Virtually free from warping, splitting and shrinking, composite lumber products out-perform conventional lumber when either face- or edge-loaded. Composite lumber is generally manufactured with the grain of each layer of veneers, strands or flakes oriented parallel to the length of the billet. Because the grade and quality of each layer can be closely controlled, variations in product properties are lower than in conventional sawn lumber products. As a result, the properties and performance of composite lumber can be more confidently predicted.
The manufacture of composite lumber represents an efficient utilization of wood resources. Large composite lumber billets are produced from a variety of tree species, including species that are relatively small, underutilized, fast-growing, and sometimes considered low-grade.
Structural properties of composite lumber are evaluated using methods specified in ASTM D 5456, Standard Specification for Evaluation of Structural Composite Lumber Products. The structural design values for composite lumber are published on a proprietary basis by manufacturers of composite lumber and are typically recognized in evaluation reports published by the ICC ES and Product Reports issued by APA.
LVL is the most widely used composite lumber product. It is produced by bonding thin wood veneers together in a large billet. The grain of all veneers is parallel to the long direction. The LVL billet is then sawn to desired dimensions depending on the construction application. The resulting product features enhanced mechanical properties and dimensional stability that offer a broader range in product width, depth and length than conventional lumber. The many uses of LVL include headers and beams, hip and valley rafters, rim board, scaffold planking, studs, flange material for prefabricated wood I-joists, and truss chords.
PSL consists of long veneer strands laid in parallel formation and bonded together with an adhesive to form the finished structural section. PSL is commonly used for long-span beams, heavily loaded columns, and beam and header applications where high bending strength is needed.
Similar to PSL, LSL and OSL are manufactured from flaked wood strands that have a high length-to-thickness ratio. The primary difference between them is that the length of strand used in OSL is shorter (up to 6 inches) than that used in LSL (approximately 12 inches). Combined with an adhesive, the strands are oriented and formed into a large mat or billet and pressed. Although LSL and OSL generally have somewhat lower strength and stiffness properties than LVL and PSL, these products offer good fastener-holding strength and mechanical connector performance. LSL and OSL are commonly used in a variety of applications, such as beams, headers, studs, rim boards, and millwork components.