In Australia, timber isn’t just sold by its species; it is rigorously graded to ensure that builders and engineers know exactly how much weight a piece of wood can support.1 This process, known as stress grading, ensures safety and consistency across the construction industry.
The grading system is primarily governed by Australian Standards like AS 1720.1 (Timber structures) and AS 1684 (Residential timber-framed construction).
1. The Two Main Grading Systems
Depending on the type of wood and how it was processed, it will usually fall into one of two categories:
F-Grades (Stress Grades)
The “F” stands for Force (specifically, the bending stress in Megapascals, or MPa).
This is the traditional grading system used for both hardwoods and softwoods.
Common Grades: F5, F7, F8, F11, F17, F27.
What it means: A higher number indicates a stronger piece of timber.7 For example, F27 is much stronger and stiffer than F5.
Used for: Mostly hardwoods (like Spotted Gum or Ironbark) and some unseasoned softwoods.8MGP-Grades (Machine Graded Pine)
MGP is a specialized system developed specifically for Australian Radiata Pine (softwood).
Common Grades: MGP10, MGP12, MGP15.
What it means: The number refers to the stiffness (Modulus of Elasticity) rather than just bending strength.
Comparison: MGP10 is the “standard” framing timber in Australia. While MGP10 and F7 are often used interchangeably, they have different technical properties regarding how much they “spring” or deflect under load.
2. How is Timber Actually Graded?
Timber mills use three primary methods to determine which grade a piece of wood belongs to:
Visual Stress Grading
A trained grader manually inspects every piece of timber.They look for “strength-reducing characteristics” such as:
Knots: Large or frequent knots weaken the wood fiber.
Sloping Grain: If the grain runs diagonally instead of straight, the board is weaker.
Shakes and Splits: Cracks that go through the wood.
Wane: Missing wood or bark on the edge of the board.
Machine Stress Grading (MSG)
The timber is run through a machine that physically bends it (usually on its flat side) to measure its stiffness.
The machine calculates the Modulus of Elasticity (MOE) and sprays a color-coded grade stamp on the wood based on the result.
This is more precise than visual grading because it accounts for the internal density of the wood that the human eye can’t see.
Machine Proof Grading
In this method, the timber is subjected to a pre-determined “proof load.” If the timber survives the pressure without breaking or bending beyond a certain limit, it is certified for that grade.
It’s essentially a “pass/fail” test for structural integrity.
3. Seasoned vs. Unseasoned Timber
A critical factor in Australian grading is the moisture content:
Unseasoned (Green): Timber with a moisture content over 25%. It is generally weaker and will shrink as it dries in the building.
It is graded with “S” groups (S1–S7).
Seasoned (Dry): Timber that has been kiln-dried or air-dried to a moisture content of 15% or less. It is stronger, more stable, and graded with “SD” groups (SD1–SD8).
| Grade Type | Typical Use |
| MGP10 / F5 | Standard wall studs and internal framing. |
| MGP12 / F7 | Higher-load framing, lintels, and floor joists. |
| F17 / F27 | Structural beams, lintels, and exposed hardwood posts. |