A Guide To Timber Stress (strength) Grading
It goes without saying that timber is a popular building material that offers a host of benefits including versatility, appearance, sustainability and more. However, not all timber is created equal in terms of strength and durability and structural properties can vary among species of wood. That’s why timber grading and strength assessments are essential to ensuring the quality and reliability of timber products for building and construction.
In this guide, we discuss what timber grading is, why it is important and how timber strength is determined and categorised.
What is Timber Stress Grading?
In construction and engineering, a timber’s strength grade refers to the ability of a piece of timber to maintain stress (force) without ‘failure’. In this case “failure” refers to the amount of downward force the timber can withstand without excessive deflection (bending / sagging).
While catastrophic failures such as breaking or total collapsing can occur, it’s very rare and usually results from a combination of factors such as poor design, lack of approvals and certification, inadequate construction methods, or improper fixings.
This is why a reliable timber strength grading system is crucial. It ensures a consistent level of quality control and empowers consumers, designers, builders and engineers to confidently select and specify timber for their specific application, knowing that it will do its job for its design life span.
Factors That Affect Timber Strength
The structural properties, strength and durability of timber can be affected by several factors, including:
- Species: Different timber species have different strength and durability properties. For example, hardwoods such as Spotted Gum and Ironbark are stronger and more durable than softwoods such as pine.
- Moisture Content: The moisture content of timber can also affect its strength. Unseasoned (green) timber is weaker than dry or kiln dried timber. This is obvious when you notice how bendy and flexible a small branch from a living tree is, but the same size branch that has fallen to the ground and dried out is much stiffer and hard.
- Defects: Natural features such as knots, splits, cracks, borer holes etc act to weaken a piece of timbers strength and lower its strength grade. The less of these defects, the stronger the piece of timber will be.
How is timber strength determined & categorised?
Once we have determined the factors above, it can be categorised into a Stress (strength) Grade. Timber in Australia is commonly graded using two methods: Visual Stress Grading and Machine Stress Grading. It is important to note that Stress grades are used only for structural timber products and different timber grades are best suited for different applications.
Stress Grades: F Rating
Most hardwoods and some softwoods are visually stress graded using an “F” grade system, with the higher the stress grade, the stronger the timber. A timber grade of F14 for example means that it can withstand a force of 14MPa (Megapascals) without excessive deflection.
Unseasoned hardwood timber is generally F14 / F17, whereas kiln dried hardwoods are usually stronger in the F22/F27 range. Pine is weaker and usually around F5/F8.
Stress rating can go as high as F34 for some exceptionally strong species that have been kiln-dried and are free of any defects.
Stress Grades: MGP Rating
Most species of plantation, structural pine are stress graded using a machine that bends each board with a set amount of force and measures its deflection (bending).
The less it bends with the set force applied, the stronger the piece is. This grading scale is represented by an MGP (Machine Graded Pine) rating which can vary from MGP10-MGP15.
What Is The Purpose Of Timber Grading Systems?
Engineers and building designers use this information to specify the right size and stress grade of timber to ensure that they can adequately withstand the force being applied to them, ie: do the job required without deflecting too much or breaking.
A Span Table is a reference guide used in construction to determine the maximum distance or span that a particular type of timber can safely cover without additional support. (see the below image for example).
It’s logical that a piece of F27 timber will span a lot further than the same size piece of weaker F14. Alternatively, you could use a smaller piece of F27 for the same span as a larger, weaker piece of F14.
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