This brief overview of the types of mass-timber systems from WoodWorks, While innovative systems continue to be developed within the mass timber category, common products currently include: 

Cross-laminated timber (CLT) – CLT consists of layers of dimension lumber (typically three, five, or seven plies) oriented at right angles to one another and then glued to form structural panels with exceptional strength, dimensional stability, and rigidity. CLT can be used for walls, floors, and roofs—as a stand-alone system or with other structural products (e.g., post and beam)— and is often left exposed on the interior of buildings. Because of the cross-lamination, CLT offers two-way span capabilities. 

Glued-laminated timber (glulam or, when used as panels, GLT) – Glulam is composed of individual wood laminations (dimension lumber), selected and positioned based on their performance characteristics and bonded together with durable, moisture-resistant adhesives. These adhesives are applied to the wide face of each lamination. Glulam has excellent strength and stiffness properties, and is available in a range of appearance grades. It is typically used as beams and columns, but can be used in the plank orientation for floor or roof decking. It can also be curved and bent, lending itself to the creation of unique structural forms. 

Nail-laminated timber (NLT) – NLT is created from individual dimension lumber members (2-by-4, 2-by-6, etc.), stacked on edge and fastened with nails or screws to create a larger structural panel. Commonly used in floors and roofs, it offers the potential for a variety of textured appearances in exposed applications. Like glulam, NLT lends itself to the creation of unique forms, and wood structural panels (WSPs) can be added to provide a structural diaphragm. 

Dowel-laminated timber (DLT) – Common in Europe and gaining popularity in the U.S., DLT panels are made from softwood lumber boards (2-by-4, 2-by-6, etc.) stacked like the boards of NLT but friction-fit with hardwood dowels. The dowels hold each board side-by-side, while the friction fit adds dimensional stability. 

Structural composite lumber (SCL) – SCL is a family of wood products created by layering dried and graded wood veneers, strands, or flakes with moisture-resistant adhesive into blocks of material, which are subsequently re-sawn into specified sizes. 

Two SCL products—laminated veneer lumber (LVL) and laminated strand lumber (LSL)—are relevant to the mass timber category as they can be manufactured as panels in sizes up to 8 feet wide, with varying thicknesses and lengths. Parallel strand lumber (PSL) columns are also commonly used in conjunction with other mass timber products. 

Tongue and groove decking (T&G) – Structural T&G decking is made from lumber at least 1-1/2 inches thick, with the flat (wide) face laid over supports such as beams or purlins for floors and roofs. Available in a variety of species, thicknesses and lengths, it is used where the appearance of exposed wood decking is desired for aesthetics or where its mass is desired for fire resistance