Laminated Veneer Lumber

LVL is commonly used in the fenestration industry as a superior core substrate in exterior stile and rail doors and as window components. When originally developed in the 1960s, however, LVL was primarily a structural product for floor joists, rim joists, scaffolding, studs, headers and roof trusses.

The LVL manufacturing process begins with rotary cutting raw logs into green veneer, which is then dried to a moisture content ranging from 5 percent to 10 percent. Various LVL manufacturers use different softwood and/or hardwood species, depending on application performance or machin-ability requirements. Each piece of veneer is visually graded, grouped according to physical defects such as knot size, coated with a waterproof adhesive and pressed under heat and high pressure.

The panels are then sawn, patched and sanded in the form of a billet, which can range in sizes up to 24 feet long. An LVL billet can be processed and machined into either finished or component products, such as those used for manufacturing windows and doors.

LVL is available in a variety of dimensions and can be custom designed and manufactured to minimize wood fiber waste. It also meets the requirements of "green" architecture, in that these veneer and gluing processes enable the end product to be made from relatively small trees of many species, which provides for efficient utilization of wood fiber resources.

The theory behind why LVL can be superior over today's solid wood stems from defect randomization. When a log is processed into thin veneers and the veneers sorted before they are glued back together, the natural defects (e.g., knots and cross grains) are randomized throughout the piece. As a result, LVL products provide greater dimensional stability due to the randomizing of natural defects.

Strength and stiffness can also be enhanced when compared to solid wood, depending on how the LVL is engineered. Structural design values are based on overall strength in addition to strength variability. LVL typically exhibits higher strength, lower variability, and therefore higher design values for structural applications. This feature allows wood fiber, in the form of LVL, to be used more wisely and more efficiently as a natural resource.

LVL is sometimes considered a commodity product when being used as structural elements in the building construction industry. LVL used for fenestration components is a specialty and requires specific design and machining properties based on end use component requirements. One LVL manufacturer provides more than 400 active lay-up designs consisting of different veneer species, veneer thicknesses and grain orientation of individual veneers. Specific sequencing of veneer types throughout the thickness of the billet defines the enhanced properties and overall performance.

When properly engineered, LVL lends itself well to the applications where solid or finger-jointed wood may not offer adequate performance characteristics. As building codes evolve and design pressure requirements increase, LVL components can be used to help make doors and windows much more robust. Common applications would include stiles and rails for entry and patio doors, mullposts, mullions, astragals, mull stiffeners and window jambs.