Risks and rewards of mass timber

By Donald Koppy, CSI, CCS, AIA, NCARB, Casey McDonald, B.S., EIT, and Tom Jaleski, ICC

As the design and construction industry shifts toward more eco-conscious building systems, mass timber has become a prominent element in this new direction. Many leading architectural firms currently favor it and the wood industry is actively promoting it.

While wood construction has never disappeared, its use as heavy timber in large buildings was easily replaced by steel and concrete in the last two centuries. With its rediscovery, it is important to remember why it originally lost its place in large building construction and, more importantly, how to overcome those shortcomings to ensure a successful project.

There are three main reasons why mass timber lost out to other materials:

Moisture

During its construction, a mass timber structure is exposed to weather and can remain exposed for a very long time. The wood species used in mass timber are classified as “softwood,” which are hygroscopic, meaning they absorb water and can also lose water. This causes mass timber to swell and shrink, often warping as it does so. Given continuous exposure to relatively small amounts of liquid water or high humidity, softwood will grow mold, rot, and decay.¹ (Many types of mushrooms grow on decaying wood.) That condition can now be counteracted only by adding representative chemicals to the natural product, which mass timber manufacturers do not use due to incompatibility with their fabrication processes.

In addition, sealers and vapor-permeable membranes are needed to protect mass timber components from weather-related water exposure during construction. However, even with the addition of these products, the need to monitor and maintain the wood’s moisture content extends well beyond the construction period and throughout the building’s lifecycle.

Combustion

Wood is combustible, which means it readily burns. So, exposure to fire, if not thick enough, can completely release the wood’s sequestered carbon into the atmosphere with some residue ash. If thick enough, mass timber will not only char but also produce vast quantities of smoke, which is the main cause of death in most building fires. Although the structure may remain standing, repairing charred structural members could exceed the cost of replacing the building.² Fire suppression/protection systems are required for most facilities. Mass timber producers currently do not use fire-treated wood for the same reason they do not use preservatives. That capacity might be counteracted in the future, but only by adding additional chemicals to the natural product, which mass timber producers currently do not use.

Staining

While the exposed wood of mass timber can be beautiful, construction activities easily stain it, and some stains are very difficult to remove. This makes its installation more of an art compared to other structural materials.

To put it more bluntly, “If someone invented wood today, it would never be approved as a building material.”³

Due to these and other minor reasons, wood, in the form of heavy timber, was basically abandoned for the structural construction of large commercial and institutional buildings, with a preference for steel and concrete. Once the building was fully enclosed and conditioned, wood was reintroduced as part of the building’s interior design. Even so, most of this reintroduced wood was in the form of hardwoods, and the only softwoods were primarily concealed and either pressure-treated or fire-treated to protect from its shortcomings.

The new wood

As a response to climate change concerns, improving availability, speed of construction, and biophilic considerations, wood in “mass timber construction” is being quickly re-evaluated for such buildings’ structures. Its implementation has moved beyond the few pioneering designers, manufacturers, and constructors and into the mainstream with much excitement and fanfare.

The question remains: Is mass timber construction ready to be adopted by the entire design and construction industry? Some see the construction of mass timber projects and harken back to images of barn-raising by low-tech and sometimes religiously no-tech constructors. It all seems simple and easy to build, but with today’s technological advancements in construction, it should now be a piece of cake.

However, as noted previously, the properties of softwood have not changed. Similar to solid sections of heavy timber, the major components of the mass timber family include cross-laminated timber (CLT), dowel-laminated timber (DLT), nail-laminated timber (NLT), and mass plywood panels (MPP), which use a combination of smaller sections of wood and adhesives or fasteners to create larger sections to provide strength and dimensional stability. However, they are all composed of softwoods.

While early buildings constructed by previous generations may have featured heavy timber post-and-beam structures, they lacked fire-rated floors, roofs, and wall systems comparable to those found in modern mass timber projects. Additionally, many of these structures were built using hardwoods rather than softwoods.

They also did not enclose air-conditioned spaces for human inhabitation. Both the softwood heavy timber of old and today’s mass timber remain hygroscopic and combustible. (Note: Neither preservative-treated nor fire-retardant-treated wood is currently used in mass timber panels.)

While softwood is not a new structural material, its use in today’s design of mass timber buildings should be considered new. The number of designers and constructors using mass timber is quickly growing; however, very few are well-experienced with the nuances of the system’s means and methods.

Concerns with means and methods

One part of the construction industry that most architects and engineers do not specify is the means and methods of construction. They cannot be blamed, as back in 1909, the American Institute of Architects (AIA) Owner-Architect agreement first excluded designers from the responsibility, and their professional liability insurers reminded them of this at the start of every policy period. Since insurance coverage is a must in today’s construction industry, both architects and engineers must avoid actions that could render them uninsured.

However, a major concern with mass timber pertains to the means and methods: mass timber is exposed to elements and usually for a longer duration than any other materials that have the same type of reactions to the changes in weather/moisture. Rainwater seeping between joints, wall openings, and protrusions can flow from floor to floor and even into wall systems. Seepage through wall system joints can run vertically and horizontally, causing multiple problems for other trades. This moisture causes the wood to swell and shrink, and it can then warp as it tries to dry out. Protection systems and methods that are not vapor-permeable can cause more harm than good, especially when encapsulating moisture in permanent weather barriers and roofing, leading to mold and rot.

It is equally important to consider the methods used to stabilize the indoor environment once the building is enclosed. Expecting an inexperienced constructor to understand all these nuances and protect mass timber components is quite the ask, and specifications for mass timber, like most construction products, state little about how the product should be protected from the elements, rather just that it should be protected. This potentially crosses into specifying “means and methods” that could be uninsurable.

Pioneers versus settlers

The initial mass timber design and construct pioneers did their research and planning to have a successful outcome, avoiding swelling, shrinking, warping, rot, decay, mold, and mushrooms.

The settlers following these pioneers must address these concerns with equal proactivity. Mass timber is in a remarkably similar place as exterior insulation and finish systems (EIFS) was in the North American construction industry during the early 1980s. Though proven in Europe, there are few standards to reference when specifying, with no defined MasterFormat name or section number. Manufacturers provide various levels of services and types of products to designers and constructors. Implementing mass timber and its environmental benefits should succeed for all involved, unlike the unfortunate decline in reputation that EIFS experienced.

Fortunately, the technical information on mass timber has been proactively addressed by the leaders in the subject. Organizations such as WoodWorks, RDH, and Mass Timber Group have provided extensive open-source and free documentation on the many technical aspects to consider when designing and constructing with mass timber. Additionally, building codes are being quickly updated to address mass timber in multiple construction types.

Despite extensive research on this subject, no CSI three-part formatted guide specification section currently addresses all the concerns associated with the various types of mass timber. The linked Section 06 17 10—Mass Timber is recommended as a starting point for designers and constructors to use confidently when using mass timber in their projects.

Additional considerations

Since not all conditions can be covered in a specification guide, the following should be addressed in one’s design documentation:

• Mass timber construction has unique regulations related to fire resistance, height restrictions, and seismic requirements. Construction and design novices must research regulations to ensure they are meeting these requirements.
• Mass timber is noted for its greater speed in construction (+/-25 percent) and subsequent reduced labor compared to traditional construction. That speed assists in limiting moisture exposure.
• While this is a given, verify mass timber can be used per the building code’s construction type, especially exterior wall systems.
• There are currently a few UL-rated top-of-wall (TOW) firestopping systems for the underside of mass timber floors/roofs above. Verify the firestopping manufacturer’s engineering judgments are acceptable to the authorities having jurisdiction (AHJ).
• Structural design of mass timber is prevalently performed by the manufacturer, requiring close coordination with the building’s other structural component engineering. Connections from mass timber structures to other structural framing systems require close coordination between engineers of record.
• The thickness of components can vary from manufacturer to manufacturer. Consider this when detailing and dimensioning mass timber components for competitive bids.
• Provisions and materials used for moisture protection of components can vary from manufacturer to manufacturer.
• Close coordination of exposed-to-view conduits, pipes, ducts, and wiring is required, especially when openings are needed in mass timber components. Consider visible routing of wiring and connections of low-voltage fixtures.
• The “wood” components may include multiple species of trees and various grades. It is essential to ensure the aesthetics match the intended vision.
• Less than 13 percent of forests in the Unite States have certifications from the Forest Stewardship Council, the Sustainable Forestry Initiative, or the American Tree Farm Systems.⁴ Verify the manufacturer uses wood only from certified forests.
• When specifying mass timber, although it may appear to have similarities, consider separate specification sections for Heavy Timber, Laminated Veneer Lumber, Wood I-Joists, Rim Boards, Shop-Fabricated Wood Roof and Floor Trusses, and Glue-Laminated Beams and Columns.
• AXA/XL Insurance’s “Risk considerations in mass timber construction” guideline identifies additional risks to be addressed when using mass timber.
• As with any wood construction, fire protection responsibilities exist during construction. Contractors/owners are required to provide active fire protection systems during construction per Chapter 33 of the International Fire Code (IFC). Codes, however, are evolving quickly to address the nuances of mass timber construction.
• Fire protection system design: Fire flow requirements from Chapter 5 of the IFC should be provided based on Appendix B for a Type IV building. Tall mass-timber buildings, Type IV-A, may use provisions for Type I and II buildings with the approval of the AHJ.

This three-part Guide Specification, 061710-mass-timber-guide,⁵ is a collaborative effort between RDH Building Sciences Inc., Jensen Hughes, and Mead & Hunt. It is available for free use on any mass timber project to support the proper implementation of mass timber construction.

Notes

¹ Refer to A Method to Characterize Biological Degradation Of Mass Timber Connections (2020) by Sinha, Udele, Cappellazzi, and Morrell.

² See Structural Repair of Fire-Damaged Glulam Timber by Chorlton, B., Gales, J. York University. American Society of Civil Engineers.

³ Review BSI-023: Wood Is Good . . . But Strange by Lstiburek, Joseph, Building Sciences Inc.

⁴ Refer to *Certified Forests, by M. Alvarez, U.S. Endowment for Forestry & Communities (2019).

⁵ See meadhunt.com/wp-content/uploads /061710-mass-timber-guide.pdf

Authors

With 40-plus years in design management, Don Koppy, CSI, CCS, AIA, NCARB, is a master construction specifier/architect who has led major construction projects nationwide. A licensed architect in 10 states, a Certified Construction Specifier, and NCARB-certified, he has excelled in roles from project director to BIM manager. His expertise ensures coordinated designs, guiding designers in product selection, detailing, and specifications.

Casey McDonald, B.S., EIT, is an associate and senior project manager at RDH Building Science. With expertise in curtain wall design, cross-laminated timber (CLT), and mass timber research, he specializes in custom unitized assemblies and pre-panelized approaches. He also conducts thermal and hygrothermal analysis and presents on various building science topics.

Tom Jaleski, ICC, is a director and managing principal at Jensen Hughes with more than 30 years of experience working with regulatory agencies. He has authored code appeals, alternates, and variances to resolve complex issues, ensuring safety while reducing costs. An expert in accessibility, fire science, egress, and predictive planning, he specializes in high-rise, healthcare, residential, and commercial facilities.

Key Takeaways

Mass timber is gaining popularity as an eco-friendly building material, yet its reemergence in the construction industry comes with challenges. Historically, mass timber was replaced by steel and concrete due to issues such as moisture, combustion, and staining. While advancements have been made, these concerns persist, requiring careful consideration during design and construction. Proper protection, fire resistance, and coordination among project stakeholders are crucial for successful implementation. The industry must also stay updated on evolving regulations and standards to ensure mass timber’s benefits are fully realized in modern construction.