Tin Shed House by Ironbark Architecture featuring spotted gum battens
Timber is a wonderfully familiar material that delights our senses, whether touch, sight, smell or sound. It is one of the most fundamental materials with which we make things and is inseparably connected to the history of mankind.
I have prepared this post to assist those seeking to find an Australian hardwood that is right for them and right for the application. It is not an exhaustive list but presents a brief overview of nine Australian hardwood species.
BLACKBUTT
The common name “Blackbutt” comes from the regular appearance of a blackened base on the trunk of the tree, caused from fires. Blackbutt primarily grows in the coastal regions of southern NSW all the way up to south east QLD. It has a quick growth rate and is easily regenerated, making it a popular species to grow in plantations and a readily available timber on the east coast of Australia. The heartwood colour ranges from a yellowish brown to light brown, with a fairly straight grain and even texture with gum veins sometimes present. Blackbutt has a very good fire rating and is one of the 7 timber species found suitable for bushfire prone areas. A hard, strong and durable timber, Blackbutt is increasingly popular choice for flooring and decking in Australia, with Parliament House in Canberra showcasing the spectacular beauty of Blackbutt by using it for its flooring.
Relic House by Ironbark Architecture featuring prefinished engineered blackbutt flooring
PROS
Readily Available in Decking
Highly durable and hard
1 of 7 hardwoods recommended for bushfire prone areas
PROPERTIES
Name
Durability
Density
BAL Rating
Termite Resistant
Lyctid Borer Susceptible
Tannin Leach
Origin
CONS
Tendency to surface checking and splitting
High tannin content – may leech when wet
Mostly supplied in random lengths
Eucalyptus Pilularis
Class 1
880 kg/m³
12.5 / 19 / 29
Yes
No
Moderate
NSW / QLD
IRONBARK
The common name “Ironbark” comes from the trees tendency to not shed its bark annually like many other eucalyptus, resulting in an accumulation of dead bark. This layer of bark protects the living tissue inside the tree from fires, and with a silvery-grey colour looks quite similar to iron metal and hence the name. Ironbark is usually seen growing, in both native forests and plantations, in the western area of NSW up to southern QLD, with some trees growing in northern VIC. Ironbark has a very high natural resistance to rot due to chemistry of the tree which helps fight off fungus. The heartwood has a deep red colour which is a great contrast to the pale yellow sapwood. Both heartwood and sapwood are often seen together in decking boards. Ironbark timber has an interlocked grain with a fine texture. Ironbarks popularity is ever increasing with more and more homes using the timber for decks, landscaping and cladding due to its high durability and fire resistance.
Ailbe by Ironbark Architecture featuring 86x19mm Grey Ironbark hardwood decking
PROS
High rot and termite resistance
Very high durability, hardness and density
Widely available
Used for over 200 years in heavy construction
Little tannin leach
1 of 7 hardwoods recommended for bushfire prone areas
PROPERTIES
Name
Durability
Density
BAL Rating
Termite Resistant
Lyctid Borer Susceptible
Tannin Leach
Origin
CONS
Hard to work, can blunt tools quickly
Boards need to be pre-drilled
Expensive
Short oil life of decking due to density
Mostly supplied in random lengths
Red ironbark is Lyctid borer susceptible
Eucalyptus Sideroxylon (red)
Eucalyptus Paniculata (grey)
Class 1
1090 kg/m³
12.5 / 19 / 29 (red)
12.5 / 19 (grey)
Yes
Yes (red)
No (grey)
Little
VIC / NSW / QLD
JARRAH
The common name “jarrah” is also the aboriginal word for both the tree and timber. Jarrah timber reflects the tones of the southwest WA landscape from where it comes from, with very deep red colours seen in the heartwood. Jarrah has an attractive grain with some incidence of wavy or interlocking grain occurring, and a moderately coarse texture. Jarrah timber is quite similar to red ironbark in which it is a very dense and hard timber, both sharing the deep red colours. Jarrah is also naturally weather, rot, termite and marine borer resistant making it a highly durable timber for outdoor purposes, however the sapwood of the timber is Lyctid borer susceptible, so the sapwood present in decking may sometimes be treated. Jarrah availability is somewhat limited due to the slow growth of the trees and the fact the timber can only be sourced from old growth forest and native forest regrowth in WA. It’s for this reason jarrah decking should be sourced from recycled means when obtainable.
Jarrah Decking Example (dry)
Jarrah Decking Example (wet)
PROS
Highly durable and dense
Rot, termite and weather resistant
Retains natural colour for longer due to close grain
Little tannin leach
PROPERTIES
Name
Durability
Density
BAL Rating
Termite Resistant
Lyctid Borer Susceptible
Tannin Leach
Origin
CONS
Limited availability
Sourced from old growth forests
Expensive
Requires pre-drilling
Mostly supplied in random lengths
Lyctid borer susceptible
Eucalyptus Marginata
Class 2
820 kg/m³
12.5 / 19
Yes
Yes
Little
WA
KARRI
The “Karri” tree is Australia’s and one the world’s tallest hardwoods growing up to 70m in height, allowing for longer lengths of timber being able to be milled, knot-free. Karri heartwood is a beautiful hue of reddish-brown, being slightly lighter in colour then jarrah. Also like Jarrah, the Karri grows in the wetter regions of southwest WA and also cultivated internationally. Availability of Karri is slightly greater then Jarrah, but still limited due to the most of the timber now in conservation reserves and regrowth forest. Karri timber has an interlocked, sometimes straight grain, with a course texture. Karri is moderately durable, with a reputation of being termite-prone, however not nearly as prone as untreated pine.
Karri Decking Example
PROS
Longer lengths of timber available, knot-free
Cheaper than other hardwoods, like jarrah and ironbark
High strength
PROPERTIES
Name
Durability
Density
BAL Rating
Termite Resistant
Lyctid Borer Susceptible
Tannin Leach
Origin
CONS
Termite-prone
Somewhat limited in availability
Pre-drilling required
Poor reputation to holding paint and impregnation of preservatives
Mostly supplied in random lengths
Not as durable as other Australian hardwoods
Eucalyptus Diversicolour
Class 2
900 kg/m³
12.5 / 19
No
No
Little
WA
REDGUM
“Forest Redgum” is a very distinctive timber in Australia, with almost everyone having come in contact with or heard of it. It has had an important part in Australia’s history being used in a range of applications from the very first settlers up until today. The timber, as the name suggests, has a striking red colour which ranges from a light to deep-dark red. Redgum has a fine and even texture with the grain being interlocked and often producing a ripple or fiddleback figure, which makes for a very attractive feature in decking boards. Redgum is a very versatile, dense and durable timber that grows in a wide range of areas from south-east VIC all the way east up until southern Papua New Guinea. Even though most of the timber has been locked away in protected forests, Redgum is still widely available on the East coast of Australia due to the broad area Redgum is found growing in. Some companies sell a mix of both Redgum and Red Ironbark (very similar timbers) often marketed as forest reds.
Redgum Decking Example
PROS
Highly durable and hard
Termite resistant
Readily available
PROPERTIES
Name
Durability
Density
BAL Rating
Termite Resistant
Lyctid Borer Susceptible
Tannin Leach
Origin
CONS
Some gum veins present in timber
Mostly supplied in random lengths
Eucalyptus Tereticornis
Class 1
960 kg/m³
12.5 / 19
Yes
No
Little
VIC / NSW / QLD / PNG
SILVERTOP ASH
“Silvertop ash”, or sometimes named “coast ash” due to its occurrence to grow along the coast of the south-eastern areas of Australia, is a moderately durable and relatively light hardwood compared to other eucalyptus like spotted gum and ironbark. The timber has a medium texture with a straight grain, also showing noticeable growth rings. The timber also often comes with natural features including gum veins and ambrosia, giving the timber a unique look. Silvertop ash is readily available through the eastern states of Australia, with its main source being from native forests, with some trees being grown internationally, in New Zealand plantations. Silvertop Ash is very popular amongst architects where being left to “silver off” in applications such as cladding and retains a smooth even finish with little to no tannin leach on surfaces below.
Alcheringa by Ironbark Architecture featuring Silvertop Ash shiplap cladding
PROS
Less expensive hardwood decking
1 of 7 hardwoods recommended for bushfire prone areas
No tannin leach
Widely available
Easy to work, excellent for nailing and screwing
Accepts coatings and preservatives well
Can be supplied in set lengths
PROPERTIES
Name
Durability
Density
BAL Rating
Termite Resistant
Lyctid Borer Susceptible
Tannin Leach
Origin
CONS
Termite-prone
Relatively prone to surface checking and splitting
Not as durable as other Australian hardwoods
Light to medium feature present in timber
Eucalyptus Sieberi
Class 2
850 kg/m³
12.5 / 19 / 29
No
No
Little
TAS / VIC / NSW
SPOTTED GUM
The common name “spotted gum” is used for four different highly durable and dense Corymbias (spotted gum was previously classified as a eucalypt, until it was changed in the mid-1990’s) that grow along the east coast of Australia, but more commonly refers to the species, Corymbia Maculata. They get their common name from the pale, smooth bark in the trees trunks that have shed in small patches leaving the tree with a “spotted” appearance. The different species commonly referred to as spotted gum only differ in appearance, not in durability or other properties. The heartwood of spotted gum ranges from a light brown to a dark reddish-brown. The sapwood of the timber is Lyctid borer susceptible, so is commonly treated to help keep the timber free of the bugs. The timber often comes with an attractive wavy grain with coarse and uneven texture; it also is noted as having a “greasy” feel when you run your hand over machined products. Spotted gum is grown throughout the eastern part of Australia and in some parts of WA and SA in both native forests and plantations. Spotted gum is currently QLD’s largest harvested native hardwood by volume, meaning that spotted gum is widely available throughout Australia, with quantities also being exported internationally.
Bounty House by Ironbark Architecture featuring Spotted Gum cladding
Bounty House by Ironbark Architecture featuring Spotted Gum decking
PROS
Readily available throughout Australia
1 of 7 hardwoods recommended for bushfire prone areas
Highly durable, dense, and hard
Widely available
Little tannin leach
Sustainably sourced
PROPERTIES
Name
Durability
Density
BAL Rating
Termite Resistant
Lyctid Borer Susceptible
Tannin Leach
Origin
CONS
Expensive
Requires pre-drilling
Mostly supplied in random lengths
Lyctid borer susceptible
Corymbia Maculata
Class 1
990 kg/m³
12.5 / 19
Yes
Yes
Moderate
VIC / NSW / QLD
TALLOWWOOD
The timber commonly known as “Tallowwood” owes its name to the greasy feel of the wood once it has been cut or machined, due to its naturally oily composition. The decking should be left to weather slightly (2-3 weeks) before giving it a sanding, slight wash with a deck cleaning product and oil. It is a highly durable and dense timber, which is highly resistant to decay due to its ability to withstand damp and wet conditions relatively well; however the softwood of the timber is Lyctid borers susceptible, so any sapwood present in the decking boards maybe have to be treated to help keep them free of the bugs. The heartwood colour ranges from a pale to dark yellowish brown, with a moderately coarse texture and interlocked grain. Availability of tallowwood is fairly common, with most of the resource being sourced from native forests on coastal regions between southern Newcastle, NSW and Maryborough, QLD. Because of it’s oily nature, it is a good choice for decking around pool areas.
Tin Shed House by Ironbark Architecture featuring Tallowwood decking
PROS
Termite Resistant
Very high durability and density
Good resistance to surface checking
High resistance to decay
Performs well in wet environments – good for pool areas
PROPERTIES
Name
Durability
Density
BAL Rating
Termite Resistant
Lyctid Borer Susceptible
Tannin Leach
Origin
CONS
Mostly available in random lengths
High tannin / oil content
Pre-drilling required
Lyctid borer susceptible
Eucalyptus Microcorys
Class 1
990 kg/m³
12.5 / 19
Yes
Yes
Little to moderate
NSW / QLD
YELLOW STRINGYBARK
“Yellow Stringybark” is the common name for three different eucalypt species, which all have a thick, fibrous bark that were once used as early European settlers for roof and wall thatching. The heartwood of the timber comes up a light yellowish-brown colour, very similar to that of tallowwood and Silvertop Ash. It has a medium to fine texture, often seen with an interlocked grain, also showing some features as gum veins and ambrosia. Yellow Stringybark mainly grows in native forests and some plantations on the coastal and tableland areas of southern NSW and eastern VIC, making it readily available in south-eastern states.
Yellow Stringybark Decking Example
Yellow Stringybark Decking Example
PROS
Readily available in south-east areas
Being grown in sustainable plantations
Termite resistant
Can be supplied in engineered lengths
Excellent result from oil based finishes
PROPERTIES
Name
Durability
Density
BAL Rating
Termite Resistant
Lyctid Borer Susceptible
Tannin Leach
Origin
CONS
Gum veins sometimes present
Eucalyptus Muelleriana
Class 2
880 kg/m³
12.5 / 19
Yes
No
Little
VIC / NSW
FINISHING TIMBER - SEALING AND OILING
A discussion about timber would be incomplete without saying something about timber finishing, which is essential not only for aesthetic appeal but also for protecting the wood from environmental factors. Different finishing methods are employed depending on whether the timber is intended for indoor or outdoor use. The different products available are extensive, but let's look at three popular products that we use in our projects.
Water-Based Urethane
Water-based urethane finishes offer a durable and versatile solution for sealing indoor timber surfaces. These finishes provide excellent protection against moisture, stains, and scratches while also enhancing the natural beauty of the wood. Architects often opt for water-based urethane finishes due to their low odor, fast drying time, and easy cleanup. Additionally, they are available in various sheen levels, allowing for customization to suit different design preferences. Unlike oil based urethane finishes, water based products will not turn yellow over time from exposure to UV. This is why the cypress floors you’ll see in 1950s to 1980s homes will appear orange, when in reality cypress has a beautiful tan brown colour.
Hardwax Oil
Hardwax oil is a popular choice among architects for finishing indoor timber surfaces. This natural oil-based product penetrates deeply into the wood, providing long-lasting protection and enhancing the timber's natural characteristics. Hardwax oil creates a durable yet breathable finish that is resistant to water, heat, and household chemicals. One of the key advantages of hardwax oil is its ease of application and maintenance, making it ideal for both residential and commercial projects. Hardwax finishes are not quite as durable as a urethane finish, but have the advantage that they can be spot repaired without the need to sand back and refinish a large area as a urethane finish would need.
Cutek Extreme CD50
When it comes to finishing outdoor timber, Cutek is the only product that I specify. A proven wood protection system, Cutek penetrates deep into the timber, forming a barrier against moisture, UV radiation, and fungal decay. It is designed to fade naturally over time, without flaking or peeling. To maintain some colour in the timber (although it will still lighten) there are Colourtone’s that can be added prior to application.
This oil-based solution allows the wood to breathe while providing long-term protection against the harsh Australian climate. Applying Cutek has a “compounding” effect, so requires two to three coats on installation and a follow up coat within the next 6-12 months. After these initial coats, the timber can be allowed to “silver-off” and the product will continue to protect the timber even though the surface looses it’s original colour. Followingf that, weathered surfaces can typically be left up to 4-5 years without recoating, potentially longer in protected settings. The maintenance of timber with Cutek Extreme CD50 does not require stripping or sanding the coat. Simply clean the surface and reapply the oil every two to seven years. This applies even if the weathering off approach is desired for a project. The maintenance intervals and rate of fading will vary depending on the wood species, the specific situation, design, and the degree of exposure to the elements.
There is one significant draw back to Cutek that is a virtue of the nature of the product. During cold, wet and cloudy weather, the product will have a significantly difficult time drying and soaking into the timber. Application of Cutek is most effective during warm, sunny weather, and coats should be applied with multiple thinner coats, rather than less but thicker coats.
Cutek Extreme CD50. Image courtesy Mortlock Timber.