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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. I have a question regarding the expansion of timber beams and joists subject to heat and moisture. If I have an exposed treated pine joist which is supported on a ledger between 2 brick walls, is there any concern expansion of the timber joist may load the walls. I was contemplating leaving a small permissible gap in the joist hanger (eg. 2mm) but looking at other similar structures this doesn’t seem to be a consideration. I read in a timber flooring answer, timber only expands across the grain. So I think I may have answered my own question. If you could just clarify please.

Yes, you are right - timber only shrinks and swells significantly across the grain. That is why we can use "green" timber in structures without serious problems. It shirnks as it dries out, but only in width and thickness. There may be some minor movement along the length, but it's unusual.

Q. We need to steam bend some 10 lengths of timber 2'1/2 X 7/8" X 44" long risng about 4" in centre My question is Which timber to use and where would I obtain it in Sydney?

We don't know of a steam bending works in Sydney, but we can refer you to one at Daylesford in Victoria. They are simply called The Timber Benders and you can phone them on (03) 5348 7767. If you intend to steam bend the timber yourselves, a CSIRO leaflet on the subject recommends myrtle beech from Tasmania or blue gum (Eucalyptus globulus). It's essential to select straight-grained material to avoid breakage. Britton Timbers stock a wide range of specialty timbers and should be able to help you.

Q. I have been asked to design and build a cabinet of Tasmanian timber to store shells. There is a known problem with timber releasing acetic acid over time which will corrode the shells. I have been trying to find a list of pH of tasmanian timbers. I have looked in Keith Bootle's book, but it doesn't include Huon Pine, myrtle and blackwood. Is there some more info available or do I need to get my own universal paper and do my own test on some sawdust?

Keith Bootle's "Wood in Australia" does actually give a pH value for Huon pine in Table 5.1 (pH 4.5). Values are also given for the Tasmanian oak species (alpine ash, mountain ash and messmate). However, we've not been able to find a reference for myrtle or blackwood. If you do your own tests we'd love to know the results!

Q. What is the best Australian timber to use to build a wooden bath tub and basin or is there more than one type of wood can you tell me?

We would suggest the boatbuilding timbers huon pine or King William pine from Tasmania. You will find lots of useful information on the net if you write "wooden bathtub" in your browser, but note that the sealer is just as important as choosing a suitable wood. Again, boatbuilding techniques will give the best results and we suggest using the epoxy saturation technique, or WEST system, which is explained in detail on this website:

Q. Which timber has the lowest tannin content or which timber will not leach?

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". Cypress pine and preservative treated pine are also free from tannin staining problems.

Q. I was wondering if you could give me the source of your Durability data for White Cypress? I'm not challenging it (it fits with centuries of experience), but having trouble finding technical source material. My background is heritage preservation, and there is surprisingly little in the literature that corresponds to this matter. A bibliographic reference would be appreciated (and acknowledged).

White cypress (Callitris spp) is regarded as durable with good natural decay resistance and a high natural resistance to termites. This was established by CSIRO in laboratory testing in the 1960's and reported in several papers which were part of a series titled "The Causes of Natural Durability in Timber". Parts XIII, XIV and XVII by lead author P. Rudman of the then CSIRO Division of Forest Products, dealt with cypress pine. The three papers were published in the German journal Holzforschung, Band 17, Heft 6 (1963), Band 18, Heft 4 (1964), and Band 19, Heft 2 (1965) respectively. (Band = Volume, Heft = Issue). The life-span of the wood depends, of course, on the severity of the location, and the resistance of cypress pine to wood rot and insect attack, as with most woods, applies only to the heartwood, not the lighter-coloured sapwood. Comprehensive field tests were also established by CSIRO in the 1960's from which natural durability ratings were derived. These are tabulated in Australian Standard 5604-2005, "Timber - Natural durability ratings". According to AS 5604, white cypress is rated Durability Class 1 outside above ground, and Durability Class 2 in ground contact.

Q. I was wondering if you could give me a little advice on a timber question or at least point me in the right direction of someone who could. I am a swimming pool installer, in particular I install above ground swimming pools. These pools are generally made from colourbond steel. Many of my clients deck around their swimming pools once they're installed and a lot of them don't like the look of colourbond steel against the timber. I would like to use some type of timber that will be machined to form the coping that goes around the top edge of the pool. Would you be able to recommend any particular species that would be best suited for this. I thought either treated pine or merbau because the majority of decks are made of that and it would blend in, but I am worried the tannins leaching from merbau would stain the pool. Would you have anything you could recommend? Secondly, what would be the best treatment for it? The coping will be exposed to sun, and chlorine or salt water. They won't be in water but naturally they will get wet a lot. I am particularly concerned about the underside, as this will be difficult to re-treat once installed. Could you recommend a treatment to best preserve the timber for as long as possible. I hope you can help me or at least point me in the right direction of someone who might. I look forward to hearing from you.

Merbau would be a good choice except, as you say, it has a high tannin content that initially will stain the water in the pool. Leaching of tannin eventually stops but it can take several months. Your clients might not want to wait, so spotted gum would avoid the problem as it has a very low tannin content. The Queensland Department of Primary Industries advises that spotted gum has "lower tannin content than most other eucalypts and therefore staining as a result of water running over unpainted timber surfaces is unlikely to occur". We suggest leaving the coping uncoated to turn grey. Exposure to sun, salt and/or chlorine will ensure that mould growth is inhibited and the coping will weather to a pleasant driftwood colour.

Q. I require a hardwood to be used externally for a school pergola feature. Brief by architect is; Doesn't leech or drip onto concrete when wet or over time. Isn't a reddy tone Durable against knocks and bumps of high school students What species should I use? I have a picture of the design if I can send that to someone?

A number of timbers will give good service in a pergola, but perhaps hardwood would be best to resist knocks and bumps. If staining of the concrete below will be a problem it would be a good idea to choose timber with a 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". We feel that the colour is relatively unimportant as it will turn grey if left without a coating. If the pergola is painted it will change the colour anyway, and in that case staining is less important because paint will seal the surface.

Q. Which reasonably available hardwoods are most stable when installed unseasoned? On the advice of the old TDA we put a couple of 125 x 125 Stringybark columns a few years ago which are still as straight as they went in. Trouble is, I cannot find a record of what variety of Stringybark we used.

Yellow stringybark is generally regarded as the best of the stringybarks for wood quality. Like all hardwoods there will be significant shrinkage as it dries and if possible it's best not to expose new wood to full sun or hot dry winds until at least some drying has taken place. It's also a good idea to avoid the centre of the log if possible as the pith tends to initiate splits. There's a good illustration of this on the net at this address:

Q. I am interested in use of Paulownia wood as decking in a hot and humid climate. Unable to find a summary of experience for outdoor use of this wood. I can see it is used for surfboards but that is not same as decking in public park . Appreciate an honest answer!

There are conflicting reports about the durability of Paulownia, where by "durability" we mean resistance to wood rot and insect attack. Some websites assert that Paulownia is highly rot resistant, but we are guided by New Zealand experience which has shown that it is non-durable in ground contact where it is no more durable than untreated radiata pine. Of course decking is not in ground contact, but performance in-ground can be regarded as an accelerated test of performance above-ground. The fact that it is sought-after for surfboards does not imply that it is durable since surfboards are in the water for a relatively short time and often sealed with an epoxy coating. The advantages for surfboard construction are that Paulownia is light in weight and dimensionally stable. Aside from the durability aspect, Paulownia is quite soft with an average density below that of western red cedar. Consequently it would be easily dented, particularly when exposed to the public.


Did you know?

Australia’s native forests, timber plantations and wood products are net absorbers of greenhouse gases, sequestering 56.5 million tonnes of carbon dioxide in 2005, reducing Australia’s total greenhouse gas emissions by nearly 10%.