Sustainable Wood Products: Mass Timber and CLT Construction

Mass timber is transforming the construction industry by enabling tall wood buildings that rival steel and concrete in performance while storing carbon and reducing environmental impact. This innovative family of engineered wood products is making wood a viable option for applications once limited to traditional materials.

What is Mass Timber?

Mass timber refers to large-format engineered wood products used as structural elements in buildings. Unlike dimensional lumber, mass timber products are manufactured by bonding multiple layers of wood together to create large, solid panels, beams, or columns.

Common Mass Timber Products:

Cross-Laminated Timber (CLT)
Glue-Laminated Timber (Glulam or GLT)
Nail-Laminated Timber (NLT)
Dowel-Laminated Timber (DLT)
Mass Plywood Panels (MPP)
Structural Composite Lumber (SCL)

Key Characteristics:

Engineered for consistency and predictable performance
Dimensional stability compared to solid wood
Fire resistance through char layer formation
Renewable and carbon-storing
Prefabricated for rapid construction

Cross-Laminated Timber (CLT)

CLT represents the most significant innovation in mass timber, enabling wood to compete with concrete and steel in mid-rise and high-rise applications.

Manufacturing and Properties

Structure:

Odd number of layers (typically 3-9)
Layers oriented perpendicular to adjacent layers
Dimension lumber bonded with moisture-resistant adhesive
Like plywood, but with lumber instead of veneer

Materials:

Primarily softwoods (spruce, pine, fir in North America)
Hardwood CLT development underway
Only two components: lumber and adhesive

Standard Panels:

Up to 10 feet wide, 60 feet long
Thickness: 3-20 inches typical
Custom sizes and shapes possible

Structural Performance:

High strength-to-weight ratio
Bi-directional load capacity
Excellent seismic performance (flexibility)
Minimal thermal bridging
Dimensional stability

Applications

Floors and Roofs:

Spanning capabilities comparable to concrete
Exposed or concealed installations
Acoustic performance can be enhanced
Service integration possible

Walls:

Shear walls for lateral resistance
Load-bearing walls
Partition walls
Can be used in hybrid systems

Building Types:

Residential (single and multi-family)
Commercial offices
Schools and institutional
Hotels
Mixed-use developments

Glue-Laminated Timber (Glulam)

Glulam has been used for decades in timber construction and remains essential for mass timber structures.

Manufacturing:

Dimension lumber glued together parallel to grain
Can be straight or curved
Custom sizes and shapes
Lengths exceeding 100 feet possible

Applications:

Beams and girders
Columns
Arches
Curved and complex geometries
Heavy timber frames

Advantages:

High strength and stiffness
Large span capability
Architectural expression
Predictable performance
Less expensive than CLT for linear elements

Code Provisions and Building Height

The 2021 International Building Code introduced significant changes enabling taller mass timber buildings.

Construction Types

Type IV-A, IV-B, IV-C:

New construction types specifically for mass timber
IV-A: Up to 18 stories
IV-B: Up to 12 stories
IV-C: Up to 9 stories

Requirements:

Specified char rates for exposed wood
Fire-resistance ratings for assemblies
Compartmentation and fire separation
Sprinkler protection
Limitations on combustible exterior walls

Traditional Type IV (Heavy Timber):

Still exists for smaller buildings
Larger minimum member sizes
Exposed wood surfaces

Fire Performance

Char Layer Protection:

Wood chars at predictable rate (approximately 1.5 inches/hour)
Char layer insulates inner wood
Structural core remains intact and cool
Performance confirmed through extensive fire testing

Testing Standards:

ASTM E119 fire resistance testing
1 to 3-hour fire ratings achievable
Mass timber can be part of fire-rated assembly
Prescriptive and performance pathways available

Design Considerations:

Exposed vs. protected wood strategies
Fire-resistance rating requirements
Connections and penetrations critical
Compartmentation to limit fire spread

Sustainability and Carbon Benefits

Carbon Storage:

1 cubic meter of wood stores approx. 0.9 tons of CO2
Carbon remains stored for building's lifespan
Mass timber buildings can be carbon sinks

Embodied Carbon Comparison: Compared to concrete/steel equivalents:

25-75% lower embodied carbon typically
Varies by product, sourcing, and methodology

Renewable Resource:

Forests managed for timber regrow and sequester more carbon
Sustainable forestry certification (FSC, SFI, PEFC)
Shorter replacement cycle than fossil fuels

Life-Cycle Benefits:

Lower manufacturing energy
Lighter weight reduces foundation requirements
Potential for reuse at end-of-life
Biogenic carbon accounting favors wood

Cautions:

Must come from sustainably managed forests
Transportation distance matters
Adhesives have environmental impact
End-of-life scenarios affect overall carbon picture

Construction and Installation

Prefabrication:

Panels manufactured off-site to exact specifications
CNC machining for openings and connections
Quality control in factory environment
Reduced site waste

Installation Speed:

Brock Commons (Vancouver): 18 stories in 70 days (9 installers)
Typical: 2-3 floors per week
Weather-independent once enclosed
Quieter construction site

Connections:

Steel brackets and plates typical
Screws, nails, or specialized fasteners
Engineered for load transfer
Fire protection of connections critical

Hybrid Systems:

Mass timber combined with concrete and steel
Concrete cores for elevators/stairs
Steel connections
Optimizes each material's strengths

Design Considerations

Structural Design:

Engineered design required (not prescriptive)
Deflection often governs (not strength)
Vibration analysis for floors
Lateral systems for wind and seismic
Connection design critical

Acoustics:

Mass and decoupling needed for sound isolation
Topping slabs or resilient underlayments
Ceiling systems for vertical sound control
Testing and modeling recommended

Moisture Protection:

Protect from moisture during construction
Enclosure strategies
Drying time if wetted
Long-term moisture monitoring in some cases

Services Integration:

Penetrations for MEP systems planned
Fire-stopping at penetrations
Cable/conduit routing considerations
Access for maintenance

Market Development

North American Production:

10+ manufacturing facilities in operation
Additional plants in development
Increasing domestic supply reducing lead times
Transportation costs favor regional production

Global Leaders:

Europe (Austria, Germany, Switzerland) pioneered CLT
Scandinavia strong in glulam and mass timber generally
Canada advancing quickly
Australia and New Zealand growing markets

Project Examples:

Brock Commons (Vancouver):

18 stories, student housing
2,400 students
Hybrid mass timber and concrete
Demonstrated speed and feasibility

Ascent (Milwaukee):

25 stories
Tallest mass timber building in the world
Residential tower
Hybrid system with concrete

Framework (Portland, OR):

12 stories
Mixed-use
Type IV-A construction
Commercial retail and residential

Economic Considerations

Cost Competitiveness:

Comparable or slightly higher than concrete/steel
Premium varies: 0-10% typically
Speed advantages offset material costs
Market maturity reducing premiums

Speed-to-Market:

Faster construction = earlier occupancy
Reduced financing costs
Revenue generation starts sooner
Significant economic advantage

Value Factors:

Aesthetic appeal commands rent premiums
Sustainability marketing value
Tenant attraction and retention
Biophilic design benefits (well-being)

Challenges and Solutions

Perception Issues:

Unfamiliarity among stakeholders
Fire safety concerns (unfounded but persistent)
Regulatory approval in some jurisdictions

Solutions:

Education and completed project tours
Fire test data and analysis
Advocacy for code acceptance

Supply Chain:

Limited manufacturing capacity historically
Lead times can be long
Transportation costs for heavy panels

Solutions:

Increasing domestic manufacturing
Regional production facilities
Improved logistics and planning

Technical Expertise:

Design and engineering knowledge developing
Fewer experienced contractors
Specialized connections and details

Solutions:

Training and education programs
Design guides and resources
Manufacturer technical support
Collaborative project delivery models

Future Outlook

Market Growth:

Compound annual growth rate 12-14% projected
Mainstream adoption increasing
High-rise and commercial expansion

Technology Development:

Hardwood CLT for underutilized species
Improved connection systems
Integrated MEP solutions
Robotic fabrication and assembly

Policy Support:

Green building incentives
Buy Clean programs favoring low-carbon materials
Forest products promotion
Climate action plans supporting mass timber

Research Needs:

Long-term durability studies
Seismic performance data
Acoustics optimization
Fire performance refinement

Conclusion

Mass timber products represent a paradigm shift in construction, proving that engineered wood can deliver the performance needed for large, tall buildings while providing significant environmental benefits. With strong structural capabilities, fire resistance, rapid construction, and carbon storage, mass timber offers a compelling alternative to conventional materials.

As manufacturing capacity increases, costs become more competitive, and technical expertise grows, mass timber will become an increasingly common choice for sustainable construction. The combination of renewable materials, reduced embodied carbon, construction speed, and architectural beauty positions mass timber as a cornerstone of low-carbon building futures.

For architects, developers, and builders committed to sustainability without compromising performance, mass timber delivers proven results today and tremendous potential for tomorrow.