The mass timber industry continues to expand, with projections showing a 15.5% annual growth rate in North America. As used in this article, mass timber means engineered wood products that are precision cut for field assembly. An example involves glue-laminated beams and panels of Cross-Laminated Timbers (CLT), which are used to erect tall timber buildings.
Mass timber projects tend to be big, architecturally significant projects, including the recent $2 billion remodel of the Portland airport. A Forbes article entitled, “Portland’s PDX Airport Dazzles With Its New Timber Roof Terminal” reflects on the beauty of the PDX project, and a research study concludes that, while further research is needed to quantify the health benefits associated with mass timber construction, the use of wood in interior environments promotes the well-being of building occupants: “Wood, as a natural and stress-relieving material, has physiological and psychological benefits that make it appealing to users.”
In addition, mass timber is appealing to the conscientious developer, including public sector developers, because it is recognized as an environmentally friendlier building material. Wood is a renewable resource, and the production of mass timber materials has a smaller carbon footprint than batching concrete or forging steel. From a practical standpoint, mass timber buildings are also quicker to build, with projects completed at about a 25% faster rate than a similarly sized concrete and steel project.
However, the mass timber industry remains a relatively small sector of the commercial construction market. The 2024 “International Mass Timber Report” notes that roughly 190 mass timber projects either began construction or were built in the U.S. in 2023, and approximately 168 projects were in the design stage.
Part of the problem with scaling up the mass timber industry is the cost of insurance. The report notes that elevated premiums for builders’ risk insurance for mass timber buildings continue to be a challenge because mass timber structures are grouped by insurance underwriters as light frame structures (think apartment construction) despite having different performance characteristics. As a result, insurance premiums are frequently high because mass timber is not fully recognized as comparable to a concrete and steel construction. Some of the panelists on the topic of risk and insurance at the 2024 International Mass Timber Conference commented that the price of insurance coverage may make the difference in whether a mass timber project pencils out.
Recommendations, If Not Standards
From the perspective of a construction litigator, these concerns are understandable, especially in wet climates, because there is no prescribed industry standard on how to manage moisture during construction on a mass timber project. This was evident from the panel discussion on moisture management at the conference, where speakers from several large, respectable contractors demonstrated an array of moisture management techniques, highlighting the cost-benefit analysis that a contractor has to go through to evaluate different approaches. At the low-cost end of the spectrum, a contractor may choose to simply rely on factory sealed edges of CLT panels. At the high-cost end, a contractor may require (and budget for) factory applied water proofing, taped seams and edges, and protected openings. To paint a picture, one contractor demonstrated a project in which mass timber components were shrouded with Gore-Tex-like breathable protective membranes, while another contractor demonstrated how their crews used squeegee mops to wipe away bulk rainwater off uncovered CLT panels.
In the meantime, academic and industry researchers continue to explore the issue of moisture hazards and moisture management techniques for mass timber projects. These include, for example:
- A study of the hygroscopic properties (effect of humidity) of wood species and how timber exposed to moisture can degrade biologically and lead to decreased mechanical properties.
- The wetting of mass timber projects during construction and how mass timber may dry out more slowly than light-frame structural lumber.
- The study of moisture safety and mold growth in CLT construction without weather protection.
Some of the studies get into the weeds, such as suggesting preventative measures in the design of a project, for example, avoiding moisture trapping during fabrication by prescribing localized coating, and during construction by sequencing work to minimize moisture exposure and to allow for drying.
The American national standard for the production of CLT panels is the PRG 320 - 2019, “Standard for Performance-Rated Cross-Laminated Timber,” published by ANSI/APA. The PRG 320 provides requirements for CLT dimensions and tolerances, performance, test methods, quality assurance, and marking for CLT panels, but it does not mandate moisture or temperature controls in the CLT construction process, except to say, “CLT panels shall be used in dry service conditions, such as in most covered structures, where the average equilibrium moisture content of solid wood is less than 16% in the U.S. and is 15% or less over a year without exceeding 19% in Canada. CLT panels qualified in accordance with the provisions of this standard are intended to resist the effects of moisture on structural performance as may occur due to construction delays or other conditions of similar severity.”
There is no explanation in the PRG 320 about how to get to the “average equilibrium moisture,” and the PRG 320 directs the reader to industry literature for guidance: “Panels shall be used in accordance with the installation requirements prescribed in the recommendations provided by the CLT manufacturer, an approved agency, and/or its trade association.”
One prominent manufacturer of CLT products cautions in its operations and maintenance materials that, like any wood elements, CLT panels may develop checks as the result of the normal process of wood elements achieving moisture equilibrium. The degree of checking that might occur will vary depending on a wide array of conditions, including a rapid change in moisture and temperature. The manufacturer recommends not exposing CLT panels to a rapid change in moisture and temperature to allow the panels to achieve their equilibrium state in a gradual manner. These concepts are also applicable in dry, hot climates, like Phoenix, where ambient moisture is less than the moisture content at the time of manufacture, and where unregulated heat may result in checking of wood finishes.
There are thoughtful advice documents in the space for design and construction professionals, including a tip sheet from the American Plywood Association, entitled, “Keep Glulam Looking Its Best With Proper Storage and Handling.” The APA cautions that glulam beams and columns are subject to the effects of changes in moisture content caused by changes in temperature and relative humidity on the job site and after installation and that these components may develop seasoning checks as the moisture content stabilizes from the level at which they were manufactured to their moisture content in-service.
The APA recommends steps that builders can take to minimize seasoning checks in glulam, including tips for covering and protecting members on the job site from direct exposure to sun and rain, especially in hot, dry climates; avoiding direct ground contact; using blocking to provide ventilation; and slitting protective wrappings on the bottom to allow for drainage of any entrapped water. After installation, the APA recommends keeping beams and columns covered and protected as long as practical, removing wrappings in the enclosed space so the moisture content of members is allowed to stabilize naturally during construction, avoid rapidly drying members with temporary heating units, and reducing the relative humidity gradually.
Similarly, the American Institute of Architects, Seatle Chapter (AIA Seattle), published a document entitled, “Mass Timber Moisture Protection Considerations for CLT Construction” to address practical concerns regarding the design and construction of CLT projects. It cautions that if a CLT building is exposed to similar moisture levels during construction and dried in the same manner typically used for projects built with standard dimensional lumber (like standard apartment complexes), significant checking or cracks may appear on the CLT panel’s surface. This is because outer portions of mass timber members may have dried to acceptable moisture content levels, but inner portions may remain at elevated levels.The guidance document contains helpful advice, including the fact that protection of the end grain is critical, but it comments that manufacturer-provided protective sheets may do more harm than good:
“CLT is typically shipped with a temporary moisture protection sheet that is loose laid and not intended for use once panels are installed. Temporary protection sheets can trap moisture and should not be relied on during construction.
“It is preferable to apply a coating or sealer at the factory that could work as moisture protection once panels are installed, but this may be a logistical challenge for manufacturers.
“A combination of approaches may be necessary, such as applying a water repellent or coating to panel edges or sealing panel joints and penetrations through panels with tape or sealant.”
The AIA document provides a table summarizing several approaches to providing field protection of CLT once it is installed, and it recommends that a moisture protection plan for CLT elements should be included in project manuals. This recommendation implicates the standard of care for design professionals working in this space.
RDH Building Science, a building enclosure firm, publishes a comprehensive Mass Timber Building Enclosure Best Practice Design Guide to help educate designers, construction professionals, and building developers on best practice enclosure design principles for mass timber buildings. It is a valuable resource document. In terms of water control best practices, RDH recommends avoiding vapor-impermeable materials exterior of mass timber because they have the potential to trap moisture within the mass timber panel and limit drying through vapor diffusion; installing a durable, fully adhered (e.g., multi-ply) roof membrane on the roof to promote long-term performance of the roof assembly, especially where temporary roof membranes over the mass timber structure are not used; and treating areas such as bathrooms, showers, laundry rooms, and food preparation areas with a waterproofing system and drainage for incidental water to reduce the moisture exposure of the mass timber floor panel.
No Rules
These industry guidance documents, while well intentioned and useful, are merely suggestive. They do not require adherence to a basic set of rules for moisture or temperature management, and, collectively, they do not provide step-by-step easy to follow instructions on how to best handle moisture and temperature. The effect of permissive guidance documents in construction litigation over mass timber failures and resulting damages is that they provide content for plaintiffs’ lawyers and experts to argue over what is—or arguably what should be—“standard in the industry.” Further, the guidance documents do not provide the insurance industry with the confidence needed to align premiums on mass timber projects similarly to the pricing of concrete and steel projects. Paradoxically, standardization of moisture and temperature management on mass timber projects may result in higher confidence and lower costs, and allow the mass timber industry to scale up.
This concept is not new. Twenty-five years ago, Charles Carll, a research forest products technologist with the USDA Forest Service, published an article entitled, “Rainwater Intrusion in Light-Frame Building Walls,” in which he addressed this very issue in the light-framed housing market. The article posits that confusion in the design and construction industries, and presumably legal risk, could be reduced by developing an engineering approach to preventing water leakage and damages. To get there, the article suggests that industry and regulators need to agree on the characterization of weather exposures for different parts of the country (Arizona building techniques being different from practices in Seattle or Detroit), a means to evaluate the moisture tolerance of building materials, and identify a desirable level of “robustness” for buildings with regard to their ability to resist water damages.
To recap, while people may have different views on the costs and benefits of regulation, for the mass timber industry, there may be economic benefits and overall growth resulting from a standardized approach to moisture and temperature management. As one recent study commented:
“There are currently no standards regulating water management for mass timber elements during construction, little knowledge of impacts of moisture exposure (wetting and drying performance, dimensional stability, checking), and few precedents serving as guidelines for monitoring moisture response of mass timber.”
Without defined standards, when failures occur on mass timber projects, litigants will likely argue about the standard of care for project management and whether a Gore-Tex-like membrane or a squeegee mop provide better protection (there are arguments supporting both approaches). A prescriptive path should help minimize risk for developers, designers, manufacturers, and contractors. By doing so, insurers can more predictably forecast risks, mass timber projects are more likely to pencil, and the mass timber industry is likely to flourish even more.