Hazard levels & stress grading

In order to provide assurance that it will meet engineering standards all timber must be classified with a Hazard Level (treatment level) and a Stress Grading. As similar species of trees can provide different quality of timber it is very important that a classification system be in place to ensure safe building practices.

Hazard levels of timber

Hazard Levels are quite simply guidelines providing information as to which type of treatment is suitable for which purpose. The hazard levels range from H1 (lowest) to H6 (highest).

  • H1 is the lowest level protecting against insects other than termites as well as Lyctiid and Anobiid attack. H1 timber can be used indoors and above ground for flooring or furniture. both CCA and LOSP treatments are suitable for use as H1.
  • H2 treatment level protects against Termites and Borers and can be used inside in an above ground application, ususally for framing and flooring. CCA, ACQ and LOSP treatments are all suitable for use as H2.
  • H3 treatment allows timber to be used outside in an above ground application. H3 protects against moderate decay, borers and termites and can be used for cladding, fascia, pergolas and decking. Treatments appropriate for H3 level are CCA, ACQ, TanE and LOSP.
  • H4 treatment protects against severe decay, termites and borers and is suitable for use outside and inground. CCA, ACQ and TanE can all be used to treat timber to H4 level. H4 timber is commonly used for fenceposts, pergolas, greenhouses and landscaping applications.
  • H5 Treatment protects against very severe decay, termites and borers. Both CCA and ACQ treatments are suitable for H5 levels. H5 timber can be used outside, inground and in contact with fresh water. It’s common applications include usage for retaining walls, house stumps and building poles.
  • H6 treatment protects timber against Marine Borer attack and decay. This treatment can be provided by CCA as well as Creosote treatments. H6 timber is appropriate for use in marine waters and is commonly used for boat hulls, jetty cross bracing and landing steps.

Structural grading of timber

In addition to Hazard Level treatments, timber is also classified according to a Strength Grading. Many factors are taken into account in the Stress Grading of timber including, length, density, flexibility, rigidity and natural imperfections. Stress Grading of timber can be done by a machine or visually.

Machine Grading of timber is a process whereby a calibrated machine computes a stress grade according to the weight and strength of the timber. This method of grading timber is less reliable than visual grading as it doesn’t take into account imperfections within the timber. Treated pine is available in grades MGP 10 and MGP 12.

Stress Grading of timber can also be done visually. The visual grader of timber evaluates the size, frequency and significance of imperfections as well as the species and other properties of the timber to calculate a Stress Grading. Visual Stress Gradings can range from F1 to F27.

Treated Pine is usually classified as either F5 or F7, F7 being seasoned treated pine and F5 being unseasoned (wet) treated pine.