Timber Grading – Australian Timber Strength Groups
If you’re planning to use timber in your next wood project – you have chosen well! It is a hard and highly durable material, which makes it perfect for demanding conditions, especially that it is capable of taking loads (even suddenly). Additionally, it will retain shape during seasoning or conversion. Providing it is of good quality, it will be free from flaws or cracks and maintain nice, uniform colour.
However, before you start ordering materials, you need to know a few things to choose right, e.g., the stress grades. Stress grading is a system of classifying timber used for structural purposes in terms of its strength, in either a visual or mechanical way. Why is it important? In short, you need to know how much stress a specific type of timber can take without failure.
For example, if you see timber marked as F17, it means that its working stress in bending is around 17 MPa (Megapascals).
To help you understand it better, we break it all down into details below.
Timber Strength Groups
In order to talk about timber grading, we need to start by understanding timber strength groups.
When it comes to the group classification (defined in the standard: AS/NZS 2878:2000. Australian/New Zealand Standard™. Timber—Classification into strength groups), we distinguish two types of timber species:
- seasoned (dry – SD group) and
- unseasoned (green – S group)
The differences between these two lie in the mechanical properties in given conditions. For example, a given species with the same dimensions and quality will usually be stiffer if seasoned.
Seasoned (dry) timber consists of 8 strength groups (from the strongest – SD1 – to the weakest -SD8) while unseasoned (green) timber – of 7 (with S1 being the strongest and S7 – the weakest).
However, groups classifications can also be differentiated as follows:
- without brackets – S and SD group classifications – refer to the groups determined by mechanical test data with the use of small, clear specimens,
- in brackets – provisional (S) and (SD) group classifications – refer to density or/and somehow limited mechanical test data.
In terms of strength properties, individual timber species are grouped according to mechanical test data (with the use of clear, small specimens). On the other hand, broader categories (e.g., mixed hardwoods) are differentiated by the species-strength rating (the lowest), but only if it has a bigger influence on the mechanical properties of the mixture.
These groups are closely related to timber stress grades when it comes to structural properties and building use.
Timber Stress Grades – What Is It All About?
Structural timber comes with stress grades, another form of timber classification (here, in terms of the structural properties) according to the Australian standards. It refers to the stress limit of a given species (when used for building applications) differentiated by either visual stress-grading or machine stress-grading.
Stress grades can be given in the following forms:
- from F4 to F34, for example, for the former, the basic working stress in bending would be around 4 MPa while for the latter – 34 MPa, and so on. This is the system usually used for native forest structural species (e.g., eucalypt),
- Machine-graded pine, namely MGP, MGP10 to 15, for example, MGP15 will refer to a minimum threshold of stiffness of around 15,000 MPa. This usually includes all exotic softwoods (pines and related species).
Timber Grading Methods
According to the Australian Standards, the strength of timber can be determined by either one of two methods.
Visual stress-grading involves determining timber grading by the strength group of a given species and the visual grade quality of its piece. The Australian Standards specifications show the limits in terms of the size or extent of some characteristics (e.g., knots or sloping grains reduce strength).
Machine stress-grading, on the other hand, refers to mechanical grading, linking strength directly to the stiffness of timber.
So, when it comes to structural timber, stress grades are given according to the strength group classifications and the grade-quality description.
Mechanically graded timber has a grade recorded on each piece by branding or colour marking (according to the Australian Standards).
What to Take into Consideration?
Structural timber must meet the required stress grade for a given purpose. It will be specified by an architect or a designer. It is worth remembering, though, that the timber stress grade will always be influenced by the characteristics of a given species (e.g., Slash Pine is always stronger than the Radiata), but the quality of the sample might not always be the same (some are more faulted than others, for example, when it comes to knots, loose gum veins, or sloping grain).
However, neither of the timber grading systems takes into consideration the appearance, durability under worse conditions, dressing, or shrinkage. While all that might have some influence on the grade, the only guarantee is the strength of the timber.
If you need a visually representative timber, you will have to look for a product that’s not only stress-graded but also labelled as select or appearance. Bear in mind that it will all depend on a given species. For example, appearance grade hardwood will always be of good quality when visually graded, but with some types, more minor visual faults might not affect the strength of the timber but still be unacceptable in terms of appearance.
Additionally, choosing by stress grade will allow you to control how much load the timber will be able to take, but it won’t ensure durability in all conditions, for example, when it comes to exposure to moisture. A durability rating will give you an idea about a given species’ resistance to fungal or insect attacks and adverse weather conditions. Your chosen timber might also need some treating, especially when selected for exposed exterior purposes. Here again, your desired appearance will be in question as treated wood might not be visually acceptable for you. As such, if you want a clear finish or lightly stained timber, you will have to decide on a species that can do well without treating.
Furthermore, a given stress grade doesn’t provide any information about the timber shrinkage. Initially, a piece will contain some moisture that can be dried out through seasons. Different species have different shrinkage properties, so they need to be taken into consideration whenever you choose unseasoned wood for structural purposes.
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Here at Integra Direct, we understand how confusing and difficult it might be to comprehend all the properties and factors essential for choosing the right solution. That’s why we will be more than happy to walk you through different types of products and laying methods as well as help you with the installation (Gold Coast only). By choosing our services, you can be sure that you are working with industry professionals who will save you both time and money with our expertise and Best Price Guarantee.