Structural Timber

Timber is a readily available, renewable building material that offers great versatility in design. Timber can allow for a cost effective build due to its relatively low material cost per square metre as well as reduced build time driven by its ease of use/manouverability and no need for setting time.

Hardwood versus softwood

Contrary to the naming convention, both hardwood and softwoods can be suitable for various structural uses. The terms themselves do not directly refer to strength but rather refer to whether the tree the timber comes from is a fruit or nut bearing broad leaved tree (hardwood) or a cone bearing tree with needle leaves (softwoods). 

Softwoods are used in structural applications such as internal framing for stud walls and ceiling rafters. Hardwoods are commonly used for applications that require significant load bearing such as roof beams or window and door lintels. 

Building standards

When using timber to build in Australia, it's important to adhere to the technical data and limitations established in the relevant building codes in your area. In particular (but not exclusively) AS 1720 provides design data covering characteristics including:

  • Strength

  • Durability and preservative treatment

  • Moisture content and shrinkage properties

  • Density

  • Hardness

  • Insect resistance

  • Installation practices

  • Finishing and maintenance

which will help you choose the right timber for your job.​

Strength and grading

Structural timber is generally sold as a graded product using the F grading system. This system uses one of three structural grading methods:

  1. Visual stress grading—a qualified expert inspects the timber (most common method in Australia)

  2. Machine proof grading—timber is subjected to various bending loads up until breaking point to establish the appropriate grade

  3. Machine stress grading—similar to machine proof grading however looking for proof of stress versus absolute failure levels 

AS1720 sets out the specific properties of each of the 10 F grades commonly used in Australia. Each grade encompasses a value for:

  • MPa bending strength

  • MPa compression strength 

  • MPa tensile strength

  • MPa shear strength

  • Elasticity of the wood

allowing users to pick the right grade of timber for the engineering demands of their project.​

Machine Graded Pine (MGP) grades are a more accurate method of assessing the structural capacity of pine than F grading and as such are more commonly used for pine products.