Design with Wood

Cross Laminated Timber (CLT)

video courtesy naturallywood.com

Cross laminated timber (CLT) is an engineered wood building system designed to complement light- and heavy-timber framing options. Because of its high strength and dimensional stability, it can be used as alternative to concrete, masonry and steel in many building types.

Having gained popularity in Europe over the past 20 years, CLT is now available to North American building designers. It offers the structural simplicity needed for cost-effective buildings, as well as benefits such as fast installation, reduced waste, improved thermal performance, and design versatility. It can be used in a wide range of applications, including mid-rise, urban infill, industrial, educational and civic structures.

Free One-on-one Project Support

If you’re working on a project where you are considering using Cross Laminated Timber and could use some free technical support—or you want to learn more about innovative massive timber construction—contact your local Technical Director or WoodWorks’ building system lead, Lisa Podesto.

Design Tools

Estimators & Calculators

Case Studies

Design Examples

Codes & Standards

Research

  • The Case for Tall Wood Buildings – A feasibility study from the Canadian Wood Council illustrates how massive timber products and systems can provide viable and sustainable alternatives for the construction of 10 to 30-story buildings. Canadian Wood Council

Videos

Availability and Cost

1. What companies are manufacturing and supplying CLT in North America?

The North American producers currently supplying CLT in the US are:

North American distribution may also be available through:

2. Is CLT more cost effective for certain types of construction than others?

Light wood-frame construction is the most economical wood system for low-rise projects. CLT becomes more competitive at higher building heights when competing with other building materials. It is most cost effective in large, regular shaped structures or in situations where fabrication is repetitive. Most industrial buildings showed similar costs for CLT when compared to concrete. Besides mid-rise and industrial, retail (1-2 stories) and educational (2-3 stories) may also prove to be cost effective for CLT.

3. What is the cost of CLT per square foot with a North American supplier?

CLT is not a commodity product and it therefore doesn’t make sense to quote per square foot. The cost is dependent on many factors so it’s difficult for manufacturers to provide a generic estimate. If you have a preliminary plan, we suggest contacting one of the suppliers to request a quote. Factors that play a role include, but are not limited to:

  • Thickness and grade of panels
  • Is the CLT for a roof, floor or wall? Walls have more openings and require more CNC time.
  • How many panels are required? A few pieces for a small job cost more per item than a truckload as the tooling, set-up and delivery costs are all distributed.
  • Is it visually exposed? If yes, on one or both sides?
  • Do you want standard material species or custom?
  • Should freight to the site be included? Where is it located?
  • Should fasteners be included?
  • Will the manufacturer be able to use the full 8’ x 40’ panels? (If the full panel size isn’t used, portions may become waste.)
  • How easy is the project to optimize from a material usage standpoint? 

About CLT/Engineering Considerations

4. What species of wood are used for CLT?

One of the advantages of CLT is that it can make use of underutilized species and grades as well as small-dimension material, so a variety of species may be used. Two companies currently producing CLT in North America are using Black Spruce (Nordic Engineered Wood) and Spruce-Pine-Fir (Structurlam), although other species or mix of species may be available on request. The standard species per ANSI/APA PRG-320 Standard for Performance-Rated Cross-Laminated Timber are Douglas-Fir-Larch, Spruce-Pine-Fir, Southern Pine and various types of MSR lumber.

5. Does panel strength vary with species and grade of lumber?

Yes, but this isn’t something a designer needs to be concerned about and requests for a specific species would likely come at a cost. Designers should simply specify the required performance based on the application (floor, wall, roof, etc.) and request the appropriate grade of CLT. There are two routes a manufacturer can take to achieve a specific stress grade—prescriptive and performance. With the prescriptive route, a species and grade layup combination is dictated because it is known to have certain structural design properties. With the performance route, any species can be used if it is proven by the manufacturer to qualify as meeting the requirements of that grade. This is the approach taken with other engineered wood products such as wood structural panels. With either route, the grade of the CLT will be third-party certified. More information is available in the ANSI/APA PRG-320-2012 Standard for Performance-Rated Cross-Laminated Timber.

6. Do CLT panels always have an odd number of layers? What are the typical sizes?

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).

Lamina thickness and number of layers varies by application and manufacturer. Thickness of individual lumber pieces may vary from 0.625 in to 2.0 in (16 mm to 51 mm) and the width may vary from about 2.19 in to 9.5 in (60 mm to 240 mm). Panel sizes also vary and typical widths are 2.0 ft (0.6 m), 4.0 ft (1.2 m), 8.0 ft (2.4 m) and 10 ft (3 m). Length can be up to 60 ft (18 m) and the thickness can be up to 20 in (508 mm). (Source: Chapter 1 of the US CLT Handbook.)

7. What provides the dimensional stability? What are the thermal/moisture expansion characteristics of CLT?

CLT’s dimensional stability is rooted in its manufacture and is the result of a couple different things:

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 small dimension lumber is 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.

In terms of how it affects the overall dimensions of the building, here are two examples:

In the Stadthaus project, a building in the UK that includes eight stories of CLT over a one-story concrete podium, the potential for shrinkage due to compression was negligible for the walls and 0.6 mm (0.02 in) for the floors. Likewise, the potential for moisture expansion was negligible for the walls and 2 mm (0.07 in) for the floors, resulting in maximum settlement for the entire building of less than 1 in.

The engineer of the two-story Long Hall in Montana said the standard he used for CLT made from Spruce was: parallel to panel plane—a dimension change of 0.01% per % change in moisture content; perpendicular to panel plane—a dimension change of 0.2% per % change in moisture content. For a 40-ft-long, 4-in-thick panel with a 5% moisture change, that would mean a 0.24-in change in the length and 0.04-in change in thickness.

Dryer lumber also affects thermal expansion, but thermal expansion coefficients for lumber at 12% MC are generally only seen initially because any heat experienced will itself also effect the moisture content, which offsets the expansion and results in overall shrinkage.

8. How are design values established for various cross-sections of CLT?

The ANSI/APA PRG-320-2012 Standard for Performance-Rated Cross-Laminated Timber provides minimum design values for a given CLT layup. If the design values for a product differ from the primary stress grades provided in the standard, they will be provided by the manufacturer. Guidelines on how to design structural elements using CLT are included in the US CLT Handbook, which is free to download or available in hard copy for a nominal fee.

9. Can CLT be manufactured with the exterior pre-finished?

CLT itself should not be left exposed as an exterior finish. In Europe, it is not uncommon for CLT manufacturers to supply prefabricated panels with the exterior finishes (insulation, siding, windows, etc.) already applied and this is one of the advantages of the pre-fabricated nature of the product. However, North American manufacturers are not yet offering this level of prefabrication. It would be up to the contractor/installer to apply finishes before or after product installation.

10. What type of adhesive is used and is it waterproof?

The type of adhesive used at both North American manufacturing plants currently producing CLT is a formaldehyde-free structural adhesive with great moisture performance. Adhesives must be qualified in accordance with ANSI/APA PRG 320 for bonding strength, moisture durability, elevated temperature performance, and heat durability.

11. How stable is CLT in changing humidity conditions? Does it need painting more often than dimension lumber?

CLT offers excellent dimensional stability (see question 7). It doesn’t need to be painted any more than another wood product; however, CLT should not be left exposed to weather or used as the exterior finish of a building.

12. What is the average thermal conductivity (k) value of CLT?

The thermal conductivity of CLT is the thermal conductivity of the wood itself. The American Wood Council provides this explanation:

What is the thermal conductivity of wood and how does it compare to other materials?

Thermal conductivity is a measure of the rate of heat flow through one unit thickness of a material subjected to a temperature gradient. The thermal conductivity of common structural woods is much less than the conductivity of metals with which wood often is mated in construction. It is about two to four times that of common insulating material. For example, the conductivity of structural softwood lumber at 12% moisture content is in the range of 0.7 to 1.0 Btu×in/(h×ft^2×oF compared with 1,500 for aluminum, 310 for steel, 6 for concrete, 7 for glass, 5 for plaster, and 0.25 for mineral wool.

In Chapter 3 of the USDA Forest Products Lab Wood Handbook, Table 3-11 entitled thermal conductivity of selected hardwoods and softwoods lists thermal properties for various species of wood.

13. What is the insulation (R ) value of CLT?

The R value of a CLT system varies depending on the thickness of the panel. The commonly used R value for wood is 1.25 per inch of thickness. So a 7-inch-thick panel would have an R value of 8.75.

One advantage of CLT is that wood is a good thermal insulator, which contributes to the overall R value of the building system. Softwood has about half the thermal insulating ability of a comparable thickness of fiberglass batt insulation, but about 10 times that of concrete and masonry, and 400 times that of solid steel. (Source: Thermal Performance of Light-Frame Assemblies, Canadian Wood Council)

14. How much do CLT panels typically weigh per gross square foot?

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

15. Is preservative treated CLT available?

Not yet. Research on the use of treatments with CLT has not yet been conducted.

16. Is CLT available with certified wood (e.g., FSC or SFI)?

CLT made from Forest Stewardship Council-certified wood is available via Nordic Engineered Wood Products and qualifies for a point within LEED. Structurlam utilizes lumber certified through the Canadian Standards Association’s Sustainable Forest Management program, which is recognized by other green building rating systems. Structurlam also utilizes lumber from Canadian forests killed by the Mountain Pine Beetle. If LEED certification is an objective, this could potentially qualify for an innovation point.

17. Are panels certified by a third-party agency?

The ANSI/APA PRG-320-2012 Standard for Performance-Rated Cross-Laminated Timber requires CLT products to be trademarked by an accredited third-party agency. The 2015 International Building Code will reference this product standard. Panels will be subject to third-party inspection just like other engineered wood products such as wood structural panels, structural glued laminated timber, structural composite lumber, or I-joists.

Building with CLT

18. What type of connections are recommended/approved for CLT?

CLT connections should adhere to the National Design Specification® for Wood Construction (NDS®) just like other wood connections. The NDS has specific provisions for commodity fasteners, which can be found in Appendix K – L. The NDS does not yet include information on how to apply its dowel bearing equations regarding grain direction to CLT. However, the existing NDS design provisions can be applied with some modification, assuming that the CLT dowel bearing strength is based on the species-specific gravity of the face ply and the loading direction relative to the grain angle of the face ply.

Self-tapping screws will likely be the most common connector used in CLT construction. These are proprietary connectors and design values and requirements would be specified by the manufacturer. The manufacturer will be responsible for providing lateral and withdrawal connection values and any information needed to explain how to use provisions of the NDS (e.g. dowel bearing strength adjustments, dowel bending strengths of the self-tapping screws, and specific application of the NDS yield equations).

Design values for proprietary fasteners and information on their approved use are available in Evaluation Reports or the manufacturer’s literature.

19. Is it possible to modify CLT in the field if changes are required?

CLT can be modified on-site pending approval by the engineer of record. That is one of the many advantages of CLT over a precast concrete wall system; modifications can be made with relative ease in the field.

20. How is mechanical, electrical and plumbing (MEP) installation handled in CLT walls?

The options for running MEP depend on whether the CLT application is exposed.

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 our building practices and need to satisfy sound insulation requirements.

When the CLT is exposed, 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 it as an exposed architectural element.
  • 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 built-up floor on top of the CLT floor deck to accommodate plumbing and mechanical, similar to the approach with heavy timber.

Note: 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 International Building Code. There may be different challenges with each of the options in order to accommodate fire and structural provisions in the code.

21. What's the highest number of stories you can build with CLT?

In the United Kingdom, there are two eight-story CLT buildings (one of which includes a one-story concrete podium), and a 10-story CLT building has just been completed in Australia. A 2012 report—The Case for Tall Wood Buildings; How Mass Timber Offers a Safe, Economical, and Environmentally Friendly Alternative for Tall Building Structures—proposes the feasibility of using mass timber building techniques and engineered wood products such as CLT to build structures up to 30 stories, though at this height designers would be looking at a hybrid system as opposed to all CLT.

Code-allowed height for combustible buildings varies around the world. Currently, US and Canadian building codes do not explicitly recognize mass timber systems, but this does not prohibit their use under alternative method provisions.

In the 2015 IBC, recently approved changes will streamline the acceptance of CLT buildings. The 2015 IBC will recognize CLT products when they are manufactured according to the ANSI/APA PRG-320-2012 Standard for Performance-Rated Cross-Laminated Timber. In addition, CLT walls and floors will be permitted in all types of combustible construction, including Type IV buildings. For more information, see the news release issued by the American Wood Council.

One of the primary challenges facing tall wood building designers is the fact that residential buildings over six stories in height (and exceeding a certain total building area) are required to be of non-combustible construction, so gaining access to this market will require well-proven fire performance. Recent fire resistance testing conducted by the American Wood Council confirmed that CLT exterior walls exceed the requirements for heavy timber construction. Fire resistance testing of CLT floor and wall assemblies conducted at FPInnovations also confirms this. Because CLT construction typically eliminates concealed spaces, this enhances its ability to meet Type IV construction requirements.

Building Code Requirements and Acceptance

22. Once a client and architect have decided to use CLT, what is the engineering process?

An engineer of record (EOR) is required for a CLT project; however, the CLT manufacturer would act similarly to a truss manufacturer in that they have staff engineers that provide shop drawings and calculations, etc. that meet the EOR’s specifications. More interaction between the EOR and manufacturer may be necessary in order to optimize the CLT system, but the manufacturer does not act as the EOR. However, the manufacturer may be able to recommend an EOR familiar with CLT design.

23. How have building codes and/or officials responded to CLT projects?

In the 2015 IBC, recently approved changes will streamline the acceptance of CLT buildings. In May 2012, APA published ANSI/APA PRG 320-2012 Standard for Performance-Rated Cross-Laminated Timber, which details manufacturing and performance requirements for qualification and quality assurance. The 2015 IBC will recognize CLT products when they are manufactured according to the standard. In addition, CLT walls and floors will be permitted in all types of combustible construction, including Type IV buildings. Type IV wall provisions require the exterior side (only) of exterior CLT walls to be protected by FRTW sheathing, gypsum sheathing, or a noncombustible material; however, there are other requirements for the exterior wall, floors, roof, etc. Floors are required to be 4-in nominal minimum and roofs 3-in nominal minimum. While this code change will not go into effect officially until the 2015 IBC is adopted by a jurisdiction, this information could be used to simplify an alternative methods argument under current codes. For more information, see the news release issued by the American Wood Council.

As with all new products that enter the market, local building code officials will give their approval providing safety and performance can be proven—as was the case with the Long Hall project in Whitefish, Montana, an all-CLT system (see case study). Although the project was just two stories, local code officials were not familiar with CLT, and project engineer Darryl Byle of CLT Solutions says it was critical to plan ahead to improve their familiarity with the system. Byle negotiated to reduce the required overall thickness of the wall assembly by taking advantage of CLT’s unique structural characteristics. Because the CLT was only exposed on one side, it could easily achieve the code-prescribed one-hour fire resistance requirement. Further, he demonstrated that the undamaged portion could still transfer loads bidirectionally around the compromised area. Byle established a reasonable fire event dimension and designed the structure to deal with that localized structural compromise (an approach that is conceptually similar to disproportionate collapse regulations used in the United Kingdom).

24. Does the content of the laminating adhesive in CLT have an impact on fire resistance?

Adhesives could have an effect on fire performance, but standards exist to prevent that from playing a significant role in the overall product performance. The product standard established for CLT, ANSI/APA PRG-320-2012 Standard for Performance-Rated Cross-Laminated Timber, requires that, for use in the US, adhesive used in the manufacturing of CLT shall meet similar requirements as structural glued laminated timber used in dry service conditions (the equilibrium moisture content of wood is less than 16%). Also, adhesives shall be evaluated for heat performance.

During recent fire tests on CLT, it was observed that the polyurethane adhesive used by the two North American CLT manufacturers (which complies with the North American standards) behaves very well at elevated temperatures. Phenolic and resorsinal adhesives should also perform very well based on longstanding use in fire situations.

The US CLT Handbook includes recommended char rate calculation procedures and supporting test data related to the use of approved polyurethane adhesive.

25. Is the system approved for use in high Seismic Design Categories?

CLT is not an approved system for seismic resistance in US building codes, in that it can’t be found in ASCE 7. It is still going through the code acceptance process, so currently relies on Section 12.2.1 in ASCE 7-10 that states:

Seismic force-resisting systems not contained in Table 12.2-1 are permitted provided analytical and test data are submitted to the authority having jurisdiction for approval that establish their dynamic characteristics and demonstrate their lateral force resistance and energy dissipation capacity to be equivalent to the structural systems listed in Table 12.2-1 for equivalent values of response modification coefficient, R, overstrength factor, Ω, and deflection amplification factor, Cd.

The US CLT Handbook provides suggested conservative seismic R values and supporting test data for engineers and building officials to determine whether the above requirement is met.

A two-story CLT building in Montana was designed in what is considered a fairly high seismic area (Seismic Design Category D). Seismic lateral loads governed the design of the project, but this wasn’t a major design hurdle as the jurisdiction readily accepted the proposed seismic R value recommended for use in the Canadian CLT Handbook. There was some special detailing at the foundation to accommodate the overall stiffness of the longitudinal walls (which is discussed briefly in the WoodWorks case study, CLT Milestone in Montana) and their reaction under seismic motion.

26. Does CLT meet ductile failure aspects of the seismic requirements of the code?

The seismic design of CLT is still being discussed, but tests of various configurations have shown that the ductile yielding of connectors occurs before the less ductile wood failure. This behavior can be ensured through the use of a capacity-based design approach where connectors are specifically designed to be yielding elements. Connections in CLT assemblies basically govern the ductility of the system.

27. Is there an ICC report for the CLT system and its connections (or equivalent third-party testing)?

ICC has not yet published any reports for CLT products or their connections; however, there are several APA Product Reports (e.g., PR-L306) that provide third-party testing and may be accepted under IBC Section 104.11.1. Moreover, the research and testing data provided in the US CLT Handbook can also be provided as an alternate means approach under IBC Section 104.11.2. Simpson Strong Tie also has testing data available on a series of CLT connectors.

28. Are there approved acoustic assemblies with STC and IIC ratings?

The maximum STC rating in the tested floor assemblies is 67 and in the tested wall assemblies is 60. Numerous assemblies have been tested and the results are included in the US CLT Handbook. Certain assemblies can reach an IIC of 65.

29. How can WoodWorks staff support architects, engineers and contractors with the code approval process in various jurisdictions?

WoodWorks has technical directors in various parts of the country whose mandate is to provide support related to non-residential and multi-family wood buildings. Among other things, we can help you access, understand and potentially present the available research on CLT. Visit the WoodWorks website to contact an expert in your region.

Research and Testing

30. What are the design and construction tolerances?

Design and construction tolerances vary depending on the panels. CLT manufacturers can provide this information for their product offerings.

31. Are ASTM E119 fire tests planned for CLT assemblies so they can have a listed fire rating?

In October 2012, the American Wood Council (AWC) conducted a successful fire resistance test on a load-bearing CLT wall at NGC Testing Services in Buffalo, New York. The test, conducted in accordance with ASTM E-119-11a (Standard Test Methods for Fire Tests of Building Construction and Materials), evaluated CLT’s fire resistance properties. The five-ply CLT wall (approximately 6-7/8-in-thick) was covered on each side with a single layer of 5/8-in Type X gypsum wallboard and then loaded to 87,000 lbs, the maximum load attainable by the NGC Testing Service equipment. The 10×10 foot test specimen lasted three hours, five minutes, and 57 seconds (03:05:57)—well beyond the two-hour goal.

Industry has taken an analytical approach to CLT, rather than the route of prescriptive fire-rated assemblies. This is because CLT assemblies, and in particular floors, will most likely never be loaded to their full design capacities, but instead be designed based on serviceability (vibration and/or deflection). Therefore, it would be overdesigning to evaluate based on a prescriptive fire-rated assembly. This also allows more flexibility in wall and floor assembly designs. The approach taken was to test CLT under different load ratios, including full design loads, in order to validate the proposed calculation method(s). Fire resistance calculation methods will be included in the next version of the National Design Specification® (NDS®) for Wood Construction and are explained in Chapter 8 of the US CLT Handbook.

32. Is there any research fire retardant treatment (FRT) for CLT panels?

The industry has not yet begun studying or evaluating FRT wood in the manufacture of CLT. FRT wood tends to have lower mechanical properties than untreated wood. Studies need to be undertaken to understand the resulting effects as well as the bonding performance of FRT.

Sustainability/Environment

33. How does CLT compare to other forms of construction in terms of environmental performance?

One of the advantages of CLT is its thermal performance, which is determined by its U-value, or coefficient of heat transfer, which relates to panel thickness. Thicker panels have lower U-values; they are better insulators and therefore require less insulation. Since CLT panels can be manufactured using CNC equipment to precise tolerances, panel joints also fit tighter, which results in better energy efficiency for the structure. Because the panels are solid, there is little potential for airflow through the system. As a result, interior temperatures of a finished CLT structure can be maintained with just one-third the normally required heating or cooling energy.1

Manufactured using wood from sustainably managed forests, CLT provides a number of environmental benefits in addition to energy efficiency. Wood is the only major building material that grows naturally and is renewable. Life cycle assessment (LCA) studies also show that wood outperforms steel and concrete in terms of embodied energy, air pollution and water pollution. And wood has a lighter carbon footprint—because wood products continue to store carbon absorbed by the trees while growing, and wood manufacturing requires less energy and results in less greenhouse gas emissions.2

In terms of resource efficiency, the distinction between light-frame and ‘heavy’ construction is important. The intent of CLT is not to replace light-frame construction, but rather to offer a low-carbon alternative to ‘heavy’ construction materials such as concrete and steel. There are building applications where light-frame construction is less appropriate, such as an industrial warehouse with 40-ft walls that need to withstand the impacts of heavy machinery, or a Class A office building where few partition walls and minimal floor vibrations are desired.

Efficient use of materials is an essential consideration for design professionals who recognize the impact they can have, both positive and negative, on the world’s resources. However, the broader and more important objective is to specify materials that perform well based on the spectrum of environmental impacts. A CLT building may require more wood than a light-frame building, but when compared to steel or concrete in applications where all three are applicable, advantages such as renewability, carbon off-sets, low embodied energy and operational energy efficiency make CLT an environmentally preferable choice.

Sources:

1) A Strategic Plan for the Commercialization of Cross-Laminated Timber in Canada and the US, Canadian Wood Council, 2010

2) The Environmental Performance of Renewable Building Materials in the Context of Residential Construction, Bowyer, J., D. Briggs, B. Lippke, J. Perez-Garcia, Consortium for Research on Renewable Industrial Materials (CORRIM), 2005; Life Cycle Environmental Performance of Renewable Building Materials in the Context of Residential Construction, Phase II, Lippke B., L. Johnson, J. Wilson, M. Puettmann, CORRIM, 2010; Synthesis of Research on Wood Products & Greenhouse Gas Impacts, 2nd Edition, Sarthe R., J. O’Connor, FPInnovations, 2010; US CLT Handbook