By Anders Carpenter and Sarah Knize
This summer, California will join the tall-wood movement with the adoption and enforcement of new code provisions for mass timber construction. Mass timber, which is an umbrella term for many types of engineered wood, offers a range of benefits including renewability, economy, and design flexibility.
Impact of Code
California’s adoption and enforcement of this code on July 1, 2021 will reduce restrictions for mass timber buildings by allowing up to 18 stories and an increase in permissible square footage of engineered wood construction. This is expected to save time in the permitting process and mitigate risk by avoiding alternate means and methods to calculate and prove equivalencies. Additionally, the new code will increase safety by requiring operational safeguards during the construction process, such as limiting the height to which the mass timber structure can be erected without exterior cladding and mandating on-site fire protection measures like fire department connections.
Type IV construction will be broken into three new categories: IV-A, IV-B, and IV-C. While carrying over the previous code’s minimum size requirement that ensures that the columns and beams are sufficiently substantial and that the roof, floor, and panel decking meet a required thickness, each Type IV category now maximizes the height and area of a building based on how much of the mass timber product is covered by fire-resistant material, such as gypsum wall board. For example, Type IV-B construction allows structures up to 12 stories, and about 20 percent of the ceiling and about 40 percent of the walls can be exposed wood. While the new types IV-A, B, and C will still require fire-rated partitions between program compartments even when they are non-load bearing (like with residential units), the code has eliminated the 1-hour fire rating requirement for noncombustible, non-bearing walls and wood stud walls, which previously required a number of additions, like fire smoke dampers and fire putty, to meet the code.
Multi-unit housing, which was limited by Type IV code to 4 to 6 stories before, will benefit significantly from the design possibilities of tall wood buildings. The difference in allowed height, area, and density will make projects that don’t “pencil out” from a schedule or cost perspective today more feasible for developers and owners come July. Oftentimes, the cost savings on the selection of a structural system aren’t derived from a direct comparison of the structural materials itself (e.g., mass timber vs. steel vs. concrete), but rather the large savings in foundation cost. Mass timber structures weigh significantly less and often require foundations one half to one third the size of their steel or concrete alternatives.
An R-2 occupancy, multi-family housing project was previously limited to five stories, but, as of July, mass timber multi-family housing can be eight stories. The allowable area has also increased from 61,500 sf to 76,875 sf under the new category of Type IV-C. This new flexibility within the code translates to more units and more stories, yielding new advantages for the building owners who can now go taller and larger while capitalizing on the speed and efficiencies of mass timber construction.
The new code will have a notable impact on market-rate, below-market, and student housing, particularly in dense urban areas. As unit sizes increase to include more bedrooms, the modularity of typical room sizes within the units and the mass timber slab widths allow critical alignments to scale up across unit sizes. These modular alignments mean there is a lot less material waste and labor in the field. Mass timber systems are predominantly prefabricated at an offsite mill using precision CNC routers, so there is very close quality control around dimensional tolerances. Most mass timber systems have tolerances within 1/16 of an inch, which means the pieces fit together in a consistent and predictable way, with little variance from governing design dimensions.
For housing projects, this is a critical distinction because accessible and adaptable clearances within units must be maintained once the final built elements are in place. Traditional concrete and steel projects have greater construction tolerances, and because of this, there are often dimensional variances in the final construction that require correction to meet code-required clearances. In contrast, mass timber projects are able to maintain close control of construction outcomes in order to eliminate the need for corrections in the field. The precision also allows for better coordination with housing project trades, like mechanical, plumbing, and electrical.
In the past code cycle, mass timber structure generally could not be exposed, thereby limiting the project types and their possibilities. With the new code, Type IV-C timber elements in the entire eight (residential) or nine (business) stories of construction do not need to be covered in order to meet code, except in select areas such as shafts, concealed spaces, and exterior walls.
By intentionally exposing the structural elements of a mass timber building, the need for materials like gypsum wall board and paint is reduced, thereby decreasing material and labor costs and improving air quality. These materials, often needed to meet fire resistance requirements, can diminish the aesthetic expression of a building’s structure while adding material cost, increasing embodied carbon, and contributing to off-gassing within buildings. In a mass timber building, the structure itself becomes the horizontal and vertical finish material, offering biophilic benefits including warmth, natural beauty, and economy.
The change in allowable concealed spaces is another advantage to designers. Previously, concealed spaces, like a standard dropped ceiling below the mass timber panels, were prohibited in Type IV construction. These types of assemblies had to be completely filled in with insulation and were not suitable for running ducts or conduit. This complicated simple design strategies like an added ceiling for acoustic mitigation. Either the ceiling could not be included, had to be filled, or required extra time and costs to allow all systems to be exposed. The forthcoming provisions allow concealed spaces if the building is sprinklered throughout, if the concealed space uses noncombustible insulation, or if surfaces within the concealed space are lined with gypsum wall board. This gives the design team greater design flexibility when ceilings are desired. This option is now possible for Type IV categories as long as one or two layers of gypsum wall board are used, depending on the construction type.
These new, clearly delineated code requirements for mass timber buildings will allow stakeholders to access the benefits in cost, schedule, and sustainability that are usually reserved for concrete and steel buildings. The code will have the most significant implications for dense mid-rise and high-rise projects like multi-unit housing and will marry economy and beauty in mass timber construction.