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Ask an Expert

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. You're correct in assuming that I have some boat-building experience, that I'm aware of the WEST system and am building to a set of plans. I asked the designer about the possibility of substituting local wood varieties for the plan specified timbers and his response was that so long as the local timbers were within certain density limits, i.e. 32 to 40 lb./cu.ft. (which they are), and that they don't contain a high percentage of natural oils or acids, they should be fine. For spars he advised that I'd need a wood that's 34 lb./cu.ft. or less, which also has long, straight grain with few or no knots (which Sapele or African Mahogany is). Could you advise where I might be able to check the oil and acidity levels of different types of wood?

We assume the designer is concerned about the possible effect of oils and acids on the epoxy materials in the WEST system, or glues used, and perhaps fastener corrosion. As you probably know, acidity is rated according to pH value on a scale of 0 to 14 where 0 is highly acidic, 7 is neutral and 14 is highly alkiline. Nearly all woods are slightly acidic, some more so than others, primarily due to acetyl content and the potential to form acetic acid. Western red cedar, Douglas fir and the oaks are quite acidic (around 3.5 pH) as determined by placing 1 part of sawdust in 5 parts of distilled water. Spruce and African mahogany are moderately acidic, while iroko is relatively low in acid content (pH around 6). It should be possible to adjust the WEST system, or vary the press time and temperature when gluing, to accommodate acidity. Fastener corrosion is unlikely to be an issue since no doubt you will be using stainless steel anyway.

Q. I have 15 acres in southern tasmania which is 90% forest. Hoping to utilise some timber to build a home, there are a number of varieties. Who can give me advice on the suitability of each variety for building purposes? I saw on 'grand designs' where a client had timber tested to confirm it could be used, do we do such tests in Aus? Please point me in the right direction.

It's certainly possible to use your own timber to build a home, but there are some steps that need to be taken. The first thing to do is identify the types of timber you have. If you intend to use some timber structurally it will need to be stress graded and to do this the species needs to be known. If you intend to use any timber outdoors, again its suitability for exposure to the elements can only be assessed when the species is known. For best results it's preferable to dry the timber, rather than use it "green". For more advice on these issues we suggest you contact the Tasmanian Timber advisory service via this link They should be able to advise you on locally available timber grading services. If you are unable to obtain the help you need, feel free to contact us again.

Q. I am seeking an article or book on the calculation of the shrinkage of timber sections. In particular timbers used as structural elements or decking, both types of elements in interior and exterior uses. Please advise.

Shrinkage values are published in a number of texts, but one of the most comprehensive is Australian Standard 1720.2-2006, "Timber Structures Part 2: Timber Properties". AS 1720.2 gives values for shrinkage in the tangential direction from green to 12% moisture content, this being the most severe case. Radial shrinkage is not tabulated but is about half the tangential value. "Tangential direction" means the direction at a tangent to the growth rings, while the radial direction is at 90° to the growth rings. Since timber is not necessarily cut perfectly tangentially or radially these values can be averaged or the tangential value can be adopted as a worst case scenario. AS 1720.2 also gives a value for the unit tangential movement, ie. the percentage increase or decrease in tangential width or thickness resulting from a 1% change in moisture content.

Q. Could you advise on the following: a) the comparative characteristics of Queensland Maple Plywood and Gaboon/Okoume Plywood, for planking in a marine environment, for the construction of a yacht tender? I have read that Gaboon ply is strong relative to it's weight but has poor decay/rot resistance. b) can 3/4" ply take wood fastenings (screws) through the end-grain, without splitting? c) Douglas Fir/Oregon Pine (34lbs/cubic foot) has been specified for internal reinforcing & external protection elements, i.e. the bow knee & thwart stiffeners, and the gunwales respectively, of the tender. Would Hoop Pine or Huon Pine (33lbs/cubic foot) be acceptable alternatives? I'm concerned about Hoop pine's strength & durability in a marine environment and the potential for knots etc in Oregon Pine. d) American White Oak (47lbs/cubic foot) has been specified for the skeg and runners, would Iroko (40lbs/cubic foot) or African Mahogany (Khaya,34lbs/cubic foot) be acceptable alternatives?, and finally, e) Sitka spruce (28lbs/cubic foot) has been recommended for spars (for a sailing option on the tender). It is said to have good strength to weight ratio, though poor decay/rot resistance and only fair fastening ability. Oregon Pine (34lbs/cubic foot)is a suggested alternative (if knot free) described as of medium weight, strong, moderate decay/rot resistance and good fastening ability. Another alternative is Sapele, although I don't know its characteristics, re weight, strength, decay/rot resistance and fastening ability. Any advice on the above would be most appreciated! Please note I scanned/searched the FAQ on your website for answers but couldn't find anything specific to the above.

Before answering your questions we make the general point that the durability requirements for timber and plywood used in boatbuilding depend on the likely use of the boat. Marine Grade plywood manufactured to Australian Standard 2272 is a very high quality material with characteristics specifically suited to boatbuilding, eg. a water-resistant glue and the ability to stand up to pounding waves. However it is not necessarily resistant to wood rot. A commonly used species for Marine Grade plywood is plantation-grown hoop pine, a species of low durability but one that has the strength and impact resistance required by AS 2272. Hoop pine plywood is suitable for boats that are taken out of the water and stored under cover after use, such as speedboats, racing dinghies, etc. but needs preservative treatment if used in situations where there is prolonged contact with water, eg. boats that are moored. This is explained on the Austral Plywood website at An alternative to preservative treatment is the WEST system (wood epoxy saturation technique) - refer "Epoxy Barrier Coating" in the WEST manual, available on line. If the boat is likely to have severe exposure to the elements, or take on fresh water, you might consider preservative treatment AND an epoxy barrier. As far as the relative merits of different types of plywood are concerned, the most important issue is to make sure they are produced to a national Standard and bear the required product stamps. In Australia the relevant Standard is AS 2272, as mentioned above, but if you are building your boat elsewhere US or British Standards may apply. Regarding different types of timber, it sounds as if you are building to a set of plans or a kit. It's difficult for us to comment on substituting other species since this may require some design input and perhaps a re-sizing of some members. Generally speaking all the timbers you mention are suitable, subject to applying the protective techniques we have mentioned. We assume you have some experience in boatbuilding, but if not there are many books and journals available on the subject.

Q. What type of wood is best for salt/pepper grinders for a lathe?

Many different woods can be turned on a lathe but for food items it's best to avoid any that might impart an odd taste to the contents. For example, it would be most unwise to use a wood such as camphor laurel, but even less obvious examples can taint food. Some of the eucalypts have quite a strong vinegar-like odour due to their acetic acid content. We suggest hoop pine, since it is an even-grained wood with little or no odour or taste.

Q. Please advise any information on camphor laurel for use in furniture, walls, flooring etc.

Camphor laurel could be used for a variety of joinery purposes. It's a fairly lightweight wood, similar in density to meranti at approx. 550kg/m³ when dry. Camphor laurel is reportedly easy to dry with quite a low shrinkage rate. We don't have a hardness rating for camphor laurel but would expect it to be slightly more resistant to indentation than pine, again similar to meranti.

Q. Can you provide me with technical information, especially grades, for merbau solid & laminated timbers.

You will find detailed information about merbau on the Wood Solutions website if you paste this link into your browser - Generally merbau is free from knots and other strength-reducing characteristics, so it achieves a relatively high stress grade. However, this is a question for a local timber merchant, since the availability of stress graded material varies from place to place.

Q. Further to my question a couple of days back on outdoor timber decking and F17 KDHW - is there a commercially available timber which could be graded to F17 KDHW as well as durability grade 1 for outdoor use?

Your question should really be put to a supplier. A timber merchant in your area could advise you on the availability of various hardwoods or, failing that, perhaps a major producer such as Boral could assist. In theory a number of hardwoods would achieve F17 grade and Class 1 durability, but whether they are available kiln-dried, in the sizes you need, in your locality, is another matter. Hardwoods such as spotted gum, blackbutt, ironbark, tallowwood and merbau are all rated Class 1 durability when used outside above ground, and all could achieve an F17 stress grade.

Q. I am interested in ebonising some timber, and would like to know some Australian timbers with high tannin content - these are the ones naturally suited to ebonising by ferric compounds. Also, what timbers are low on tannins - they will need to be soaked in tannic acid before ebonising

We don't have details of the specific tannin content of various species, but most hardwoods contain tannin to a greater or lesser degree although one Australian hardwood that is notably low in tannin is brush box. In general, white woods have a lower tannin content, whereas woods with a lot of yellow, red, or brown colouration to them tend to contain more tannin.

Q. Which is a better timber to use for window frames exposed to the weather, cedar or Vic ash. and if one is better then the other is the difference small or significant. I have been told that cedar timber is better if you live near the sea (salty enviroment) otherwise cedar and Vic ash as are just as durable, is this correct?

To answer questions like this we refer to Australian Standard 5604, "Timber - Natural durability ratings". We find that western red cedar is rated Class 2 durability outside, above ground, while Vic ash is rated Class 3. Ratings are on a scale of 1 to 4 where Class 1 is most durable and Class 4 is least durable, so clearly cedar is considered the better choice from a durability point of view. This assumes that neither timber is treated. If Vic ash windows received an effective preservative treatment that would alter the equation, or if the windows were protected from the weather by verandahs it's much less important what kind of timber you use. A salty environment is not particularly harmful in itself, which is why many seaside structures are made from wood (jetties, boats, boardwalks, etc.) Fresh water is actually more hazardous. However, in a marine environment you must make sure that all metal fasteners are highly corrosion resistant, preferably stainless steel.


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%.