Visual Stress Grading
Visual grading relies on the ability of a visual inspection by a trained timber grader to visually identify any imperfections that may reduce the structural capacity of the timber. Generally, visually graded timber will be at the lower end of the timber grading scale, and being in the ‘F’ structural grade range. The problem with this method is that a timber grader may miss structural deficient imperfections which may result in the timber being incorrectly graded. This method of grading timber for structural use is conducted by visually inspecting a combination of the elements in the timber (Eg: Timber species, Unseasoned/Seasoned timber, knot size/location, machining hit/miss, checking, bow/cup/twisting) and cross referencing with the relevant Australian Standard. These timbers are then rated to a ‘F’ structural rating (F4, F5, F7, F8, F11, F14, F17, F22, F27, F34 and F43). Timber that is structurally graded is marked with a specific colour after grading, which helps to visually identify the graded classification.
AS 3818.1 Timber Heavy Structural Products – Visually Graded – Part 1 General Requirements
AS 3818.10 Timber Heavy Structural Visually Graded- Part 10 Building Poles
AS 2082 Timber – Hardwood – Visually Stress Graded for Structural Purposes
AS 2858 Timber – Softwood Visually Stress Graded for Structural Purposes
This method of grading timber for appearance use is conducted by visually inspecting a combination of the
elements in the timber (Eg: Timber species, Unseasoned/Seasoned timber, knot size/location, machining
hit/miss, checking, bow/cup/twisting and other associated timber deficiencies) and cross referencing with the
relevant Australian Standard.
Appearance graded timber is usually associated with products such as Flooring, Decking, Cladding and Joinery
Common terms for Appearance Grading timber:
Select/Clear grade: This is the best visual appearance grade and is virtually free from visual defects. This grade is
used in furniture or where a feature may be made of an exposed timber.
Standard grade: This is the middle range grade and is used for various purposes such as decking, flooring or
Utility/Merchant grade: This is a non-structural grade and is used for economy reasons where a structural or
appearance grade is not required. This timber may contain knots, sapwood and colour variations.
AS 2796.1 Timber Hardwood Sawn and milled products – Part 1 Product Specification
AS 2796.2 Timber Hardwood Sawn and milled products – Part 2 Grade Description
AS 4785.1 Timber Softwood Sawn & Milled Products – Part 1 Product Specification
AS 4785.2 Timber Softwood Sawn & Milled Products – Part 2 Grade Description
Mechanical Grading Method
Mechanical Grading relies on timber being passed through a machine and stress placed on the timber in a
downward action, the deflection of the timber is measured and therefore the timber grade determined on this
amount of deflection.
The stress placed on the timber is usually on its Minor Axis.
Machine Graded timber is classified into further structural grade classifications.
These are ‘F’, ‘MG’ (Machine Graded, eg; MGP10).
The general timber grading classifications are:
Unseasoned timber: Seasoned timber: (Kiln Dried)
Machine Proof Grading
Machine proof grading begins with timber being sorted into ungraded groups.
Timber is then tested by applying a pre-determined weight load (Bending Load) known as the ‘Proof Load’. As
this ‘Proof Load’ is applied along the timber axis it is constantly stressed until it reaches the pre-determined
stress grade limit.
The timber is graded according to its ‘Proof Load’ stress limits which are classified in:
AS 3519:2005 Timber – Machine proof grading.
In determining the Machine Proof Grading the timber is also classified according to its cross-sectional
dimensions which give the timber its proof load.
MGP10, MGP12 and MGP 15 have their grading classification stamped along the length of the timber,
instead of a colour as per the ‘F’ Grade timbers.
Differences between Mechanical grading and Machine Proof Grading:
Mechanical Grading Machine Proof grading
Timber loaded about minor axis (on flat) Timber loaded about major axis (on edge)
Small loads applied – timber is loaded at
much less than design strength
High load applied – timber is loaded at loads
near the design strength
Intention is to find poor pieces by measuring
Intention is to find poor pieces by breaking
High speed operation – timber in the machine
for typically 1 second
Low speed process – timber may take 15 to
20 seconds to pass through the machine