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Wood species & their properties

Like to know what wood to use building a pergola or framing a barn? How about the difference between tung oil and polyurethane as a floor finish? How to revive decking or deal with merbau stains? Or how to meet the building code or bushfire standards? Or ask about the environmental advantages of wood or forest certification?

Q. Please can you advise which timbers are the most durable as decking around a swimming pool.

If the decking is likely to be splashed regularly we suggest a hardwood such as tallowwood or ironbark. These are durable hardwoods which have a high level of rot resistance. If tannin staining is going to be a problem on paving or on the pool surround, spotted gum would be a safe choice. All three are rated Class 1 Durability outdoors above ground.

Q. We are a primary zinc smelter. At the moment we use celery top pine for our copper busbar supports (corbels) that run alongside our cells. These cells are where the electrowinning of zinc from solution takes place. Currently we are finding it difficult and expensive to source celery top pine and are after a substitute material. We are looking at a range of materials, but timber has a range of benefits for our application. Is there an alternative timber to celery top that would have similar durability and mechanical properties? The copper busbars generally run at approximately 80degC and radiate heat onto the corbel. The corbel is in an acidic environment where it can be splashed or have drips of acidic solution. The solution is mildly acidic at 190 to 250g/l of H2SO4.

Numerous softwoods other than celery-top pine could be used in a mildly acidic environment. Softwoods are generally more resistant to degradation by acids than hardwoods, and all timbers are more resistant to acids than alkalis. In fact most wood is naturally mildly acidic with a pH between 3.0 and 5.5. The higher the concentration of the acid, and the higher the temperature, the more aggressive the environment for wood. However, if you have found by experience that celery-top pine performs well under the conditions you describe we feel that hoop pine would give similar service at lower cost. Hoop pine has a lower durability rating, but rot resistance is not a requirement in this situation. In the past, hoop pine has been used successfully for battery separators.

Q. 1. Is meranti considered to be a 'Kiln Dried Hardwood' ? Is there a definition of KDHW in any of the Australian Standards? 2. Is meranti suitable timber to be used on timber doors as edge strips? I understand its density can vary from 350 -700kg/m3 3. Is meranti a sustainably logged timber from plantations or does it come from S.E. Asian rainforests?

In answer to your questions, 1. Meranti is mostly used for joinery and is generally kiln dried to ensure dimensional stability. It is classed as a hardwood, which means it comes from the group of trees classed as hardwoods (angiosperms), not all of which necessarily have "hard" wood. 2. Meranti is divided into two groups on the basis of density, light red meranti and dark red meranti. The density of light red meranti usually ranges from around 400kg/m³ to 640kg/m³, while dark red ranges from 560 kg/m³ to 865 kg/m³. Both light red and dark red are comprised of groups of species, rather than single species, and there is some overlap between the two groups. We consider light red meranti to be suitable for edge strips on doors in domestic situations, but harder timbers (eg dark red meranti) might be preferred in commercial work or high wear areas. 3. Meranti comes from natural forests, not plantations. Sustainable forest management is at various stages of development in SE Asian countries, with some more advanced than others. For example, Malaysia has a relatively robust system which you can read about via this link:

Q. Hope you can help with this query. We are currently getting fencing quotes and are getting conflicting information regarding fence posts...some say hardwood (Grey Gum quoted) post...some say treated pine posts best...others say cypress...all seem to say what they use is best and others not worth using. What fence posts are best...should we use treated pine palings/plinths/rails but hardwood (grey gum) posts...or best to use treated pine post too?

Grey gum is rated Durability Class 1 in ground contact (the highest rating) and according to the Australian Standard this equates to a probable life expectancy of greater than 25 years. How much greater will depend on the local climate, whether there is a watering system adjacent to the fence line that might encourage decay, and other factors. Some suppliers of treated pine provide a warranty. For example, one company gives a 50-year warranty on their H4 treated pine - for more details, refer to this site: This doesn't necessarily mean treated pine is the better choice, and grey gum may achieve a similar life under favourable conditions. Regarding palings, plinths and rails, treated pine palings need to be well restrained to prevent distortion. For that reason we recommend three rails for the usual height of a dividing fence, one rail at the top, one in the middle, and one at the bottom. But there is little to choose between grey gum and treated pine for above-ground situations with the probable life expectancy of grey gum above ground rated at greater than 40 years.

Q. Hi, I am looking for a suitable hardwood to use for in ground posts for a picket fence. I want 115x115 posts hence the hardwood. I don't think there is any H4 pine at that size. What hardwood would you suggest and would I need to soak it in creosote. thanks...Steve

We suggest a Class 1 Durability hardwood such as ironbark, tallowwood, grey box, etc. for your fence posts. Soaking the in-ground portion in creosote is good additional protection but perhaps more important in the long term is to install the posts correctly. There is a very informative article explaining the pros and cons in the March 2014 newsletter published by Outdoor Structures. You can access it on the net via this link: Also be aware that heavy garden watering adjacent to the fence line will shorten the life of the timber.

Q. Can you please tell me the R value for Spotted Gum cladding eg 20mm vertical shiplap boards? If that information is not available then its thermal conductivity.

We don't have an R-value specifically for spotted gum but it's a function of density - the denser the wood the less air is trapped in its cell structure and the less efficient it is as an insulator. Karri is a species of similar density to spotted gum and has an R-value of 0.12 in a thickness of 25mm. So by simple ratio 20mm would have an R-value of 0.10.

Q. Could you please advise me if Swamp Gum Tas is suitable for boat building. I have seen one report that it rots quicker than rotting fish.

It would last a bit longer than that, but the data is not encouraging. Australian Standard 5604, "Timber - Natural durability ratings", rates swamp gum Durability Class 4 which is the lowest rating. There are actually several different species called swamp gum - Eucalyptus ovata, E. camphora and E. yarraensis all go by the same common name, but none are considered very durable. So we wouldn't recommend swamp gum for boatbuilding. You might like to read a letter to the editor of The Mercury published on 28 June, 1876, protesting about the poor quality of swamp gum. It's available on line via this link:

Q. Hi, I am looking to build a beehive, and am wondering which woods have the best thermal properties? The most important wood attributes for my project will be thermal performance, aesthetic appeal, rot resistance and ease of working. Thanks very much.

The lower the density of the wood the better its thermal resistance, since there is more air trapped in the cell structure. Balsa wood is the best insulator, but impractical for a bee hive because of its low durability. The best balance between good thermal properties, rot resistance and workability would be provided by western red cedar. You could also consider heat-treated Scandinavian softwood, marketed as ThermoWood, which is claimed to have similar durability properties to western red cedar. More information can be obtained by writing ThermoWood in your browser.

Q. Could you please advise me on suitable timber for bench seats for a Sauna. They are currently western red cedar lining, but keep rotting every two years or so. I was thinking of something much more dense and wondered if Merbau (70x19mm) would be suitable. Considering expansion and contraction. Or can you recommend other timbers except cedar?

It's quite remarkable for western red cedar to decay after only two years. It must be very damp inside the sauna as cedar generally gives good service. Perhaps merbau isn't the best alternative as it has a high tannin content which is likely to cause brown stains in a damp environment. So maybe it would be better to choose a timber that is known for its low tannin content. A data sheet published by the Queensland Forest Service recommends spotted gum because it "has a lower tannin content than most other eucalypts, therefore staining of paintwork, brickwork, etc. as a result of water running over unpainted timber surfaces, is unlikely to occur".

Q. We are Landscape Architects currently designing an entry wall for an estate in Mickleham, Victoria. The wall is to consist of a combination of steel and timber logs. The 500-600mm long logs will be stacked horizontally (with each end exposed) and secured on a steel frame, therefore sitting fully above ground. Sustainability is paramount (particularly in terms of timber durability and low maintenance requirements), so a suitable species needs to be selected. Could you please provide advice / further information (regarding durability, issues, possible treatments etc.), specifically for this application, on the following. - Golden Cypress (Cupressus macrocarpa): This species may be readily available near the site. We have so far been advised that it is very durable (approx. Class 2) but that logs cannot technically be rated due to sapwood content. A supplier has also expressed concerns about rapid drying out / splitting to each end of the logs. - Any potential hardwood species? (sustainable sourcing is important) - Treated Pine - Any other species ideal for this application.

Logs are inclined to develop radial splits towards the centre as they dry out. This can be controlled to some extent by running a sawcut along the log to the depth of the pith. Rather than having splits at random locations, one large split will be located wherever you choose. The sawcut will open slightly as shrinkage progresses, but it can be placed where it won't be seen - preferably on the underside of the log so it doesn't collect water when it rains. Bear in mind that the logs will shrink in diameter as well as circumference, so they need to be able to settle, otherwise there will be gaps between logs. Regarding the durability of Cupressus macrocarpa, New Zealand research concluded that four cypress species, including Macrocarpa, grown in NZ as exotics could be placed in the upper end of Durability Class 3. This implies that above ground, "in horizontal or jointed components fully exposed to the weather, an average life exceeding 15 years with occasional earlier failures is likely". As you are aware, durability ratings only apply to heartwood, not sapwood. Perhaps it would be possible to arrange for a woodturner to turn the cypress logs so that (a) the sapwood is removed, and (b) they are perfectly round making assembly easier. When they are cut to length we suggest standing them in a water-repellent solution to allow absorption into the end-grain. Products such as XJ Timber Protective, Cabot's Bar D-K, etc. are suitable for this purpose. This should extend the life of the timber beyond the New Zealand estimate, depending on severity of exposure, local climate, etc.


Did you know?

About 6% of Australia’s 147 million hectares of native forests are public forests potentially available for timber harvesting. Timber is harvested from about 1% of those public native forests each year.