California Catches up on Mass Timber

New Code Summary

The new code’s clear articulation of what is allowable in mass timber construction will save time in the permitting process and mitigate risk by avoiding alternate means and methods to calculate and prove equivalencies. 

It  also enhances safety by requiring operational safeguards during the construction process –setting limits on the height to which the mass timber structure can be erected without exterior cladding, and mandates on-site fire protection measures, like fire department connections. 

What was formerly known as 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 the columns and beams are sufficiently substantial and that the roof, floor and panel decking meet a required thickness; each Type IV category 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 around 20% of the ceiling and around 40% of the walls can be exposed wood. While The new types IV-A, B, C will still require fire rated partitions between program compartments (eg. residential units) even when they are non-load bearing, they have 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.

Design Implications: Concealing or Exposing Mass Timber

Changes in the building code often mean a new set of constraints on how we design and build. However, in the case of the July changes and additions to the California Building Code, a wealth of design opportunities is created through the definition of clear constraints around mass timber construction methodology. This is a methodology that many jurisdictions in California have struggled to interpret during code analysis and permit review, thereby limiting its perceived viability as a design solution.

Although wood is commonly used as an exterior material to bring a warmth and approachability to urban projects, it is rarely used for structural elements like columns and slabs. For architects, an exciting advantage of Type IV-C is that the structural 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. In the past code cycle, leaving so much of the mass timber structure exposed could only be done for a building of just a few stories, thereby limiting the project types and their possibilities.

Wood visually conveys a sense of warmth through its texture, color, and biophilic quality as a natural material. Exposed wood is often an appropriate material to capture the sense of place for projects in a more natural setting. Spaces that can effectively make a connection to nature have been proven to lower heart rates and reduce stress in its inhabitants. As the building industry is  increasingly thoughtful about sustainability, embodied carbon, and healthy materials, the use of exposed wood on a project can immediately reflect a building owner’s or institution’s prioritization of the planet and people through a considered approach to design, materials, and construction. 

By intentionally exposing the structural elements of a mass timber building, the need for materials like gypsum wall board and paint is reduced–subsequently cutting costs on material, labor 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 both 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 relief. Either the ceiling could not be done, 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 there for Type IV categories as long as one or two layers of gypsum wall board are used–depending on the construction type. 

New Heights Extend Mass Timber Efficiencies to New Building Types

Previously limited to between four to six stories, multi-unit housing–including market- rate, below- market, and student housing product offerings in general–as well as construction of a range of building types in tight urban areas, will benefit the most from the new design possibilities of tall wood buildings. This difference of height, area, and density can make projects that don’t ‘pencil out’ from a schedule or cost perspective today, more viable for developers and owners after July. Oftentimes, the cost savings on the selection of a structural system aren’t derived from a direct comparison of the structural materials itself (mass timber vs. steel vs. concrete), but rather the large savings in foundation cost. Mass timber structures weigh significantly less and 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. The new flexibilities translate to more units and more stories, yielding new advantages for the building owners who can now go taller and larger, while capitalizing on speed and efficiencies in mass timber construction. 

Mass timber’s modularity and flexibility of scale across unit types is a key advantage of mass timber in multi-family or student housing. 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 offset mill using precision CNC routers, so there is very close quality control around dimensional tolerances. Most mass timber systems have tolerances within 1/16”, 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. However, 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.

Construction of denser program types  in constrained, urban areas can require a surgical approach. Mass timber’s logistical benefits bring an additional advantage beyond the material’s aesthetics. The modularity of these products streamline the on-site construction. As a tightly quality-controlled kit of parts, the mass timber structural panels arrive to the site sequentially organized on a flatbed truck so that the building can be assembled with a crane and a handful of workers–facilitating speed, accuracy, and predictable site logistics. This process allows increased control of construction cost and schedule which can be especially challenging on urban sites where unforeseen field corrections can be costly in time and materials. 

When the contractor wraps up their work, the General Conditions savings from the months shaved off the project can be significant. Whether this translates to lease income from early occupancy or an extra semester of student residence; developers and contractors need to look at the project lifecycle to identify the savings, not just the construction costs.