Challenges Facing the Development of Hardwood Cross-Laminated Timbers

By Curt C. Hassler (, Joseph F. McNeel (, Levante Denes (, and Jonathan Norris (, WVU Appalachian Hardwood Center and Wood Science and Technology, West Virginia University

Interest in mass timber products for construction has swept both Europe and North America in recent years with new manufacturing facilities cropping up across both continents. This growth has been sustained by readily available softwood species in the sizes and grades required in CLT (Cross-laminated Timber) manufacturing. Whether the raw material is western U.S. softwoods, European spruce, or southern pines, the necessary raw material requirements are easily met and sustained. The only major confounding factor recently has been the significant run up in softwood lumber pricing (which has fallen back to more traditional levels over the last year or so), with the Producer Price Index (PPI) for softwood lumber doubling between 2019 and 2022 (Statistica, 2022).

As with any industry, hardwood lumber manufacturers have generally viewed Mass Timber as a significant opportunity to develop new markets for hardwood lumber. The research community in the Appalachian region has taken up the challenge to investigate the opportunities for hardwood CLT manufacturing.

Unfortunately, the interest in developing a hardwood CLT industry has neglected to consider certain unique, but obvious, aspects of hardwood raw material requirements and manufacturing differences that do not exist with softwood CLT manufacturing. This article defines and quantifies some of the more obvious challenges and issues that must be addressed for a viable hardwood CLT industry to be developed in Appalachia.

Softwood Versus Hardwood Lumber Grading

Softwoods are the primary species for satisfying dimensional, structural lumber markets. Lumber is available in standard sizes (e.g., 2×4, 2×6, etc.) that are surfaced and dried to moisture contents compatible with CLT board requirements. Additionally, these softwood products are graded either visually or using machine stress rating (MSR) systems for structural purposes, in accordance with the rules set forth by the recognized grading agencies (American Lumber Standard Committee 2022), with regular inspections of the grading process at individual mills by a certified grading/inspection agency.

Structural lumber grades are based on the size and location of a variety of defects, including knots, wane, decay, splits, holes, shake, warp, crook, twist, cup, slope of grain, and skip. The visual grade designations for structural lumber are Select Structural, No. 1, No. 2, and No. 3 and all lumber considered for use in CLT panels must receive the appropriate structural grade in order to be manufactured into CLT panels. Each structural grade has its own specific limitations for the defects listed above. One example of the definitions and specifications for visually grading lumber are promulgated, maintained, and administered by the Northeastern Lumber Manufacturers Association, Inc. (NELMA). In fact, NELMA (2021) is the only organization that includes grading rules and design values for eastern hardwoods.

Additionally, the NELMA grading rules provide for non-destructive testing via machine (Machine Stress Rated or MSR) with an accompanying visual override for certain board characteristics that cannot be evaluated by machine. The operable measure for MSR-based grading is the modulus of elasticity (MOE).

For the purposes of CLT production, the boards within each panel must meet certain structural lumber grade requirements, according to the American Panel Association (APA) publication, PRG 320, “Standard for Performance-Rated Cross-Laminated Timber” (APA 2019). For most panel layups based on visual grading of boards, the minimum grade for the longitudinal layers must be visual structural grade No. 2, while the minimum grade for the inner, transverse layers must be visual structural grade No. 3.

Most important, softwood lumber with the specific dimensions and grades needed for CLT manufacturing are readily available in the marketplace. A CLT manufacturer can simply place an order with a producer or broker for the desired dimensions and grade of lumber for their manufacturing activities. The only issues may be price and timing of the shipment.

Hardwoods represent an entirely different market situation. Hardwood markets are generally focused on appearance, with the amount of clear wood in a board being the key to establishing a grade. The greater the proportion of clear wood in a board, the higher the grade and the more expensive the board. This hardwood grading protocol is administered solely by the National Hardwood Lumber Association (NHLA) (2019). Additionally, these appearance-graded boards are, for the most part, not used in their graded form. They are surfaced, ripped, and crosscut into clear wood pieces of various dimensions by the consumers of these boards and then glued, finger jointed, etc. for the manufacture of furniture, cabinets, millwork, etc.

Specifically, hardwood products are not typically manufactured and graded for structural applications. Part of the reason for this is that appearance graded lumber generally has a higher market value than structural lumber, which is a low margin, commodity product. As a result, hardwoods are rarely considered a viable alternative to graded structural softwood dimension lumber.

However, some industrial products sawn from hardwoods are graded, such as railroad ties and bridge timbers. For instance, the Railway Tie Association (2003) publishes “Specification for Timber Crossties and Switch Ties” which details defect limitations such as knots, decay, holes, shake, split, checks, slope of grain, bark seams, and manufacturing defects. Yet, these types of graded hardwood products do not lend themselves to effective and efficient CLT production in their marketed form.

From a lumber grading perspective, then, structural grading and appearance grading are incompatible for manufacturing hardwood CLT panels. Currently, there is no market where a CLT manufacturer can order structurally graded hardwood lumber. What is the alternative? One option is to investigate how NHLA grades of hardwood lumber might yield structural grades of hardwood lumber.

This approach was taken by West Virginia University’s Appalachian Hardwood Center (WVU-AHC) for their investigation of hardwoods for CLT manufacturing. Specifically, WVU-AHC has procured packs of hardwood lumber graded as NHLA No. 2A and below to determine what these boards might yield in terms of structural lumber. These lower grades were selected because they are significantly less expensive to procure than the higher value NHLA lumber grades (No. 1 Common and Better).

Hardwood Lumber Sizes for CLT Production

From a CLT manufacturing perspective, the typical dimensions of softwood boards that are used in CLT panels include a consistent finished thickness of 1-3/8 inches, with a width ranging from 2.4 inches to 5 1/2 inches to 7 1/2 inches and even wider. Hardwood board thickness isn’t even specified in the same manner; rather, thickness is expressed in quarter-inch classes. For instance, 4/4 indicates a 1-inch board. Other typical thicknesses produced include 5/4, 6/4, and 8/4, with thicker pieces ranging up to 16/4. Further, these boards in a green condition generally have an additional 1/8 inch of thickness to account for shrinkage during drying. In a rough kiln-dried condition, these boards require surfacing to achieve a consistent thickness, since the kiln drying process may result in different shrinkage of boards and result in inconsistent thickness.

Additionally, boards are commonly produced at the sawmill in random length and random width pieces. The only standard size material produced at a hardwood sawmill are from the log hearts where the production of higher grades of lumber is minimal. These heart-centered products include, but are not limited to, railroad ties (e.g., 7.5 inches x 9 inches x 8.5 feet) and pallet cants (e.g., 3.5 inches x 6 inches in various lengths).

The most commonly sawn hardwood board thickness is 4/4 (1-inch). In considering how to best approach raw material procurement for hardwood CLT, the logical choice would be to investigate the purchase of commonly available raw material. This approach was taken by researchers with WVU-AHC as part of a study investigating the feasibility of using yellow-poplar for CLT manufacturing, with the objective of gaining certification under the APA PRG 320 Standard for Performance-Rated Cross Laminated Timber (APA 2019).

A sample population of 4/4 boards was obtained from a cooperating West Virginia sawmill and sawn from 7.25-inch-thick flitches. This is the mill’s standard process for sawing log hearts into pallet boards; saw logs to a 7.25-inch flitch then process the flitch through a gangsaw. The higher-grade boards (NHLA grades 1 Common & Better) are sorted for appearance markets, while the remaining lower grade boards (2A, 2B, 3A, and 3B) are ordinarily sorted out for pallet boards. Further, the mill sorted out the 10-foot boards that were requested by WVU-AHC for the CLT research. Specifically, additional effort was required by the cooperating mill to supply boards with a “standard dimension” similar to that provided by the softwood structural market. Projecting this situation into a marketing effort, this additional work ultimately adds more cost to the production of hardwood lumber earmarked for CLT manufacturing.

This effort can be burdensome for a hardwood sawmill that does not have a gang saw capable of producing fixed width boards from a flitch. For example, a hardwood mill without a resaw or gang saw will have to rely on the edger operator to identify boards of a certain grade to saw to CLT required widths, which may be tenuous at best. A similar situation would be operative in a mill with a resaw producing a single board at a time, where the production of boards of a set width may be more difficult to achieve.

Structural Lumber Yields from Hardwoods

The evaluation of structural lumber yields from NHLA low grade yellow-poplar boards was conducted by WVU-AHC. Results of this investigation for yellow-poplar are detailed in Azambuja et al. (2021). Boards provided by the cooperating West Virginia hardwood sawmill included lumber in four NHLA grade categories: No. 2A, No. 2B, No. 3A, and No. 3B. The boards were provided as 4/4, rough, kiln-dried. They were then surfaced and ripped to a fixed width of 6.25 inches. The final dimension for CLT manufacture was 6 inches x 0.75 inches x 10 feet. Visual structural grading results, based on the original NHLA grade (before processing) and the structural grade after surfacing, are contained in Table 1.

In general, the yield of boards for longitudinal layers that require structural No. 2 and Better from NHLA graded boards was very poor, ranging from 45.6% (No. 2B) down to 18.5% (No. 3B). In total, only 36.9% of the hardwood boards visually graded for structural characteristics (No. 2 and Better) was found acceptable for longitudinal laminates.

From a CLT procurement perspective, the basic approach is to determine how many 4/4, rough, kiln-dried boards are required to yield one thousand feet (surface measure) of CLT-ready boards. Taking the No. 2A NHLA graded boards, for instance, 441 boards yielded 194 boards of structural grade No. 2 and Better boards, or 970 feet of surface measure (each finished board has 5 feet of surface measure @ 6-inches wide x 10 feet length). Solving for 1,000 feet of surface measure for CLT-ready boards requires 455, 4/4, rough, kiln-dried boards. Since a 4/4, rough, kiln-dried board contains 6 bf, it would require 2.73 MBF of 4/4, rough, kiln-dried boards to yield 1,000 feet of surface measure of finished boards. Table 2 summarizes the volume of 4/4, rough, kiln-dried yellow-poplar footage required to yield 1,000 feet of surface measure in CLT ready boards.

It is apparent that procuring 1,000 feet of surface measure of CLT-ready boards from 4/4, rough, kiln-dried boards will incur a substantial cost, from 2.634 to 6.516 times the cost per MBF of the rough, kiln-dried lumber.

Since these boards were essentially mill run, the obvious option is to incorporate a visual override protocol at the mill to eliminate boards that would obviously not make at least a No. 2 structural grade (and perhaps a No. 3 structural grade). This would help reduce the footage necessary to achieve the desired outcome in CLT-ready boards. This option was not studied as part of the WVU-AHC research.
The other potential avenue for additional research is to evaluate No. 1 Common boards for CLT production. Since lower grade lumber (No. 2A and No. 2B) will require about 2.6 times the cost to achieve a desired volume of CLT ready lumber, the difference in cost between one MBF of No. 1 Common and 2.6 MBF of No. 2A or 2B Common will be less significant.

But could the yield of higher-grade structural boards be more economically efficient using No. 1 Common lumber, rather than these lower grades? This option was not explored by WVU-AHC but does offer sufficient merit for study.

WVU-AHC has also assessed the yields of structural boards from low grade soft maple and red oak, using 4/4, rough, kiln-dried boards. Preliminary results for these species indicate similar yield patterns as those noted for yellow-poplar.

Earlier research at WVU-AHC evaluated the yield of structural lumber of six hardwood species (McDonald, et al. 1996, Pahl et al. 1992) processed from graded railroad switch ties and ungraded mill-run pallet cants. The difference with this study and the study by Azambuja et al. (2021) was that the boards produced in the latter study were sorted from the processing of flitches through a gangsaw.

In the former research, 2×7-inch boards were sawn from the graded switch ties and 2×6 inches from ungraded pallet cants. All resulting boards were visually graded for structural application. Table 3 shows the results of that research.

Fifty-three percent of the visually graded boards were No. 3 and Below Grade for the mill-run pallet cants, a result similar to the structural grade results from Azambuja et al. (2021) for NHLA grades No. 2A and No. 2B (56 and 55%, respectively). However, the processing of graded railroad switch ties into 2×7 boards raised the yield of No. 2 and Better structurally graded boards to 89%, a dramatically different result, generally attributed to using graded switch ties in the study.

These results infer that the production of structural lumber from graded, heart-centered cants may be a better option for securing structural hardwood lumber. This allows the hardwood sawmill to continue manufacturing higher-grade, more valuable lumber, from the outer portions of logs. The tradeoff then becomes an economic decision between simply producing cants for existing markets or processing them into nominal 2-inch-thick boards. Of course, the latter decision must account for the additional processing and grading needed to meet structural lumber specifications for No. 2 and Better structural, as required by PRG 320 in the parallel laminates.

Structural Grading Options

The foregoing discussion has been in the context of visual grading of structural lumber. In the case of mill-run lumber from flitches (Azambuja et al. 2021) and from mill run pallet cants (McDonald et al. 1996 and Pahl et al. 1992), the resulting board grades were heavily skewed to No. 3 and Below Grade boards.

Previous research has indicated that machine stress rating of hardwoods can yield better results than visual grading. Green et al. (1994) performed both Machine Stress Rating (MSR) and visual grading of 803, 2×8 mixed oak boards. Results showed that while “only 1% of the lumber qualified as Select Structural by visual grading, 36% of it could be assigned an MSR grade with properties equal to or greater than those of Select Structural” (Green et al. 1994).

Another study, unpublished by Green, Wolcott, and Hassler (Senalik and Green 2020), compared visual grading and MSR grading of 2×6 lumber sawn from log heart cants for several hardwood species. These were the same boards produced in the Pahl et al. (1992) study. Results showed that MSR ratings of these boards were 10% to 20% higher than visually graded properties. “Thus, research to date indicates the possibility of achieving higher yields for a specified set of allowable properties using the MSR process” (Senalik and Green 2020).

More recent research conducted at WVU-AHC with yellow-poplar for production of CLT showed similar results to these earlier studies. Azambuja et al. (2021) subjected 1,135 yellow-poplar visually graded boards to non-destructive proof-loading to determine their modulus of elasticity (MOE). Since the minimum MOE required for boards used to produce CLT panels is 1.20 x 106 psi (11,4445 MPa), that figure was chosen to determine what proportion of the boards could meet the minimum MOE specifications required for boards used in CLT panels (APA 2019). Table 4 summarizes those results.

A total of 39 boards (3.4%) out of the 1,135 boards tested did not meet the MOE threshold specifications, which implies that machine stress rating may ultimately be the better alternative for evaluating hardwood boards for CLT production.

WVU-AHC is currently evaluating and comparing visual grading and machine stress rating for soft maple and red oak, again in the context of CLT production. Preliminary results indicate similar outcomes for both species to those noted for yellow-poplar relative to grade yield.

CLT Panel Options

The selection of 4/4 boards to conduct CLT research at WVU-AHC necessarily translates to the production of 5-layer CLT boards (three parallel layer and two perpendicular layers). With the final thickness of boards being 0.75 inches, the panel thickness would be 3.75 inches. This is very similar to European CLT layups with boards 20mm, 30mm, and 40mm thick (roughly 3/4, 19/16, and 1.5 inches, respectively).
A CLT panel manufactured of 2×4, 2×6, or 2×8 softwood boards would contain at least three layers (two parallel layers and one perpendicular layer). Given the thickness of the softwood boards at 1.5-inches, the softwood CLT panel would be approximately 4.5-inches (approximately, since a small amount of wood must be removed prior to CLT panel layup, as a fresh surface is required for gluing).

Producing an equivalent three-layer hardwood CLT panel at 4.5 inches thick, would obviously require 7/4 boards (finished thickness would be 1.5-inches). Unfortunately, 7/4 lumber is not a typical thickness sawn by hardwood sawmills and would require another significant change in the conventional lumber manufacturing process used currently and would not be well accepted by an industry faced with an uncertain CLT market. This is further evidenced by the fact that the third-party pricing publications do not provide pricing for 7/4 lumber (Hardwood Market Report 2022; Weekly Hardwood Review 2022).

Drying Hardwood Lumber for CLT Production

According to APA PRG 320, CLT boards must have a moisture content (MC) of 12% ± 3%. For softwood CLT manufacturers, this is not an issue since softwood lumber can be easily purchased in the marketplace with an MC of 15% or less and marked “MC-15” or

By comparison, hardwood lumber is generally dried to between 6 and 8% MC for the furniture, cabinet, and millwork markets (Simpson 1999). Thus, hardwood mills and kiln operations would have to adjust their drying schedules to achieve a higher MC. This may not be an issue since it would presumably reduce the cost of kiln drying, assuming that enough demand was available to achieve full kiln charges with CLT destined lumber.

Depending on the hardwood species being considered for CLT manufacturing, the length of time to dry could be an issue. Senalik and Green (2020) state that a typical drying schedule for southern yellow pine structural lumber is one to two days. By comparison, Wang and Simpson (2020) discuss drying of red maple for structural purposes and found that a kiln schedule that is more severe than that allowed for appearance grade processing, could be accomplished in five days, without any additional structural degrade compared to the milder schedule. Presumably, similar results could be obtained when drying 4/4 yellow-poplar lumber.
Due to economic considerations, as outlined above, the approach to incorporating hardwoods into CLT manufacturing is to use No. 2A and below lumber. However, No. 2A and lower grade lumber is not typically kiln-dried by the producing sawmill, but sold green to pallet and flooring manufacturers. The question is whether hardwood mills would be willing to consume kiln space with low grade lumber for CLT manufacturers at the expense of drying their higher-grade, high value lumber.
The situation could be further exacerbated if a CLT manufacturer was able to procure 6/4, 7/4, or 8/4 boards of NHLA No. 2A Common and lower and the accompanying increased kiln residence time to reach the desired moisture content.

Current Availability of Hardwoods for CLT Manufacturing

For hardwoods to be competitive with softwoods for CLT manufacturing, there must be an equivalent level of available raw material (i.e., boards). As stated earlier, softwood lumber is readily available in the marketplace, in commonly required sizes, surfaced (i.e., S4S, or surfaced-4-sides), and at the suggested moisture condition, in grades required by the softwood CLT manufacturers.

What is the current state of hardwood lumber for CLT manufacturing? The most commonly available hardwood lumber, in quantities that can be ordered and delivered in a similar timeframe to softwood lumber, is 4/4, unsurfaced, kiln-dried, with an assumption that it can be procured in specific lengths without paying a premium and that mills will be amenable to drying the lower grades of hardwood lumber. Setting aside the less likely possibility of procuring thicker material, what characteristics are required to make them suitable for CLT manufacturing?

  • Boards must be conditioned to approximately 12% MC to meet PRG 320 requirements and to achieve adhesive efficacy.
  • They must be surfaced on four sides.
  • They must be structurally graded to yield No. 2 and No. 3 structural grade boards.

The most efficient first step in the preparation process is to grade the boards in the rough condition using structural grading standards. Research at WVU-AHC has shown that procuring NHLA No. 2A and lower grade boards resulted in 55 to 81% of the boards grading out as either No. 3 or Below Grade (see Table 1).

Purchasing hardwood lumber at 6 to 8% MC, reconditioning that lumber to a moisture content of approximately 12%, and surfacing boards prior to grading means additional costs incurred for boards that will not meet grade specifications. Unfortunately, following moisture conditioning and surfacing, boards must be regraded to ensure that no below grade boards remain. Additionally, if the perpendicular layer boards are targeted as No. 3 structural grade, it would be very difficult to achieve the proper proportion of structural grade No. 2’s and No. 3’s for manufacturing purposes.

The other option is to apply MSR testing to the boards rather than visually grade them. Applying a visual override of the boards in the rough, kiln-dried form and then using MSR testing on the boards could be an economically viable alternative to simply using a visual grading process. As illustrated by Azambuja and others (2021), only 3.4% of the boards failed to meet the minimum MOE for CLT manufacturing (without the benefit of a visual override, which could have further reduced the number of failures).

Obviously, each of these steps will add costs to the lumber. Can the various steps needed to procure acceptable hardwood CLT lumber result in a competitive price with the readily available softwood CLT lumber? This question MUST be answered if hardwood can successfully penetrate the softwood dimension market.


Because CLT represents a new, value-added opportunity for hardwoods, there has been a rush to explore the viability of hardwood CLT manufacturing. While commendable, there are at least two critical issues that need to be addressed:

  • Gaining certification approval of hardwoods (yellow-poplar initially) by the American Panel Association under PRG-320 standards.
  • Incorporating some level of production focused on producing structurally graded boards using the conventional lumber manufacturing approaches common to the hardwood industry.

Realistically, the first issue is perhaps an easier one to address. While there is a great deal of panel testing and documentation that goes into certification, that effort is more narrowly focused than the latter issue. Convincing the hardwood industry to produce structural lumber, in a way that is competitive with the softwood industry, is a more monumental task. Without a well-defined market for structural hardwoods, the hardwood industry will necessarily be reluctant to move forward under such uncertainty. It will ultimately take one or two champions within the industry to recognize the potential for hardwood-based CLT panels and move forward to produce structural grade hardwoods for the CLT marketplace.

Will hardwood CLT panels ever become a reality? We hope so, and the hardwood industry has been able to convince the American Panel Association (APA), the agency that certifies CLT panel manufacturing and quality standards, to consider adding hardwood to the APA PRG-320 Standards (APA 2019). The APA Technical Committee that oversees these standards is currently considering the addition and a decision should come within the year. What this will mean to our industry is hard to predict, but there is a significant and growing market for CLT construction material in the U.S. and Appalachian hardwoods could be a solid contributor to that market.

Literature Cited

  • American Lumber Standard Committee, Inc. 2022. Frederick, MD.
  • American Panel Association. 2019. Standard for Performance-Rated Cross-Laminated Timber, ANSI/APA PRG 320-2019. Tacoma, WA. 40pp.
  • Azambuja, R., D. B. DeVallance, and J. McNeel. 2021. Evaluation of Low-Grade Yellow-Poplar (Liriodendron tulipifera) as Raw Material for Cross-Laminated Timber Panel Production. Forest Products Society, Forest Prod. J. 72(1):1–10.
  • Green, D.W.; Ross, R.J.; McDonald, K.A. 1994. Production of hardwood machine stress rated lumber. In: Proceedings of the 9th international symposium on nondestructive testing. Madison, WI: Forest Products Society. pp. 141–150.
  • Hardwood Market Report. 2022. Memphis, TN.
  • McDonald, K. A., C.C. Hassler, J. E. Hawkins, and T. L. Pahl. 1996. Hardwood Structural Lumber from Log Heart Cants. Forest Products Journal, Vol. (46), No. 6, pp. 55–62.
  • Northeastern Lumber Manufacturers Association, Inc. 2021. Standard Grading Rules for Northeastern Lumber. Cumberland Center, Maine.
  • Pahl, T. L., J. E. Hawkins, C. C. Hassler, and J. Slahor. 1992. Efficient Utilization of Hardwoods for Timber Bridges. Final Project Report, submitted to USDA Forest Products Laboratory, Madison, WI. 78 pp.
  • Senalik, C. A. and D. W. Green. 2020. Chapter 4, Grading and Properties of Hardwood Structural Lumber. Undervalued Hardwoods for Engineered Materials and Components, Second Edition. General Technical Report FPL-GTR-276. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 108 pp.
  • Simpson, W.T. 1999. Drying and Control of Moisture Content and Dimensional Changes, Chapter 12. Wood handbook—Wood as an engineering material. Gen. Tech. Rep. FPL–GTR–113. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 463 pp.
  • Statistica. 2022. The Railway Tie Association. 2003. Specifications for Timber Crossties and Switch Ties. Fayetteville, GA. 9 pp.
  • Weekly Hardwood Review. 2022. Hardwood Publishing. Charlotte, NC.