Structural hardwood in Australian construction: species, F-grades, and durability classes

TL;DR

Structural hardwood is solid sawn timber from Australian eucalypt and equivalent species, visually stress-graded under AS 2082:2007 and rated for natural durability under AS 5604:2022. The two specification dimensions are F-grade (structural capacity: F14/F17 for unseasoned, F22/F27 for kiln-dried, F34 for premium kiln-dried specialty) and durability class (1 to 4, with Class 1 most durable: Ironbark, Spotted Gum, Grey Gum). Used across Class 1a houses, Class 2 low-rise apartments, and Class 3-9 commercial work wherever the element is exposed to weather or in-ground service. Applications where hardwood is the right call: in-ground or splash-zone bearers and posts (Class 1 or 2 durability), exposed feature beams in heritage and natural-finish architecture, decking and pergola structures, and any element exposed to weather where preservative-treated softwood is unacceptable. Where softwood LVL or glulam covers the load, hardwood is rarely the cost-effective choice; where weather, in-ground exposure, or appearance drive the spec, hardwood is the default (verified 2026-05-13, QTimber durability ratings).

What it is

Structural hardwood is solid sawn timber from broadleaf evergreen tree species, in the Australian context primarily eucalypts. The “hardwood” classification is botanical (angiosperm vs gymnosperm) rather than a strict measure of mechanical hardness, but in practice Australian hardwood timbers are materially denser, harder, and more durable than the softwood pines that dominate light-framed construction.

The structural grade is set by visual stress grading under AS 2082:2007: a trained grader inspects every length for slope of grain, knot size and location, splits, checks, machining defects, and dimensional variation, and assigns the timber to an F-grade and species combination. The grade stamp is the legal basis for structural use; ungraded hardwood cannot be used as structural timber.

Hardwood for structural use is supplied in three moisture conditions:

Condition	Description	Typical F-grade range
Unseasoned (green)	Above 25% moisture content; supplied straight from the saw mill or air-dried only	F14, F17
Air-dried	12 to 20% moisture content from outdoor stack drying	F17, F22
Kiln-dried (KD)	10 to 15% moisture content, controlled-kiln process	F22, F27, F34

Kiln-dried hardwood is more dimensionally stable, takes a finish reliably, and reaches higher F-grades because the kiln process removes defects (collapse, checking) that would downgrade air-dried timber. Most premium hardwood for exposed feature work is kiln-dried.

Common Australian species

Species	Typical F-grade range	AS 5604 durability class (in-ground / above-ground)	Where used
Spotted Gum (Corymbia maculata)	F22 to F27 KD	2 / 1	Decking, posts, exposed beams, flooring
Blackbutt (Eucalyptus pilularis)	F22 KD	2 / 1	Decking, posts, joists, flooring
Grey Ironbark (Eucalyptus paniculata)	F27 to F34 KD	1 / 1	Posts, bearers, in-ground structural, marine
Red Ironbark (Eucalyptus sideroxylon)	F27 KD	1 / 1	Posts, bearers, sleepers, in-ground
Tasmanian Oak (Eucalyptus regnans/obliqua/delegatensis)	F17 to F22 KD	3 / 3	Internal structural, flooring, joinery (not exposed)
Victorian Ash (Eucalyptus regnans/delegatensis from Vic mainland)	F17 to F22 KD	3 / 3	Internal structural, flooring, joinery
Jarrah (Eucalyptus marginata)	F17 to F22 KD	1-2 / 1	Posts, bearers, decking; WA-supplied premium
Merbau (Intsia bijuga, imported)	F22 to F27 KD	1 / 1	Decking, joinery; imported, scrutinise origin and CITES
Stringybark species (Eucalyptus obliqua, etc.)	F17 to F22 KD	3 / 3	General framing where green hardwood is acceptable

The durability rating is for untreated heartwood. Sapwood of any species has materially lower durability (typically Class 4) and is unsuitable for exposed use unless preservative-treated. Specifying “Spotted Gum” without specifying heartwood-only invites sapwood timber that will rot in 2 to 5 years exposed.

F-grade vs species: how they combine

A timber’s F-grade is set by the visual grading rules, which are species-specific. The same visual rules applied to Spotted Gum and to Stringybark produce different F-grades because the base mechanical properties differ. AS 1684 span tables and AS 1720.1 design methods both use the F-grade (not the species name) for capacity calculations, so a span table reading for “F22” applies whether the supplied timber is Spotted Gum, Blackbutt, or Stringybark of equivalent grade.

The species choice matters where durability, density, hardness (for flooring), or appearance drives the specification. The F-grade matters where the structural span and load drive it. Most hardwood orders specify both: “F22 Spotted Gum, KD, heartwood, structural decking” gives the supplier enough to deliver a usable product.

Where structural hardwood is the right call

In-ground posts and bearers: Class 1 durability hardwood (Ironbark, Grey Gum, Jarrah heartwood) outlives any preservative-treated softwood in direct soil contact. The expected service life for Class 1 hardwood in-ground exceeds 25 years; for H4-treated softwood, 15 to 25 years.

Splash zone bearers under verandahs and decks: where rain and splash bring the timber up to fibre saturation periodically, Class 1 or 2 hardwood is the durable choice. Spotted Gum, Blackbutt, or Jarrah at F22 or F27 covers nearly all cases.

Exposed feature beams: any beam visible in the finished space where natural timber finish or stained timber is the architect’s intent. Spotted Gum and Blackbutt are the volume choices for an exposed beam with a natural finish.

Decking: 90 × 19 mm or 90 × 22 mm Spotted Gum, Blackbutt, or Jarrah is the standard decking spec. KD hardwood with 9 to 10 mm spacing for ventilation.

Heritage matching and restoration: replacing hardwood in pre-1970 housing means matching species (or close substitute) to maintain the original character. Tasmanian Oak and Vic Ash are common drop-in matches for interior hardwood; exterior matches need species-specific sourcing.

Where engineered timber falls short: any element exposed to weather without effective edge sealing, where the glue-bonded chemistry of LVL or glulam becomes the failure point. Solid hardwood has no glue lines to fail.

Where hardwood is wrong

Hidden structural beams where load is the only concern: LVL is typically half the cost at the same load capacity. Hardwood premium isn’t paying for anything that matters when the beam is boxed in.
Long-span beams beyond solid timber’s natural reach: a hardwood beam over 6 m is rare, expensive, and weighty. Use LVL or glulam.
Where dimensional stability matters and cost is constrained: kiln-dried LVL is more dimensionally stable than any solid timber after fixing. For internal lintels and joists in finished interiors, LVL is more predictable.
Curved or tapered sections: hardwood is sawn from a straight log. Glulam is the right product for curved or tapered geometry.
Routine ridge beams under standard roof loads: F22 LVL is cheaper than hardwood at equivalent capacity. Save hardwood for visible or exposed-to-weather use.

Sizing: AS 1684 vs engineer’s design

AS 1684 span tables include hardwood F-grades and cover routine residential bearers, joists, lintels, and posts in non-cyclonic areas. Read the table for the F-grade, span, and load condition. The F-grade in the table must match the F-grade on the timber stamp; mismatched timber is non-compliant.

Structural engineer’s design under AS 1720.1 is required for the same situations as LVL: outside-table spans, multi-load conditions, curved geometry (rare in solid hardwood), or unusual support conditions.

The stress grade is fixed at the saw mill, not on site. A timber stamped F17 cannot be revised to F22 by site inspection; if a higher grade is required, the supplied timber must be replaced.

Connections

Hardwood connects to surrounding framing via bolts, lag screws, structural screws, and joist hangers:

Bolts through hardwood: pre-drill the hole at bolt diameter (no oversize, no undersize). Hardwood splits readily without pre-drilling; the bolt cannot be driven through a too-tight hole.
Coach screws (lag screws): pre-drill the shank and the thread separately per AS 1720.1 (pilot hole = 70% of root diameter for the thread, full diameter for the shank). On Class 1 hardwood the pilot is critical: under-driven coach screws snap before reaching design embedment.
Joist hangers: galvanised steel hangers (Pryda Triple Grip, Multinail) with nail counts to match the load. Hot-dip galvanising over standard zinc plating is recommended for any hardwood in exterior or splash exposure.
Posts to footings: stainless steel or hot-dip galvanised post bases (HoldDown HD, Pryda Post Base) lift the timber clear of splash and water-pooling zones. Direct in-ground hardwood requires Class 1 species and a minimum 600 mm embedment (deeper for tall posts).

Common defects and on-site issues

Sapwood specified or supplied as “Spotted Gum”: the supplier delivers what’s in the stack. Specify heartwood-only on any exposed application and inspect the timber on arrival; sapwood is visibly lighter and lower-density.
Unseasoned hardwood used internally: green hardwood at 25% MC moves materially as it dries to internal equilibrium (10 to 12%). Joists, lintels, and posts shrink, twist, and check. Use KD timber internally; green hardwood is for external structural where movement is less critical.
Splits and checks at cut ends: hardwood checks at exposed end-grain over time. End-grain sealer applied at the cut is good practice; for visible beams, the architect should accept some end-grain character or the timber must be capped.
Wrong durability class for application: a Class 3 species (Tas Oak, Vic Ash) used as an external post rots in 5 to 10 years. Spec must match the exposure.
Galvanic corrosion with stainless steel fasteners: dissimilar metals (mild steel coach screw, then later a stainless steel bracket) into wet hardwood drive accelerated corrosion. Match fastener material to the application.
Heavyweight hardwood lifted unsafely: solid hardwood is 800 to 1100 kg/m3 density. A 200 × 65 × 4000 LVL beam weighs about 22 kg; the same hardwood beam in Spotted Gum is about 65 kg. Manual handling is a HRCW and lift planning issue.

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

Section / product	Species	Per linear metre
90 × 19 KD decking	Spotted Gum	$14-22
90 × 19 KD decking	Blackbutt	$14-22
90 × 22 KD decking	Jarrah (WA-supplied)	$18-30
150 × 38 F22 KD post	Spotted Gum	$25-40
200 × 65 F22 KD bearer	Spotted Gum	$80-130
100 × 100 KD post	Ironbark, Class 1	$40-65
200 × 100 F27 KD beam	Ironbark	$130-200
Custom-sawn or unusual section	Any species	Premium 40-100%, lead time 4-12 weeks

Regional pricing varies materially: WA Jarrah is cheaper at source in Perth; QLD Spotted Gum is cheaper in QLD; Tas Oak is cheaper in Tasmania and Victoria. Volume-builder pricing through Big River or major hardwood yards is typically 10 to 25% below merchant pricing for stock sections.

Standards and references

Standards Australia, AS 2082:2007 Timber, Hardwood, Visually stress-graded for structural purposes. https://store.standards.org.au (verified 2026-05-13).
Standards Australia, AS 1720.1:2010 Timber structures Part 1: Design methods. https://store.standards.org.au/product/as-1720-1-2010 (verified 2026-05-13).
Standards Australia, AS 5604:2022 Timber, Natural durability ratings. https://store.standards.org.au/product/as-5604-2022 (verified 2026-05-13).
Queensland Department of Agriculture and Fisheries (QTimber), Natural durability ratings. https://qtimber.daf.qld.gov.au/guides/natural-durability-ratings (verified 2026-05-13).
Australian Building Codes Board, NCC 2022 ABCB Housing Provisions (timber structural references). https://ncc.abcb.gov.au/editions/ncc-2022/adopted/housing-provisions (verified 2026-05-13).