Strength grading of sawn timber: visual, machine and the C-classes

Why grade for strength

Boards from the same log differ widely in strength because of knots, grain slope and shakes. Strength grading sorts them so a designer can rely on a stated strength.

Two ways to grade

• Visual grading assesses features such as knots, slope of grain and shakes. Quality still varies considerably between pieces within a grade.
• Machine grading measures a property such as stiffness or density that correlates with bending strength. It lets designers use the full design strength of each piece.

The European system

In Europe, strength grading follows EN 14081. Common softwood classes are C16, C18, C24 and C30. The number is the required 5th-percentile (characteristic) bending strength in N/mm². A C24 board therefore has a characteristic bending strength of 24 N/mm².

Other regions have their own systems. In the United States the American Lumber Standard Committee sets the framework; in Canada the National Lumber Grades Authority writes the grading rules.

What the sawmill has to do

• Grade by an accredited method, with the grading machine or inspector under control of a certification body.
• Mark each piece so the strength class, species group and moisture condition can be traced.
• Deliver the moisture content that the product standard expects. Dimensions and strength both change with moisture.

Practical advice

Ask the buyer which standard and class applies before quoting. A price for "C24" and a price for "visually graded, no class" are not comparable.