Finger-joined lumber is becoming a very popular and desirable product in the construction industry’s repertoire of engineered wood products. Its popularity can be attributed several factors:
- dimensional stability
- interchangeability with non-finger-jointed lumber
- efficiency in wood fiber use
Along with this growth comes the need to educate architects, designers, engineers, building inspection officials, and end users about the advantages and acceptability of this product.
- Check out Performance of Adhesives Used in Finger-Joined Lumber in Fire-Resistance-Rated Wall Assemblies
What is Finger-joined Lumber?
Finger-joined products are manufactured by taking shorter pieces of quality kiln-dried lumber, machining a “finger” profile in each end of the short-length pieces, adding an appropriate structural adhesive, and squeezing the pieces together to make a longer piece of lumber. Although finger-joining is used in several wood product manufacturing processes including the horizontal joints for glulam manufacture, the term finger-joined lumber applies to dimension lumber.
The two major advantages of this product are its straightness and dimensional stability. The straightness factor is the result of stable short length pieces of lumber being combined in the manufacturing process. With finger-joining, the length of a piece of lumber is not limited by tree size. In fact, the process may result in the production of joists and rafters in lengths of 12m (40′) or more. Another advantage is the greater value derived from the forest resource since the short-length pieces can be cut out of lower grade lumber.
The finger-joining process allows the removal of strength reducing defects to produce a product with higher engineering properties. The strength of the joints is controlled by stipulating the quality of wood which must be present in the area of the joint.
The structural properties are confirmed through a comprehensive quality assurance program with independent third party verification. Daily structural tests are certified to verify that the product meets the requirements as set out by the North American lumber grading system. Each piece must be comprised of species from the same species group, and strict tolerances are established for the machining of the fingers; the quality, the mixing, and the curing of the adhesive. Depending on the type of finger-joined lumber being manufactured, edge and flat bending tests and tension tests are performed on each piece to ensure the joint can meet the design value for the lumber.
There are two categories of finger-joined lumber depending on the intended end use. The first category, sometimes referred to as a structural fingerjoint, uses a phenol-resorcinol formaldehyde adhesive, such as used in panel products, or in glued-laminated timber. This allows the product to be used in either vertical or horizontal load applications. The second category, VERTICAL STUD USE ONLY, typically uses a polyvinyl acetate adhesive and, as indicated by its name, is for vertical use only [i.e. studs]. Both products may be used interchangeably with solid sawn lumber in terms of strength and end use. A description of both products is provided in Table 1 below.