When buying green oak timber, it is important to assess the quality of the wood. Some wood may look strong but have knots and built-in weaknesses that will sacrifice its strength or make it difficult to cut. Two types of grading exist to make decisions when buying wood. First, appearance grading is designed for non-structural applications such as furniture. Second, strength grading is meant to grade the ability of the wood to withstand pressure and stress.
Types of Defects
The most common type of defect in green oak is a knot. Knots in wood can be difficult to overcome when cutting, but there are several other potential defects to be aware of. These can include tension wood, spiral grain, ring shakes and fungal degrade. It is the job of the lumber supplier to make sure that these are not present in their commercial wood, but some suppliers sell wood with these defects.
The most important characteristic to look for when buying timber is the frequency of knots. Knots make wood weaker, more susceptible to warping and can make it difficult to create strong joints between boards. When choosing timber, remember that some of the potential defects that will affect your dry board will not be able to be seen yet because the timber is green. Splints, for example, will not be visible on green wood.
Green oak is given one of three grades -- A, B or C -- based on the bending strength ratio. The bending strength ratio is measured as the strength of bending the given log divided by the strength of bending a perfect log. A grade of A is the highest rating with a bending strength ratio of 71 per cent or more and is normally given only to wood designed for exceptional structural duties. A grade of B corresponds to a bending strength ratio of between 62 and 70 per cent. And a grade of C corresponds to a bending strength ratio of between 48 and 69 per cent.
Application of Grades
It is often inefficient to use grade A wood for each aspect of a project. In a normal construction job, most of the wood should conform to a B grade. Wood that will be subject to less stress can be of C grade. Remember that the grading process is not an exact science, and deciding which wood to use for which applications will require judgment and careful consideration.