(c) Image courtesy of RDH Building Science
Risks and Rewards of Mass Timber
As the design and construction industry moves toward more eco-conscious systems to construct the buildings we work and live in, mass timber has made it to the forefront of this change in direction. It is currently a favored building material for many leading architectural firms and is being strongly marketed by the wood industry.
While wood construction has never gone away, its use in large buildings was easily replaced by steel and concrete in the last two centuries. Now 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.
Three reasons mass timber lost out to other materials
- Moisture: The wood species used in mass timber are classified as “softwood,” meaning they can both absorb and 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 even high humidity, softwood will grow mold, rot, and decay. We can now counteract that condition, but only by adding chemicals to the natural product, which manufacturers of mass timber simply don’t use. Even with the addition of these products, the need to monitor and maintain moisture content extends from construction throughout the lifecycle of the building.
- Combustion: Wood is combustible, which means 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 only char, but will produce vast quantities of smoke which is the main cause of death in most building fires. However, the cost of repairing charred structural members may well exceed the cost to replace the building. Fire suppression/protection systems are required for most facilities. Mass timber producers currently do not use fire-treated wood. In the future, maybe we can counteract that capacity, but only by adding more chemicals to the natural product.
- Staining: While the exposed wood of mass timber can be beautiful in appearance, it is easily stained by construction activities, and some stains are very difficult to remove. Because of this, its installation is much more of an art than other structural materials.
Due to these and other more minor reasons, wood was basically abandoned for construction of large commercial and institutional buildings, with a preference toward steel and concrete. Once a 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: Mass Timber’s Resurgence
As a response to climate change concerns improving availability, speed on construction, and biophilic considerations, wood in mass timber construction is being quickly reevaluated for building structures. Its implementation has moved beyond the few pioneering designers, manufacturers, and constructors, and into mainstream with much excitement and fanfare.
But is mass timber construction ready to be used by the entire design and construction industry?
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.
As we seek new ways to reduce greenhouse gases in our facility designs, it is imperative their implementation is addressed properly in construction documentation. This is a global design construction industry concern, and we need to share our knowledge to benefit everyone. This is why we are proud to release the 3-part Guide Specification, “Section 061710 – Mass Timber.”
Download the “Section 061710 – Mass Timber” and “Risks and Rewards of Mass Timber Addendum”
The 3-part Guide Specification is a collaborative effort between RDH Building Science, Jensen Hughes, and Mead & Hunt to assist in the proper implementation of mass timber construction.
• Section 061710 – Mass Timber Guide Specification »
• Risks and Rewards of Mass Timber Addendum »
Special thanks to Casey McDonald, Senior Project Engineer at RDH Building Science, and Tom Jaleski, Director at Jensen Hughes, for their contributions to this effort.
