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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. I was wondering if I construct a deck to meet the span tables in 1684.2 then how much weight can the deck support per m2? Thank you in advance for you assistance.

If you use the tables for floor joists and bearers (Tables 5 & 6), as recommended for domestic decks up to 1.0m above ground, the design load is 1.5 kPa. If you use Tables 49 & 50 for domestic decks more than 1.0m above ground, the design load is 2.0 kPa. Commercial decks, or domestic decks supporting significant dead load such as a spa, might need special design beyond the scope of AS 1684.

Q. In respect to laminated Hardwood posts, is there a structural standard ? ie GL18 etc. If so, what is needed to get the posts certified to this grading?

Glulam manufactured by members of the Glued Laminated Timber Association of Australia (GLTAA) is certified under the GLTAA's Quality Assurance Scheme, as explained on their website at Glulam can't be graded in the same way as a stick of solid timber since its performance depends on the quality of the glue bond. This is assessed by ongoing sampling and testing ("process controls") which underpin certification and provide an assurance that performance targets are being met.

Q. We are replacing F7 H3 pine rafters on a carport. Due to increased roof load the engineer has shown the 240 x 45 mmm rafters as LVL / H3 ( 5900 mm single span ). Is there an 'appearance' grade LVL product that suits the above spec ? What are the best options for non-engineered timber at the spec given that would exceed F7 ?

We don't know of an appearance grade LVL - typically LVL has dark-brown gluelines visible on the face and is primarily a structural product. However, kiln-dried hardwood is available in stress grades up to F27, and sizes up to 290 x 45, that should equal or exceed the capacity of LVL. You don't mention which State you are in, but Barrenjoey Timber in NSW can supply dressed F27 hardwood suitable for appearance grade situations. You can visit their website at this location:

Q. I have a Balinese hut and one of the four timber support posts has developed a series of deep to very deep vertical cracks which extend the full length of the post. The depth of the cracks increase from 35mm at top of post to 70mm at middle and bottom of post. The width of the cracks vary up to a maximum of around 8mm. I am concerned that the structural integrity of the hut is being compromised by these cracks or may become so in the future if the cracks worsen.

While the cracks in your post are obviously quite severe, the effect on strength is much less in a post than it would be in a beam. It sounds as if the cracks are "heart shakes", ie. they radiate out from the centre of a post that contains the pith or heart of the original log. You don't mention the size of the post, but as long as the deep crack is not matched by another on the other side (ie. the post would then effectively be cracked right through) we would not be overly concerned. On the other hand, since the other three posts are evidently OK, perhaps you might want to replace the cracked post for appearance reasons.

Q. I work in the Timber Preservation Industry, (Arch Wood Protection) and was recently asked by a customer how timber is graded, and the grades verified. Some of my contacts indicated that the mill must develop an SOP to show how they will achieve the standard for each MGP grade and this is then verified by an independent certification process and third party destructive testing each year. Could you please clarify the actual process requirements, i.e What Standards need to be adhered to (AS1720 etc) and who undertakes the third party testing, so that I can gain a better understanding for when I am talking to our customers.

The process you describe is correct. For example, to set up a machine stress grading operation the following steps are required: selecting a parent population of timber, eg. pine from a particular region; sorting the population into sub-populations and assigning a stress grade in accordance with AS/NZS 1748; evaluating the structural properties of the stress-graded timber in accordance with AS/NZS 4063 by testing stiffness and strength; carrying out ongoing monitoring of the graded product to AS/NZS 4490. Periodic monitoring is required to ensure that the properties of the graded timber do not drift away from the values determined under AS/NZS 4063. Sampling is required at intervals of not more than 12 months at a rate of 1 in 10,000 of each grade/size combination. Testing is done under laboratory conditions eg. by a NATA-accredited organisation. For more detailed information the report available at this location might be of interest:

Q. Are there any span tables for outdoor structures (ie verandahs, carports and pergolas) available? My understanding is that the span tables in AS1684 are only suitable for roofs with ceiling joists, collar ties etc.

We are currently in the process of developing software called Timber Solutions which will provide more flexibility in design. Meanwhile, the tables in AS 1684 can be used for flat roofs and low pitch roofs by selecting the single-span rafter option and applying a 10kg/m² roof mass. For a pergola with a lightweight roof (or no roof) this roof mass will be somewhat conservative, but about right for a verandah or carport.

Q. I'm planning to install shade sail 6m x 8m and use treated H4 timber posts 100 x 100 x 3600 or 100 round in ground 600mm. Will the timber be strong enough to hold such a big sail?

Your question really needs some engineering input taking into account exposure of the site and the likely wind load. At a guess, if there are only four posts (one in each corner) we would say that 100 x 100 treated pine is unlikely to be adequate. Maybe the supplier of the sail can provide engineering advice on this matter.

Q. What is the stress grade for gum tree? Would you specify that for me.

It's not possible to nominate a stress grade unless you can tell us the kind of wood you are dealing with. Once we know that we can tell you the range of grades that kind of wood would achieve. From that range the specific grade will depend on the quality of the wood - that is the purpose of grading it, to assess its quality and put it into a grade.

Q. I need to know structural strength characteristics of Australian hardwood from the Coffs harbour area. A client wants to use local timber (un milled) to build a large garage. I need to do a roof & wall design.

Several different hardwoods are available in the Coffs Harbour area, all with different strength properties. If your supplier can nominate the type and stress grade of the timber, your design can take full advantage of its structural properties. However, if you are being offered mixed hardwoods, the design assumptions are on the conservative side. According to Australian Standard 1720.1-2010, Timber Structures, Part 1: Design Methods Table H2.3, mixed hardwoods from southern NSW fall into Strength Group S4 (unseasoned) and SD4 (seasoned). Australian Standard 2082, Timber - Hardwood - Visually Stress-Graded for Structural Purposes, then allocates the stress grades F7 to F14 for timber of S4 Strength Group, and F11 to F22 for timber of SD4 Strength Group, depending on the quality of the timber, ie. size and position of knots (if any), sloping grain, etc.

Q. I have a roof awning that will carry a ceiling. I am using radiata pine as rafters. The roof area is 7 metres wide and 4.5m long. The rafters will be running the 4.5 metres plus a 600mm overhang so the rafters will be 5.100 metres long (maximum span 4.5m). I will be spacing the rafters max 400mm apart. What is the minimum size of timber I need to use if I am using a standard metal roof. The weight of the metal roof and ceiling material combined will be 6kgs p/m2. Thanks in advance for your advice.

The absolute minimum size would be 120 x 35 MGP10 pine. However, the 35mm is a bit narrow to accurately nail into without splitting the wood, and for this reason we suggest 120 x 45mm. This assumes you are in a non-cyclonic built-up area which is rated wind speed N1/N2.


Did you know?

A government report showed there is no evidence proving that harvesting timber from native forests has reduced overall forest biodiversity or led to the extinction of any species of plant or animal.