Lumber grading is a system for evaluating the appearance or strength of a piece of wood. The grade indicates the purpose for which the lumber is considered acceptable or the percentage of non-defective or clear wood available from a board of specific dimensions. Defects commonly include knots, stains, and splits that compromise either the appearance of the board or its structural integrity. Hardwoods such as oak and maple are graded differently than softwoods, and individuals who perform lumber grading on hardwood require no official certification. Softwoods, such as pine and spruce, are often used in building construction and must be evaluated by certified graders.
In the US, standards for hardwood lumber grading are established by the National Hardwood Lumber Association while the US Department of Commerce issues the US Softwood Lumber Standards. The lumber grade will usually appear on the lumber itself. Softwood is generally more consistently marked than hardwood.
Depending on the amount of defect-free wood required, different grades of hardwood lumber are commonly used for different projects. Long pieces of decorative molding require longer pieces of clear wood, while shorter pieces are needed for furniture and cabinetry. Likewise, defects in appearance are less important if the wood is to be painted.
Hardwood is graded based on the percentage of clear, or defect-free, surface area that can be obtained from the most defective side of a board. For example, the highest grade hardwood board must have a minimum length of 8 feet (2.43 m) and a minimum width of 6 inches (15.24 cm), and it must be possible to cut the board into four pieces of clear wood ranging from 3 x 7 inches (7.62 x 17.78 cm) to 4 x 5 inches (10.16 x 12.7 cm). If both faces of the board meet this requirement, it is graded as firsts and seconds (FAS).
Lumber that is graded FAS-1 face (FA1) or select is often used in place of FAS lumber. Grade FA1 lumber has one side which meets the FAS criteria, but the side with the lowest percentage of clear wood meets only the number 1 common grade. Select lumber meets the surface area requirements for number 1 common, but otherwise meets the FAS criteria.
Common grade numbers 1, 2A, 2B, 3A and 3B for hardwood require less total surface area and a lower percentage of clear wood than FAS lumber. The percentage of clear wood required decreases as the grade decreases, and beginning with grade 2B common, the clear wood may include "sound" defects, including stains and intact knots. Number 1 common is also called construction grade, and is the grade most often used to build furniture, while number 2A is considered acceptable for cabinetry and 2B for painting.
Softwood lumber grading can be based solely upon structural integrity or upon a combination of structural integrity and appearance. Stress-graded or dimension softwood is used mainly in the construction of buildings and is evaluated only on the basis of mechanical properties, such as bending strength. Woodworkers and craftsmen most often use either nonstress-graded or appearance-graded softwood, both of which are graded on a combination of strength and appearance. Unlike hardwood lumber grading, softwood lumber grading is based on least defective or best side of the board.
Non-stress or common graded softwood is graded for strength and appearance, though strength is the primary consideration and appearance is given less weight as the grade decreases. The highest grade is number 1 and has the fewest defects. Grade 3 and below are used only if appearance is not important.
Softwood that is graded primarily for appearance is called select lumber and is graded as select A, B, C, or D as the number of defects in the board increase. Even grade D select, however, should contain only small defects. When offered for sale, grades A and B are sometimes combined and marked "B&BTR," which stands for B and better.
Strength and various other mechanical criteria are used to grade dimension softwood. Appearance is not taken into account. This lumber will be used only for construction and will not be visible when the construction is complete.