Visually Stress-Grading

In Australia, visually stress-graded structural timber is classified using F-grades (e.g., F5, F7, F11, F17, F22, F27). The “F” number represents the characteristic bending strength of the timber in megapascals (MPa), which is a measure of how much stress the timber can safely carry in structural applications. The grading is determined by visual inspection, considering features such as knots, slope of grain, checks, splits, and other natural growth characteristics that affect strength. A higher F-number means stronger timber with higher structural capacity. These grades apply to both softwoods and hardwoods used in Australian construction.

Common Visual Stress Grades (AS 1720 / AS 2082):

F5 – Very low strength grade, often used for non-structural or light structural purposes.

F7 – Common in plantation softwoods (like Radiata Pine). Used for light framing where loads are modest.

F8 – Slightly stronger softwood, sometimes used in light framing.

F11 – Entry point for structural hardwoods. Strong enough for standard beams, joists, and posts.

F14 – Stronger hardwood option, commonly used in residential and light commercial work.

F17 – A widely used grade of structural hardwood, balancing strength and availability. Often used for heavier beams and posts.

F22 – High-strength hardwood, suitable for demanding structural applications.

F27 – Very strong grade, often seen in durable Australian hardwoods like Spotted Gum or Ironbark. Used in high-load applications.

✅ In short:

The F-number = characteristic bending strength in MPa.

Higher numbers mean greater load capacity.

Determined by visual inspection of defects and grain quality.

Used in both hardwood and softwood for structural applications in Australia.

🔹 Technically, F-grades could run from F1 upwards (since the “F” number is just the bending strength in MPa).
🔹 In practice, only certain grades are used and standardised in Australia, because:

Very weak grades like F1, F2, F3, F4 would represent timber with only 1–4 MPa bending strength — this is so low that such material isn’t suitable for structural use. It would split, sag, or fail under even light loads.

The minimum grade used structurally is F5. Anything weaker than that is not considered structurally reliable.

Not every number is used — the system “jumps” (e.g., F5, F7, F8, F11, F14, F17, F22, F27). These are the grades codified in AS 1720.1 and AS 2082 for design and supply.