Mass Timber and CLT Construction: The Wood Structure System Expanding Into Taller Commercial Buildings
Mass timber construction has moved rapidly from European specialty and North American niche into mainstream commercial construction. IBC 2021 introduced Types IV-A, IV-B, and IV-C allowing mass timber buildings up to 18 stories. CLT (cross-laminated timber), glulam beams and columns, NLT (nail-laminated timber), and DLT (dowel-laminated timber) form structural systems with distinctive aesthetics, faster erection than concrete, and embodied carbon benefits versus traditional materials.
Contractors working with mass timber face specific coordination challenges — panel specification, connection details, fire protection, weather protection during construction, and tolerance-sensitive erection. Understanding mass timber fundamentals supports effective delivery. This post covers mass timber coordination for GCs.
Several engineered wood products serve structural use:
Mass timber products
- CLT (cross-laminated timber) — layered perpendicular lamellas, walls and floors
- Glulam — dimensional lumber laminated with grain parallel, beams and columns
- NLT (nail-laminated timber) — dimension lumber nailed edge-to-face, floors
- DLT (dowel-laminated timber) — wood dowels replace nails
- MPP (mass plywood panels) — plywood layers
- LVL (laminated veneer lumber) — for beams in specific applications
Each product has specific structural uses. CLT forms walls and floor plates. Glulam forms long-span beams and columns. NLT/DLT for floor systems. Selection depends on structural requirements, aesthetic, and sourcing. North American manufacturers have grown substantially.
IBC 2021 expanded mass timber heights:
IBC Type IV categories
- Type IV-HT (Heavy Timber) — traditional, limited heights
- Type IV-A — up to 18 stories, fully protected mass timber
- Type IV-B — up to 12 stories, mostly protected, some exposed
- Type IV-C — up to 9 stories, more exposed timber allowed
- Specific fire-resistance ratings and finish requirements
- Sprinklers required
Type IV categories distinguish by amount of protected vs exposed mass timber. Type IV-A requires encapsulation; IV-C allows more exposure but limits height. Specific fire ratings for each. Compliance with IBC provisions enables tall wood buildings impossible under older codes.
Fire performance is critical:
Mass timber fire performance
- Char rate predictable (approx. 1.5 inches/hour)
- Char provides insulation protecting interior
- Oversized members for fire resistance
- Encapsulation where required (Type IV-A)
- Penetration firestopping
- Connection protection
- Sprinkler systems required
Mass timber char behavior differs from dimensional lumber. Char rate is calculable, leading to predictable fire-resistance ratings based on member size. Oversized members achieve required ratings. Encapsulation with gypsum or other materials provides additional protection where required.
Connections determine performance:
Connection types
- Self-tapping structural screws
- Steel brackets and hangers
- Wood-to-wood connections
- Concealed connections (aesthetic)
- Bolted connections
- Glued connections (some cases)
- Specific manufacturer hardware
Connections are specialized. Long self-tapping screws common. Steel brackets hidden or expressed. Concealed connections produce clean aesthetic but complicate assembly. Manufacturer hardware specific to their panels. Engineering coordination essential.
Panels fabricated and shipped:
Panel logistics
- CNC fabrication with precise dimensions
- Openings and connections pre-cut
- Panels shipped to site
- Size limitations (truck transport)
- Handling equipment (cranes, specialty rigging)
- Sequencing matches erection schedule
- Weather protection during transport
CNC fabrication produces precise panels with openings and connection hardware pre-installed. Shipping to site requires coordination. Panel sizes limited by transport. Sequencing aligns with erection — just-in-time delivery common to minimize storage.
Mass timber erects quickly:
Erection characteristics
- Fast erection (days per floor on repetitive structures)
- Crane-dependent
- Weather-sensitive
- Minimal shoring typically
- Connections inspected after placement
- MEP rough-in often during erection
- Sub-grade waterproofing before structure
Mass timber erection is often faster than steel or concrete. Prefabricated panels placed and connected. Weather days affect substantially. Crane logistics important. Coordination with MEP trades who may start early.
Weather protection during mass timber construction is critical. Panels exposed to sustained moisture can suffer damage, discoloration, or dimensional changes. Dry-in schedule, temporary roofing, and covers during overnight or weather events protect the investment. Construction phasing that minimizes weather exposure supports quality.
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MEP Coordination
MEP integration differs from steel/concrete:
MEP in mass timber
- Dropped ceiling common where timber exposed
- Routing exposed for MEP below deck
- Penetrations through panels pre-planned
- Fire sprinklers typically dropped
- Electric and data chases designed
- Less flexibility than steel frames
- BIM coordination critical
MEP routing must work around structural mass timber. Exposed timber aesthetic often drives ceilings below or expressed systems. Penetrations pre-planned in shop — field drilling limited. BIM coordination essential to avoid field conflicts.
Acoustic considerations specific:
Mass timber acoustics
- Impact noise transmission (walking, dropping items)
- Floor topping (concrete) for impact reduction
- Resilient channels for airborne isolation
- Acoustic underlayments
- Design to STC and IIC targets
- Testing verifies
- Topping weight affects structure
Bare CLT floor transmits impact noise. Concrete topping (2-4 inches) improves impact and airborne isolation substantially. Acoustic underlayments supplement. Design coordinates structure, topping, and finishes to meet acoustic targets for intended use.
Exposed timber is aesthetic feature:
Aesthetic considerations
- Exposed wood grain and color
- Protection during construction from damage
- Finish selection (stained, natural, clear coat)
- Panel joint appearance
- Connection aesthetic (concealed vs expressed)
- Consistent wood species and appearance
- Repair of damage challenging
Exposed timber is selling point of mass timber. Protection during construction is essential — dings and stains are difficult to repair. Finish selection affects appearance. Project teams that protect exposed timber substantially produce better final result.
Environmental benefits drive adoption:
Mass timber sustainability
- Carbon sequestration in wood
- Lower embodied carbon than concrete or steel typical
- Sustainably sourced certified wood
- FSC or SFI certification
- Reduced construction emissions (faster erection)
- End-of-life considerations
- Biophilic benefits
Carbon sequestration in wood plus lower embodied carbon of timber vs concrete/steel drives sustainability interest. Responsibly sourced wood (FSC, SFI certified) supports story. Environmental performance marketing differentiates projects. Biophilic benefits well-documented.
Mass timber construction — CLT, glulam, NLT, DLT — has become mainstream option for commercial buildings. IBC 2021 Types IV-A/B/C authorize up to 18 stories depending on protection level. Fire performance through predictable char rate and oversized members. Specialized connections using screws, steel brackets, and manufacturer hardware. Precise shop fabrication with CNC. Fast erection with careful logistics. MEP coordination specific to structure. Acoustic performance requires topping and design. Aesthetic of exposed timber requires construction protection. Sustainability benefits drive owner interest. For contractors developing mass timber capability, expertise compounds across projects. Mass timber is growing segment deserving sector-specific attention. The combination of speed, aesthetic, and sustainability makes it compelling for appropriate projects.
Written by
Marcus Reyes
Construction Industry Lead
Spent twelve years running AP at a $120M general contractor before joining Covinly. Lives in the world of AIA G702/G703, retainage schedules, and lien waiver deadlines. Writes about the construction-specific workflows that generic AP tools get wrong.
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