Cross-Laminated Timber in Tall Buildings: Material Properties That Make Mass Timber a Viable Structural System

Cross-laminated timber (CLT) has moved from experimental curiosity to a legitimate structural framing system for mid-rise and high-rise construction. Architects and engineers are specifying CLT for buildings that push well beyond the traditional height limits of wood construction, leveraging its strength-to-weight ratio, prefabrication precision, and environmental benefits. For builders and specifiers evaluating mass timber for their next project, understanding the building performance advantages of CLT along with its structural behavior and code pathways is essential. This article examines how cross-laminated timber performs as a tall building material and what construction professionals need to know about designing with it.

How Cross-Laminated Timber Is Manufactured and Why It Works at Height

Cross-laminated timber is an engineered wood product made by stacking layers of dimension lumber at 90-degree angles and bonding them under pressure with structural adhesives. The perpendicular layering gives CLT its defining characteristic: dimensional stability and two-way spanning capacity that solid timber cannot match.

The Lamination Process

CLT panels are manufactured in a controlled factory environment using kiln-dried lumber, typically spruce-pine-fir (SPF) or Douglas fir. The process follows these steps:

1. Individual lumber boards are graded, finger-jointed, and planed to consistent thickness.
2. An odd number of layers (typically three, five, or seven) are arranged with alternating grain orientation.
3. Structural polyurethane or melamine-based adhesives are applied between each layer.
4. The stacked assembly is pressed under hydraulic or vacuum pressure for several hours.
5. Cured panels are trimmed, sanded, and cut to final dimensions with CNC precision.

Why CLT Suits Tall Buildings

The orthogonal layering produces a panel with strength properties in both directions, unlike sawn lumber or glulam which are strongest only along the grain. This bidirectional strength enables CLT to serve as floor diaphragms, shear walls, and roof decking in the same assembly. A CLT panel weighs roughly one-fifth of an equivalent reinforced concrete slab while delivering comparable load-bearing capacity. This weight reduction translates directly into smaller foundations, reduced seismic mass, and faster crane cycles during erection.

Panel Sizes and Connection Systems

Standard CLT panels range from 1.2 to 3 meters wide and up to 16 meters long, limited only by transportation constraints. Panels are connected with self-tapping screws, steel splines, or proprietary bracket systems that transfer shear and uplift forces between adjacent panels. The connections are a critical design element: the panel-to-panel joint detailing determines the overall diaphragm stiffness and the building’s ability to resist lateral loads from wind and seismic events.

Structural Performance and Fire Resistance of CLT Systems

Concerns about structural performance and fire safety have historically limited wood buildings to six stories or fewer under most building codes. Modern CLT systems address both concerns through char-layer fire design and rigorous testing programs.

Fire Performance Through Char Layer Design

When exposed to fire, CLT panels form a protective char layer that insulates the remaining unburned wood and maintains structural capacity. The 2015 National Design Specification (NDS) for Wood Construction introduced a calculated char rate methodology that allows engineers to size CLT members with a sacrificial char zone. The key points:

• Char forms at approximately 0.65 mm per minute during standard fire exposure.
• A 30 mm sacrificial layer provides roughly 45 minutes of fire resistance.
• Connections must be protected with intumescent coatings or encapsulated within the CLT cross-section.
• Encapsulation requirements in the International Building Code (IBC) mandate fire-rated gypsum board coverage for taller mass timber buildings.

Seismic and Wind Performance

CLT’s light weight compared to concrete or steel provides a distinct advantage in seismic design. Lower building mass reduces the base shear demand imposed by earthquake ground motions. Full-scale shake-table testing at the University of California San Diego demonstrated that a ten-story CLT building can meet stringent seismic performance criteria without supplemental damping systems.

Notable Tall CLT Projects and What They Demonstrate

Several landmark projects have demonstrated the feasibility of CLT in tall buildings and provided valuable data for the wider construction industry.

Framework at Block 75, Portland Oregon

At 12 stories, Framework was designed as the tallest mass timber building in the United States at the time of its approval. The project uses a CLT lateral-force-resisting system combined with glulam beams and columns. The structural design relies on CLT shear walls and floor diaphragms working together as a unified system. The project underwent rigorous fire testing that demonstrated a mass timber assembly using CLT and glulam members can meet fire-resistive requirements for buildings of the same height and occupancy classification.

UMass Amherst Design Building

The four-story, 87,200 square foot Design Building at the University of Massachusetts Amherst features an exposed glulam post-and-beam system with CLT shear wall cores and roof decking. A CLT zipper truss spans the building commons area at the second level. The building targets LEED Gold certification and serves as both a teaching tool and a demonstration of sustainable structural engineering. The project was informed by the university’s ongoing research in building technology and timber engineering.

CLT Modular Classroom Pilot Program

In Washington state, five school districts are participating in a $5.5 million pilot program using CLT modular construction for K-3 elementary classrooms. The CLT is sourced from beetle-killed fir and pine trees that would otherwise pose a wildfire hazard in the region. This program demonstrates how CLT manufacturing can create local economic value from otherwise wasted forest resources while reducing fire risk through managed thinning.

Codes, Standards, and Specification Pathways for CLT

Specifying CLT requires referencing the correct product standards and knowing how the building code treats mass timber construction.

Applicable Standards

The primary manufacturing standard for cross-laminated timber is ANSI/APA PRG 320, Standard for Performance-Rated Cross-Laminated Timber. This standard establishes:

• Layer thickness and grade requirements for each ply
• Adhesive performance criteria for wet-use and dry-use conditions
• Quality control testing protocols for production facilities
• Marking and certification requirements for code compliance

Design Values and Connection Provisions

The 2015 NDS for Wood Construction added a dedicated chapter on CLT design that includes:

• General connection provisions revised to accommodate CLT in the dowel-type fastener chapter
• New sections for wood screw and nail withdrawal from the end grain of CLT panels
• Dowel bearing strength determination methods for fasteners installed perpendicular and parallel to CLT faces
• Placement provisions for lag screws and other threaded fasteners in CLT assemblies

Design values for specific CLT products are obtained from manufacturer literature or ICC-ES code evaluation reports. Because CLT is a manufactured product with proprietary layups and adhesive systems, generic tabulated values are rarely sufficient for final design. Engineers should obtain the specific product report from the CLT supplier and verify that the values match the intended structural application.

Building Code Pathways for Tall Mass Timber

The 2021 International Building Code introduced three new construction types for mass timber (IV-A, IV-B, and IV-C), allowing exposed timber structures up to 18 stories under specific conditions. These types require:

1. Encapsulation of mass timber elements with fire-rated gypsum board (Type IV-A: all surfaces; IV-B: most surfaces; IV-C: limited surfaces)
2. Sprinkler protection throughout the building
3. Non-combustible protection for exit stair enclosures and elevator shafts
4. Enhanced connection fire protection at panel joints and steel hardware locations

Sustainability and Embodied Carbon

CLT’s role in reducing the embodied carbon footprint of buildings is one of its strongest selling points. A cubic meter of CLT stores approximately one tonne of CO2, making mass timber buildings carbon-negative when the sequestered carbon in the wood is accounted for. Skidmore, Owings & Merrill’s Timber Tower Research Project demonstrated that a high-rise mass timber structure can reduce embodied carbon by 60 to 75 percent compared to an equivalent concrete building. CLT buildings also pair well with other sustainable strategies such as vegetated roof systems that improve building performance through enhanced stormwater management and thermal insulation.

For specifiers evaluating CLT, understanding the interaction between the structural system and the building envelope air and vapor control layers is essential. Mass timber buildings require careful moisture management during construction and throughout the service life of the assembly.

Specification Checklist for CLT Projects

When writing specifications for a cross-laminated timber structure, include the following elements:

• Product standard: ANSI/APA PRG 320 with the required stress grade and appearance class
• Panel thickness and layer count based on structural and fire-resistance calculations
• Connection system type and corrosion protection requirements for steel hardware
• Moisture protection plan including construction sequencing and weather enclosure requirements
• Acoustic performance criteria for floor-ceiling assemblies in residential and mixed-use occupancies
• Finish expectations: exposed architectural surfaces require tighter fabrication tolerances and may need additional sanding or coating

The design process considerations for CLT buildings differ from conventional steel or concrete structures in important ways: panel layouts must be coordinated with MEP penetrations in advance, connection tolerances are tighter, and the erection sequence must account for the panel curing and fastening schedule. Early engagement with the CLT manufacturer and the structural engineer of record is critical to a successful project.

For longer spans or where CLT alone does not provide sufficient structural capacity, composite systems using CLT with a concrete topping slab can extend span capabilities while adding acoustic mass and fire resistance. These hybrid systems are increasingly common in taller mass timber buildings where floor vibration and acoustic separation are design drivers. Understanding material selection principles across different building systems helps specifiers make informed decisions about the most appropriate assembly for each project condition.

Cross-laminated timber has proven itself as a viable structural system for tall buildings through rigorous testing, successful real-world projects, and code recognition. As CLT manufacturing capacity continues to expand across North America, the material will become an increasingly accessible option for builders and developers seeking sustainable, code-compliant framing systems.