ironbark, Jarrah, Merbau all qualify	Stricter installation detail (no concealed cavities)
BAL-40	Ironbark; very limited other species	Confirm with bushfire consultant; engineered solution may apply
BAL-FZ (Flame Zone)	No timber decking permitted in flame-zone deck location	Non-combustible alternative required

The BAL rating for the property is determined by a Bushfire Attack Level assessment from a qualified consultant. The decking species must match; using BAL-12.5-rated timber on a BAL-29 property is a compliance defect.

Maintenance

Hardwood decking needs ongoing maintenance to retain its colour and surface integrity:

Oil-based protective coatings (Cabot’s Decking Oil, Intergrain, Sikkens) applied at install and every 6 to 18 months thereafter, depending on exposure
UV silvering: untreated or unmaintained hardwood greys to silver over 6 to 24 months. Aesthetically acceptable in many architectural contexts.
Sanding and re-oiling: rough boards from weather exposure can be sanded and re-oiled rather than replaced
Slip risk: oiled or wet hardwood is slippery; consider anti-slip oil grades or grooved boards for high-traffic decks

The maintenance schedule is the largest hidden cost in a hardwood deck. Composite alternatives skip the recurring oil cost but typically don’t reach the same character.

Common defects and on-site issues

Sapwood in the pack: visibly lighter, less-dense boards mixed with heartwood. Reject sapwood boards or downgrade the affected sections to non-exposed locations.
Cupping: boards lay concave-up after fixing; cause is moisture differential face-to-face. Use seasoned timber, allow acclimatisation before fix, install heart-side down where appearance allows.
Gaps closing in summer / opening in winter: incorrect initial spacing. Re-install with 8 to 10 mm gap if movement is excessive.
Mould and black staining: humid climate; insufficient ventilation under the deck. Increase sub-floor clearance, improve airflow.
Iron staining around screws: low-spec fasteners react with timber tannins. Use stainless 316 grade for coastal sites; hot-dip galvanised for inland.
Fixing through sapwood edge: screws pull through softer sapwood. Pre-drill heartwood only.
End-checking: cut ends crack from end-grain moisture loss. Seal end grain with end-grain sealer at install.

Pricing (2026 indicative, ex-GST, ex-Sydney metro yard)

Section / species	Per linear metre
90 × 19 KD Spotted Gum	$14-22
90 × 19 KD Blackbutt	$14-22
90 × 19 KD Jarrah (WA-supplied)	$18-28
90 × 19 KD Merbau (imported)	$12-18
130 × 19 KD Spotted Gum	$22-35
140 × 19 KD Spotted Gum	$25-40
90 × 22 KD Ironbark	$24-38
Hidden clip decking premium (pre-grooved + clips)	+$4-8/LM

Installed cost on a typical deck (excluding sub-frame) runs $180 to $260 per square metre for Spotted Gum, $220 to $300 for Jarrah, and $250 to $400 for hidden-clip premium installations.

Standards and references

Standards Australia, AS 5604:2022 Timber, Natural durability ratings. https://store.standards.org.au/product/as-5604-2022 (verified 2026-05-13).
Standards Australia, AS 3959:2018 Construction of buildings in bushfire-prone areas. https://store.standards.org.au/product/as-3959-2018 (verified 2026-05-13).
Standards Australia, AS 2082:2007 Timber, Hardwood, Visually stress-graded for structural purposes. https://store.standards.org.au (verified 2026-05-13).
Meyer Timber, Timber in Bushfire Zones Fact Sheet, October 2022. https://meyertimber.com.au/wp-content/uploads/Factsheet_TimberInBushfireZones_OCT22.pdf (verified 2026-05-13).
Australian Building Codes Board, NCC 2022 ABCB Housing Provisions and bushfire references. https://ncc.abcb.gov.au/editions/ncc-2022/adopted/housing-provisions (verified 2026-05-13).