CLT panels are usually manufactured with an odd number of layers, which creates a direction of greater strength for a specific application (floor, roof or wall). Although panels have strength capacity in both directions like a plate element, the direction parallel to the grain of the outside laminations is typically the stronger axis (i.e., major strength direction). Exact lamination thickness and number of layers varies by application and manufacturer.¹⁵

CLT panels Photo courtesy of FPInnovations

CLT's dimensional stability is rooted in its manufacture and is the result of two basic factors:

• The lumber itself has an average moisture content (MC) of about 12%, which is relatively dry. Dryer lumber means less overall shrinkage when the lumber acclimates to the service condition. Also, because smaller-dimension lumber can be used, the moisture content tends to be consistent throughout the product.

• The nature of cross lamination is also a major contributor. Wood expands and contracts five to ten times more tangential to the grain than parallel to the grain. The alternating orientation of layers in CLT has the effect of restricting overall expansion and contraction of the panel in both directions.

The exact weight of CLT is dependent on the density of the species used and thickness of the panel (for values, see the National Design Specification® (NDS®) for Wood Construction), but may be estimated at about 35-36 pounds per cubic foot. Manufacturers provide detailed information for their product offerings.

Structural design is discussed in the next section, but in general there are a number of simple details that can be used to establish roof/wall, wall/floor, and inter-story connections in CLT assemblies, to connect CLT panels to other wood-based elements, or to connect CLT panels to concrete or steel for a hybrid assembly. The type of hardware depends on the assemblies to be connected, on the panel configuration, and on the type of structural system used in the building. Some of the more common lateral load transfer connectors include metal 'L' brackets for shear or traditional strap hold-downs used for overturning.

Proprietary self-tapping screws or a variety of commodity nails can be used to connect panels in plane or at intersections. Ease of installation along with high lateral and withdrawal capacity make self-tapping screws especially popular because they can take combined axial and lateral loads.

CNC technology is also driving the development of new and innovative fastening systems. For example, glued-in rods can be used for connections under high longitudinal and transverse loads, and epoxied shear connectors can be used to create composite floors with structural concrete over CLT.

Detailed guidelines on how to design structural elements using CLT are included in the U.S. CLT Handbook.

CLT and Mechanical/Electrical/Plumbing

When the CLT is exposed on the interior, these are some of the ways MEP is being addressed:

• Fur out a wainscoting on the lower half of the wall to accommodate electrical outlets and plumbing pipes.
• Run the conduit, pipes and chases on the face of the CLT and use them as exposed architectural elements.
• Add stick-framed partition walls on the interior of the structure to accommodate MEP.
• Create a drop ceiling to conceal the MEP.
• Gap the CLT panels to accommodate plumbing and mechanical and add a thin faux panel on the face.
• Create a non-structural built-up floor on top of the CLT floor deck to accommodate plumbing and mechanical, similar to the approach with heavy timber.

When the CLT is covered, there are two basic approaches:

• Rout and bore the panel to take the electrical conduit, plumbing pipes and mechanical chases. This can be done on site with traditional carpentry tools or in the factory using CNC technology, but the latter would require more coordination and would increase the cost of the product.
• Fur the wall out to accommodate MEP. This is likely to be the more common method in North America based on building practices and need to satisfy sound insulation requirements.

Local MEP codes may have an impact on the options available. In addition, note that “concealed spaces” are not allowed in Type IV (heavy timber) construction, which is where CLT is being added under the IBC.