In a landmark decision for the construction industry, the Washington State Building Code Council (SBCC) approved code changes that permit mass timber as a structural system in buildings up to 18 stories tall. Washington became the first state in the nation to integrate tall mass timber provisions directly into its building code without requiring alternative methods approval. This regulatory shift marks a pivotal moment for structural engineers, architects, and builders working with engineered wood products in commercial and multi-family construction.
The code updates incorporate all of the International Code Council’s Tall Wood Building Code proposals into the IBC 2015 framework. They establish three new construction classifications—Type IV-A, IV-B, and IV-C—that allow mass timber at 18, 12, and 9 stories respectively. These classifications follow more than two years of comprehensive research and testing by the ICC’s Ad Hoc Committee on Tall Wood Buildings, which included full-scale fire tests that validated the performance of mass timber assemblies under realistic fire scenarios.
The Washington Precedent and National Implications
Washington’s adoption of tall mass timber codes creates a template that other states are already beginning to follow. Oregon, California, and several northeastern states have cited Washington’s framework as they develop their own tall wood building provisions. The significance of Washington’s approach lies not in the height allowance alone but in the regulatory path: by amending the adopted IBC directly rather than relying on performance-based alternate methods, Washington established a legally predictable pathway for mass timber construction.
How Washington’s Code Differs From Alternative Methods
Before this change, any building over six stories seeking to use mass timber would need an alternative methods approval—a case-by-case review process that adds time, cost, and uncertainty to project financing. By codifying the tall wood provisions directly, Washington eliminated these barriers. The three-tier system (Type IV-A, B, C) gives design teams a clear framework with predefined fire-resistance ratings, allowable heights, and area limitations.
Key Structural Requirements in the Washington Code
- Type IV-A buildings can reach 18 stories (270 feet) with a 4-hour fire-resistance rating for the primary structural frame
- Type IV-B buildings can reach 12 stories (180 feet) with a 3-hour fire-resistance rating for shafts and 2-hour for frames
- Type IV-C buildings can reach 9 stories (85 feet) with 2-hour ratings for shafts and structural frames
- All exposed mass timber surfaces in Type IV-A buildings must be protected by fire-rated gypsum board or equivalent encapsulation
- Type IV-B permits limited exposed timber areas; Type IV-C allows the most exposed timber, subject to sprinkler and occupant density criteria
These classifications reflect a measured approach to fire safety that balances the aesthetic demand for exposed wood with the life-safety requirements of tall buildings. The graduated system allows design teams to choose the appropriate level of encapsulation based on building height and occupancy, rather than applying a one-size-fits-all standard.
Mass Timber as a Structural System in Tall Buildings
Mass timber encompasses several engineered wood products used as primary load-bearing elements. Cross-laminated timber (CLT) panels, glued-laminated timber (glulam) beams and columns, and nail-laminated timber (NLT) form the structural vocabulary of tall wood buildings. Unlike light-frame wood construction, mass timber assemblies rely on the inherent fire resistance of large-section timber—a char layer forms during combustion that protects the structural core and maintains load-bearing capacity for the required fire duration.
Structural Performance Characteristics
| Property | Mass Timber (CLT/Glulam) | Reinforced Concrete | Structural Steel |
|---|---|---|---|
| Weight per cubic foot | 28-35 lb | 150 lb | 490 lb |
| Compressive strength (parallel to grain) | 1,600-2,400 psi | 3,000-6,000 psi | 36,000-50,000 psi |
| Fire resistance behavior | Char layer forms, predictable burn rate | Spalling possible under extreme heat | Rapid strength loss above 1,000 F with applied fireproofing |
| Carbon footprint (kg CO2 per m3) | -700 to -1,000 (carbon stored) | 400-600 | 12,000-15,000 (virgin production) |
| Typical floor-to-floor cycle | 3-5 days | 7-14 days (curing) | 4-7 days |
| On-site labor requirement | Low (prefabricated panels) | High (formwork, pouring, curing) | Moderate (field welding, bolting) |
The weight advantage is particularly significant for tall mass timber buildings. A mass timber superstructure weighs roughly one-fifth of an equivalent concrete frame, which reduces foundation demands, seismic mass, and column sizes. For projects built on sites with poor soil conditions or in high-seismic zones, this weight reduction translates directly into cost savings on deep foundations and lateral-force-resisting systems.
Connection Design and Lateral Systems
Tall mass timber buildings typically employ hybrid structural systems. Steel or concrete cores provide lateral stability through elevator shafts and stairwells, while mass timber columns, beams, and floor panels carry gravity loads. Connection detailing is critical: bracket connections at beam-to-column joints, self-tapping screws for panel-to-panel shear transfer, and hold-down systems at wall bases all require engineered solutions calibrated to the orthotropic properties of wood. Recent updates to wood construction standards under the National Design Specification for Wood Construction and the Special Design Provisions for Wind and Seismic have provided more specific guidance on these connection types.
Fire Testing That Proved Tall Wood Works
The code changes rest on an extensive testing program conducted by the ICC Ad Hoc Committee on Tall Wood Buildings between 2016 and 2018. The committee conducted full-scale fire tests on mass timber assemblies, including a multi-story furnace test that subjected a two-story CLT compartment to a fully developed fire. The results confirmed that properly designed mass timber assemblies can achieve fire-resistance ratings equivalent to non-combustible construction.
Key Findings From Full-Scale Fire Tests
- Char rate consistency: CLT chars at a predictable rate of approximately 1.5 inches per hour under standard fire exposure, allowing engineers to calculate the residual structural section after any fire duration.
- Self-extinguishment: Once sprinklers activate and the fire load is consumed, mass timber surfaces self-extinguish rather than contributing to sustained flaming, provided the char layer remains intact.
- No structural collapse: In the full-scale tests, CLT floor and wall assemblies maintained structural integrity throughout the fire exposure and cooling phases, with no signs of imminent collapse.
- Encapsulation effectiveness: Tests demonstrated that fire-rated gypsum board encapsulation prevents any contribution to fire growth, effectively converting mass timber assemblies into performance equivalents of non-combustible construction.
- Delamination risk management: The committee identified manufacturing quality standards that prevent adhesive failure at layer interfaces, which could accelerate char penetration. These standards are now part of the code requirements for CLT used in tall buildings.
These findings directly address the most common concern raised by code officials and fire marshals regarding tall wood buildings: the question of whether engineered wood can match the fire performance of concrete and steel. The data supports the conclusion that it can, provided the mass timber elements meet the adhesive bond quality, minimum thickness, and connection detailing specified in the code.
Construction Efficiency and Project Delivery
The speed of mass timber construction is one of its most compelling advantages for developers and general contractors. The Brock Commons project at the University of British Columbia—an 18-story wood-hybrid student residence that served as a reference for Washington’s code committee—was completed in fewer than 70 days from the arrival of prefabricated components on site. A conventional concrete frame would have required four to five months for the same structural scope.
Prefabrication and Tolerances
Mass timber components arrive on site as prefabricated panels and members with CNC-machined connection pockets, pre-drilled holes for services, and factory-applied finishes. This precision reduces on-site cutting and fitting, which in turn lowers labor costs and construction waste. Modern high-rise design and construction methods increasingly incorporate prefabricated building systems, and mass timber represents one of the most mature prefabrication technologies available today.
Sequencing and Crane Requirements
- Floor panels are typically installed in a repeating sequence: columns or walls on one crane lift, followed by beams, then floor panels
- Smaller crew sizes (four to six workers for panel installation) reduce labor costs and site congestion
- Lighter crane capacity requirements (mass timber floors weigh 80-90% less than concrete equivalents) allow smaller, more readily available crane models
- Dry construction eliminates curing time between floor placements, maintaining a consistent three-to-five-day cycle per floor regardless of weather
- Mechanical, electrical, and plumbing rough-in can follow immediately behind the structure without waiting for concrete curing or formwork removal
The dry construction nature of mass timber also has significant implications for project scheduling in climates with cold or wet conditions. Concrete placement becomes difficult or impossible below freezing temperatures, but mass timber erection continues year-round because all connections are mechanical (screws, bolts, brackets) rather than chemical (wet concrete, epoxy). Wood preservation and performance considerations during construction are well understood, and manufacturers deliver components at the proper moisture content for immediate installation.
Cost Considerations for Structural Engineers
While mass timber often carries a material cost premium of 10 to 20 percent over concrete or steel on a per-square-foot basis, the total constructed cost frequently breaks even or comes in lower when the following factors are accounted for:
- Reduced foundation costs due to lighter superstructure weight, particularly on sites requiring deep foundations or pile-supported mat slabs
- Shorter construction schedules that reduce general conditions, site overhead, and temporary protection costs
- Lower crane and equipment costs due to lighter lifts
- Reduced temporary shoring and formwork compared to cast-in-place concrete
- Smaller seismic force demands that translate into fewer shear walls, smaller base connections, and simpler lateral load paths
Sustainability certification programs such as LEED Zero increasingly recognize the embodied carbon benefits of mass timber. The carbon stored in the wood itself, combined with the lower carbon emissions from manufacturing and transport compared to steel and concrete, positions mass timber as a preferred structural system for projects pursuing net-zero carbon certification.
Looking Ahead for Tall Mass Timber Construction
Washington’s adoption of tall mass timber codes has created a regulatory precedent that is reshaping the national conversation around wood construction. The model has been influential in the development of the 2021 IBC provisions, which incorporated the tall mass timber types on a national level. Architects and structural engineers who develop expertise in mass timber design now position themselves at the leading edge of a construction method that combines structural performance, construction speed, and environmental benefits in ways that conventional building systems cannot match.
As more states follow Washington’s lead, the availability of mass timber products and the depth of the engineering talent pool will continue to grow. For structural engineers, the key skills to develop include understanding char-layer fire design methodology, connection detailing specific to mass timber assemblies, vibration serviceability analysis for long-span timber floors, and hybrid structure coordination with concrete or steel lateral cores. The Washington code framework provides the regulatory certainty needed to invest in these capabilities with confidence.