organic / Timber / Structural Timber

Spotted Gum timber cladding/screening, various grades & profiles (19mm, 21mm, 25mm, 32mm)

Premium Australian eucalyptus hardwood with exceptional structural properties and natural durability. Density 950 kg/m3, Janka 11.0 kN, MOE 19.8 GPa, Durability Class 1 above ground, BAL-29 fire compliance, stress grades to F34. Ideal for structural cladding and screening in all Australian climate zones.

Atlas code

ORG-TBR-ST-001

organictimberstructural-timberspotted-gumhardwoodeucalyptuscladding

Spotted Gum timber cladding/screening, various grades & profiles (19mm, 21mm, 25mm, 32mm)

At-a-glance signals

Overview

Executive summary

Spotted Gum (Corymbia maculata) is Australia's premier structural hardwood for cladding and screening, combining exceptional mechanical strength with natural durability. This eucalyptus species achieves Durability Class 1 above ground (40+ years, AS 5604), Strength Group SD2 with stress grades to F34 when seasoned, and BAL-29 bushfire compliance without additional treatment at 18mm+ thickness. Density of approximately 950 kg/m3 at 12% moisture content delivers a Janka hardness of 11.0 kN, superior fixing retention, and excellent impact resistance. The distinctive wavy interlocked grain produces prized fiddleback figuring, with heartwood colours ranging from light brown to dark chocolate. Available in cladding profiles (shiplap, tongue-and-groove, weatherboard) and screening battens (42x19mm to 90x42mm) from sustainably managed native forests across eastern Australia.

Best when…

Exceptional structural strength — Strength Group SD2, stress grades F17-F34 (seasoned, AS 2082)
Durability Class 1 above ground (40+ years service life per AS 5604)
BAL-29 bushfire compliance without treatment at 18mm+ thickness (AS 3959 Appendix F)
Janka hardness 11.0 kN — superior impact and abrasion resistance
Natural termite resistance (heartwood)
100% Australian grown and processed from sustainably managed native forests
Distinctive wavy grain with prized fiddleback figuring
Excellent weathering characteristics — silvers gracefully when left unfinished
Carbon-sequestering material — stores approximately 850 kg CO2/m3
Long service life 50-80+ years with minimal maintenance

Top advantages

01 Exceptional structural strength — Strength Group SD2, stress grades F17-F34 (seasoned, AS 2082)
02 Durability Class 1 above ground (40+ years service life per AS 5604)
03 BAL-29 bushfire compliance without treatment at 18mm+ thickness (AS 3959 Appendix F)
04 Janka hardness 11.0 kN — superior impact and abrasion resistance
05 Natural termite resistance (heartwood)

Top limitations

01 Premium pricing ($115-300+ AUD/m2 depending on profile and grade)
02 High density (950 kg/m3) mandates pre-drilling for all fixings
03 Natural colour variation from light brown to dark chocolate requires sorting for consistent facades
04 Interlocked wavy grain causes machining difficulties and potential tearing during planing
05 Sapwood susceptible to lyctid borer attack — must be excluded from exposed applications

Technical

Physical ·8

Density: 950 kg/m3 Air-dry density at 12% moisture content. Range 940-990 kg/m3 depending on growth conditions and provenance. C. citriodora (Lemon-scented Gum) and C. henryi are closely related species often marketed together. Sources: WoodSolutions, The Wood Database, QTimber.
Specific gravity: 0.95 Specific gravity at 12% moisture content. Calculated from air-dry density of 950 kg/m3. One of the densest commercially available Australian hardwoods.
Porosity: 8-12 % Diffuse-porous structure typical of eucalyptus species. Moderate porosity allows penetration of oil-based finishes but limits water-based coating absorption. Estimated from timber science literature for eucalyptus hardwoods.
Water absorption: 10-14 % Equilibrium moisture content (EMC) for Australian climate zones. Typically 10-14% in coastal southeastern Australia, 8-12% in drier inland regions. Kiln-dried to 10-12% MC before delivery. Source: AS 1684, timber industry standard.
Hardness: 11.0 kN Janka hardness rating. Substantially harder than Jarrah (8.5 kN), Blackbutt (9.1 kN), and Merbau (8.6 kN). Equivalent to approximately 2,330 lbf (ASTM). Sources: WoodSolutions, QTimber, Eco Timber Group.
UV resistance: Moderate Moderate UV resistance. Unfinished timber will silver/grey over 6-12 months of exposure. Natural extractives provide some UV protection but exterior finishes (oil or coating) recommended for colour retention. Recoat every 12-24 months for oils, 3-5 years for film-forming finishes.
Chemical resistance: Good Good chemical resistance. Natural heartwood extractives provide resistance to mild acids and alkalis. Avoid prolonged contact with strong alkalis (cement, lime) which can cause staining. Tannin leaching may stain adjacent concrete or render — use barrier membrane or allow initial weathering before adjacent finishing.
pH tolerance: 4-8 pH Natural timber pH tolerance range. Heartwood pH approximately 4.5-5.5 (mildly acidic). Compatible with most construction materials. Avoid direct contact with ferrous metals in wet conditions — use stainless steel or hot-dip galvanised fixings.

Mechanical ·5

Tensile strength: 100-130 MPa Tensile strength parallel to grain. Estimated from MOR and typical hardwood tensile/flexural ratio. Not commonly tested independently — structural design uses characteristic bending strength per AS 1720.1.
Compressive strength: 55-70 MPa Compressive strength parallel to grain for seasoned timber. Strength Group SD2 provides characteristic compressive strength of approximately 55-70 MPa per AS 1720.1. Source: WoodSolutions structural data.
Flexural strength: 142 MPa Modulus of Rupture (MOR). Characteristic bending strength for structural grading per AS 1720.1. The Wood Database reports 141.8 MPa (20,550 psi). Among highest MOR values for Australian commercial hardwoods. Sources: The Wood Database, WoodSolutions, QTimber.
Impact resistance: 80-110 J/m Excellent impact resistance — the primary Australian species for tool handles subject to high-impact forces (axe handles, hammer handles). Interlocked grain provides resistance to splitting under impact. Joint Group JD1 (seasoned) per AS 1720.1. Source: QTimber, WoodSolutions.
Creep resistance: Excellent Excellent creep resistance. High density and Strength Group SD2 classification provide superior long-term load resistance. Suitable for structural applications where sustained loading occurs (pergola beams, structural screening supports). Source: AS 1720.1.

Sustainability & Health

Embodied carbon & energy ·7

Embodied carbon: 150-350 (gross); net negative including sequestration kg CO2-eq/m3 Cradle-to-gate embodied carbon for sawn Australian hardwood. Includes harvesting, transport, kiln drying, and milling. Timber stores approximately 850 kg CO2/m3 through carbon sequestration (biogenic carbon). Net carbon balance is typically negative (carbon-storing). No individual EPD available for Spotted Gum — values based on Australian hardwood industry averages. Source: Timber Development UK data, IStructE guidance.
Carbon footprint: -500 to -700 (net, including sequestration) kg CO2-eq/m3 Including biogenic carbon sequestration. Spotted Gum at 950 kg/m3 stores approximately 850 kg CO2/m3. Net carbon balance: approximately -500 to -700 kg CO2-eq/m3 (carbon negative). This carbon remains stored for the service life of the timber (50-80+ years). Source: Wood science carbon accounting methodology.
Embodied energy: 500-1500 MJ/m3 Primary energy for harvesting, kiln drying (most energy-intensive step), and milling. Kiln drying accounts for approximately 60-70% of total embodied energy. Air-drying alternatives reduce embodied energy significantly but increase lead times. Source: Timber industry LCA data.
Water footprint: 5-20 L/kg Natural forest growth — negligible irrigation requirements. Processing water for kiln operations is minimal and typically recycled. Overall water footprint very low compared to manufactured building materials. Source: Timber industry environmental data.
Recycled content: 0 (new); up to 100 (reclaimed) % New-sawn timber from sustainably managed native forests contains 0% recycled content. Reclaimed/salvaged Spotted Gum is available from demolition sources at premium pricing. Reclaimed timber can be 100% recycled content.
Renewable content: 100 % 100% renewable natural material from sustainably managed native forests. Harvest rates regulated by state forestry agencies (Forestry Corporation NSW, HQPlantations QLD) to ensure sustainable yield.
Circular score: 9 /10 Excellent circular economy credentials: 100% renewable, 100% biodegradable at end of life, reusable/salvageable, recyclable as mulch/biomass. Long service life (50-80+ years) maximises material value retention. Carbon-storing throughout service life.

Compliance & Fire

Fire performance ·6

Combustibility class: Combustible — Group 3 (AS 5637.1), BAL-29 compliant (AS 3959) Combustible material — Group 3 per AS 5637.1. However, achieves BAL-29 bushfire compliance per AS 3959:2018 Appendix F without additional treatment when boards are 18mm+ thick and density exceeds 750 kg/m3 at 12% MC. Spotted Gum at 950 kg/m3 comfortably exceeds this threshold. Sources: AS 3959:2018, Mortlock Timber, Coach House Timbers.
Fire resistance level: 30-60 minutes Fire resistance depends on section size and char rate. Australian hardwood char rate approximately 0.5-0.6 mm/min (lower than softwoods at 0.65 mm/min). 32mm board: approximately 50-55 minutes to char through. Actual FRL depends on complete wall assembly. Source: AS 1720.4.
Ignition temp: 300-350 degC Piloted ignition temperature for dense eucalyptus hardwood. High density provides higher thermal inertia, requiring more energy to reach ignition. Source: Fire safety literature for Australian hardwoods.
Flame spread index: 100-150 FSI Flame Spread Index for dense Australian hardwood. High density provides relatively slow flame spread. Group 3 material per AS 5637.1. Source: Timber construction fire safety literature.
Smoke dev. index: 150-250 SDI Smoke Development Index for dense eucalyptus hardwood. Moderate smoke production during combustion. High density results in slower, more complete combustion with proportionally less smoke than lighter species.
Heat release rate: 150-250 kW/m2 Peak heat release rate for dense hardwood timber. High density (950 kg/m3) provides slower combustion rate than lighter species. Based on cone calorimeter testing data for dense eucalyptus hardwoods.

Cost & Lifecycle

Capex & lead time ·6

Material cost (range): 95-300+ AUD/m2 Supply only, ex-GST. 83x14mm shiplap: $95-120/m2. 125x19mm cladding: $130-180/m2. 42x19mm screening battens: $100-150/m2. Premium select grade or large profiles: $180-300+/m2. Mortlock Timber quotes $240-300+/m2 for their standard cladding range. Sources: JBM Timber, Adelaide Timber Cladding, Mortlock Timber, SE Timber Supplies.
Material cost (per unit): 2500-8000 AUD/m3 Approximate volume pricing. Premium structural hardwood — varies significantly by grade, profile, and quantity. Rough-sawn: $2,500-4,000/m3. Dressed/profiled: $4,000-8,000+/m3. Source: Industry pricing.
Lead time: 5-28 days Standard stock profiles (83x14mm, 125x19mm): 5-10 days. Custom profiles or large orders: 14-28 days. Premium select grades may require longer lead times. Availability generally good from multiple national suppliers. Source: Industry suppliers.
Lifecycle cost: 180-400 AUD/m2 50-year lifecycle cost including installation, maintenance, and replacement. Excellent lifecycle value due to Durability Class 1 and 50-80+ year service life. Assumes oil finish recoat every 2 years at $8-12/m2, or unfinished (silver) with zero coating maintenance.
Annual maintenance: 0.50-8 AUD/m2/year Annualised maintenance cost. Unfinished (natural silver): $0.50-1.00/m2/year (inspection only). Oil finish: $4-8/m2/year (recoat every 1-2 years). Film-forming finish: $2-4/m2/year (recoat every 3-5 years). Source: Industry estimates.
Market availability: Good Good to excellent availability nationally. Harvested from sustainably managed native forests in NSW and Queensland. Multiple suppliers across all major cities. Stock profiles generally available ex-warehouse. Custom profiles and large quantities may require mill scheduling.

Service life & durability ·3

Expected lifespan: 50-80+ years Durability Class 1 above ground per AS 5604 — expected service life exceeding 40 years. In practice, well-detailed and maintained Spotted Gum cladding achieves 50-80+ years. Heritage examples in bridges and wharves exceed 100 years. Source: AS 5604, WoodSolutions.
Maintenance interval: 1-5 years Maintenance depends on finish type. Penetrating oils: recoat every 1-2 years. Film-forming coatings: 3-5 years. Unfinished (natural silver weathering): inspect annually for structural fixings, no coating maintenance required. Source: Timber industry best practice.
Warranty period: 15-25 years Typical supplier warranty for structural hardwood cladding. Mortlock Timber offers structural warranties. Warranty coverage varies by supplier, application, and maintenance compliance. Some suppliers warrant 15-25 years for above-ground external applications with proper installation and maintenance.

Layer D

Where it's used

External wall cladding (horizontal shiplap/weatherboard)

Privacy screening (vertical batten systems)

Pergola and outdoor structure cladding

Bushfire-prone area construction (BAL-12.5 to BAL-29)

Commercial and institutional facades

Interior feature walls and ceiling linings

Fencing and boundary screening

Coastal construction applications

Where it's used

Compare with