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Structural timbers & engineering

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. My elderly sister has recently had some major renovations done to her house. The "builder" has disappeared to Qld. The quality of the work is appalling and now I'm wondering if it is actually safe. Her house is timber highset on hardwood piers spaced. She now has a large living area underneath her house with no supporting posts or interior walls. The span is 5.5m and from what I can see it's support is 210mm hardwood bearers at roughly 460mm centres. Can you tell me if this is sufficient for the span please. The cosmetic stuff I can fix for her but this is a bit of a worry.

We feel you need to engage a building consultant or engineer to check the construction. It's difficult to comment without knowing the type and grade of timber, type of roofing material, etc. Presumably the house has a trussed roof if there are no internal walls, so the outer bearers are going to be working pretty hard. A bearer size of 210mm is unusual, and we weren't sure if the bearers are actually what we would call joists, since they are at fairly close centres. On the face of it the timber does seem a little undersize, although it might be OK if it's a particularly high strength kiln-dried hardwood, or perhaps an engineered timber product such as LVL. But it sounds as if the house needs an overall assessment, including checking the supporting piers, bracing and other structural aspects.

Q. I need to know the structural & fire rating on kwila posts, 90 x 90 & 115 x 115? We are building a house in a very busy area & need it to be fire rated.

It's more usual for the local authority to nominate the fire resistance level (FRL) required. In the Building Code of Australia's terminology this could be 30/-/- for a free-standing post, where 30 represents the period for which structural adequacy is required (in minutes). The dashes indicate there is no requirement for integrity or insulation. Or it might be 60/-/- depending on the situation. Once the FRL is known, an engineer can calculate the effective depth of charring after exposure to fire, and then the effective residual section, ie. how much timber is left. It's then necessary to see whether the timber remaining after exposure to fire can carry the load. Effective depth of charring is calculated in accordance with Australian Standard 1720.4-2006, "Timber structures Part 4: Fire resistance for structural adequacy of timber members". The strength of the residual section is calculated in accordance with Australian Standard 1720.1-2010, "Timber structures Part 1: Design methods".

Q. I am looking for some information on where I can get my timber rated. I am owner builder and wish to use the Ironbark off our own property to build but we must first have it verified and rated. Can you please direct me? I live in Moore, Queensland.

It's a good idea to use your own timber, but it will need to be graded to pass certification. This can be done visually, in accordance with the relevant Australian Standard. We don't have details of people in your area who are experienced in grading timber, but Timber Queensland might be able to direct you to a local grader. You can phone them on 3254 1989. You will also have to decide whether to dry the timber or use it "green" (unseasoned). Ironbark is slow to dry and hard to work with when seasoned, so if you only intend to use it for framing you might wish to use it unseasoned. On the other hand, it will then shrink as it dries out whereas seasoned timber is not prone to shrinking. Perhaps you should discuss this with your carpenter or builder and see whether they are happy to work with seasoned ironbark framing. If so, you could air dry the timber, but you will need to allow up to a year depending on weather conditions, and depending how large the pieces are that you want to dry.

Q. I am trying to ascertain the equivalent size in Structural Pine to Hardwood. Our Company currently uses 100mm x 75mm hardwood in between roof trusses in the roof cavity to which is bolted through the fascia a metal riser bracket from 300mm to 1200mm high on which the electrical supply is run to the residence/shop/factory. The Company needs documented information as to the size of structural pine needed to achieve the same properties as the hardwood.

What you need is certification from a consulting engineer to show the size of pine needed. It might be slightly bigger than the hardwood you are currently using, but there seems no reason why pine couldn't be substituted. However, it is beyond the scope of our service to offer this advice. There should be a small to medium size engineering practice in your area that could advise you.

Q. I am building a verandah 3.6m wide with posts 4.5m apart. What size timber do I need to span the 4.5m between the posts? The verandah has a colorbond roof over and hardiflex lining.

This application is described as a "verandah beam". Assuming you are not in a high wind or cyclonic area, and you will be using F5 grade treated pine, a size of 240 x 45 is suitable as a single span between posts.

Q. I'm looking for an engineer who can peruse and certify (or edit) my plans for a mortice-and-tenon timber frame. Don't mind where they are as email and telephone solves the geographic issues. Any suggestions?

We understand you are located in South Australia. Your job requires a small engineering practice with expertise in timber design. We suggest Core Engineering at 113 Anzac Highway, Ashford, phone (08) 8351 3344.

Q. I am building a decking in South Australia. I am planning to use 240 x 42 LOSP timber beams for the Joists. I need to know what the maximum span is for this beam before it needs a support.

The maximum span will depend on the spacing of the joists, which in turn will depend on the maximum recommended span of the decking, according to whether it is hardwood or softwood. Span tables have been published by Design Pine that take these variables into account. They can be downloaded from the Wood Solutions website at

Q. I am designing a bench seat for use in a restaurant. I would like to use lengths of timber dowel spanning the length of the seat, betweeen the bench seat frames. Can you please advise maximum span for 32mm hardwood timber dowel.

We need to make a few assumptions to answer your question. First we assume the dowels are kiln-dried hardwood of F17 stress grade or better, eg. Victorian ash or similar. Then we need to assume the weight they will carry. A reasonable assumption for one person is 120 kg, but to be on the safe side we might assume one person has another sitting on their lap, which means 240 kg. However, the weight will be shared between at least three dowels so we can say 80 kg per dowel. Taking all this into account we recommend a maximum span of 750 mm.

Q. I am a structural engineer involved in the development of a product that will be bolted to a timber member and will result in impact loads (moment and shear) being transferred to the member. We intend to test the product in a test facility. Part of the test will be to determine if the timber has failed. Beyond the obvious signs of splitting, permanent deformation, tearing etc. is there any guidance for how failure of timber members in service can actually be determined?

Since impact loads are by nature sudden, if they are sufficient to cause failure it will be visually obvious. An interesting frame-by-frame photographic record of impact failure is available on the net via this link: A paper that might be relevant, titled EFFECT OF TUP GEOMETRY ON IMPACT BEHAVIOUR OF PARALLEL STRAND LUMBER (PSL), can also be downloaded from the net.

Q. We would like to use some reclaimed Oregon for a timber pergola. The local council has asked us to advise the stress grading of the timber and if unknown to seek advice from an engineer. If we do need to go to an engineer to provide an estimate of the grading, are you able to advise what the fee for this service may be as I have absolutely no idea? I don’t think a full report is needed, and email will probably suffice from an expert.

It might be best to engage someone who is experienced in grading timber. Various people provide this service depending which State you are in. In Victoria you could contact John Hay on Mob. 0407 043 185. In New South Wales State Forests no longer provides an inspection service, but their former inspectors have formed a private company and you can access their details via this link: In South Australia you can contact a company called Timber Consulting Pty. Ltd. To find out the cost you will need to contact the relevant grading service.


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.