In this edition of Technical Matters, we will explore the need for lumber grading and the methods used to achieve different grades for different end uses.
As a general rule with all lumber that is sawn from a log, there is a need for some method of sorting or grading of the sawn boards into categories as they are produced. This allows the end user or customer to be able to choose or differentiate the selected boards as being fit for the purpose for which they intend to use them. Hardwood lumber is usually graded on the basis of how much clear (free of defect) wood can be cut from a sawn board when it is cut up and manufactured into a hardwood product, such as a door or cabinet. Different grades suit different purposes, and the grade acts as a description of what the lumber should be expected to look like.
Both North American and European temperate hardwoods (and tropical hardwoods) are almost always graded on appearance, as they are generally used for more decorative purposes than softwoods. All wood is, by its very nature, a natural material and by definition has natural defects and characteristics, which determine how it looks. When we talk about grading on appearance, we are determining how each board ‘looks’ in terms of the number of defects or characteristics that there are on the surface of each individual board. The higher the percentage of clear wood that is on the board, the higher the grade of the board. Of course, the grade of the board also determines its value. The clearer the board is with fewer defects, the higher its value.
There are two methods of determining the grade of hardwood lumber by appearance. The first is by visually assessing the defects in a sawn board by an individual lumber grader. At this point, each board is assigned a grade solely on the judgement of the grader on the basis of how the board looks. This ensures that each individual board within a pack of lumber can be sold ‘as seen’ and can be said to have been graded without any consideration of its intended end use. This method is arbitrary and depends entirely on the experience of the grader and is open to individual interpretation. This can produce significant discrepancies in what is supposed to be the same grade from different sawmills and producers. This system is how European temperate hardwoods tend to be graded. Different countries within Europe which supply hardwood lumber, such as France, Ukraine and Croatia may all have different interpretations of how their production meets a particular grade.
The second type of hardwood grading is by the ‘cutting unit’ method, as used by the North American hardwood lumber industry based on the National Hardwood Lumber Association (NHLA) rules for grading hardwood lumber. These NHLA rules give a clearly defined mathematical calculation of determining the grade of a sawn hardwood board by measuring the amount of clear wood in each board. These rules provide both the buyer and seller with a consistent set of guidelines with which to specify and trade in North American hardwood lumber.
The NHLA rules were designed with the furniture trade in mind, in order to provide a measurable percentage of clear or defect-free wood for each grade. For the highest grade, known as FAS (Firsts & Seconds) 831/3 % or 10/12 must be clear wood (i.e. wood free of defect). The cutting unit method restricts the number of defects and their location on the board and also limits the total number of cuttings allowed in any board to remove those defects. The larger the dimension of the board, then the more cuttings you can take to get the clear pieces required to make the grade. Another consideration is that the clear pieces of wood must meet the minimum specified lengths and widths for that particular grade. For FAS, the minimum size is 4”x 5’ and 3”x 7’ - long enough to make door stiles, for example.
As the grade of the board gets lower with the presence of more defects and less clear wood, the yield requirement to achieve the grade also reduces along with the minimum specified lengths of the pieces. The grade after FAS is known as No.1 Common. For this grade, the percentage of clear wood needs to be 66 2/3% or 8/12, and the minimum size that the pieces need to be is 4”x 2’ or 3”x 3’. This grade is ideally suited to the needs of the cabinet industry. It was originally known as the ‘cabinet grade’. For a more rustic grade, then No.2 Common is the one to choose, needing only 50% clear wood and minimum-sized pieces of 3”x 2’.
The cuttings obtained from the lower grades will be the same clear wood and of the same quality, just in smaller pieces. It is important to investigate the use of the lower grades where possible, as it is hugely beneficial to the hardwood industry to utilize all of the boards that are cut from a log. At the same time, it is important for the end user to make sure that they are not using too high (and too expensive!) a grade for a particular application.
The NHLA rules described above and adopted by the North American hardwood industry can be a daunting prospect to fully understand, but they do establish the comparable value of boards, and provide the buyer and end user with a consistent standard by which they can purchase timber for a particular end use. These standard grading rules generally apply to all North American hardwood species that are commercially available from the United States with just a few exceptions.