What Is Mass Timber Construction (CLT)?
Mass timber builds structures from engineered wood panels and beams — cross-laminated timber (CLT), glulam, and nail- or dowel-laminated timber — at scales previously reserved for steel and concrete. CLT panels (lumber layers glued crosswise, like giant plywood) form floors and walls with the strength for 8-18+ story buildings. Mass timber sequesters carbon, erects fast, and exposes beautiful wood structure — the 2021 IBC's tall-wood provisions (Types IV-A/B/C) codified its mainstream use.
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The material system
CLT: 3-9+ layers of dimensional lumber glued crosswise into panels up to 10x60 feet — floors, walls, roofs. Glulam: laminated beams and columns for the frame. Connection design is the engineering: steel connectors, proprietary screws, and the fire design (mass timber chars predictably at ~1.5 in/hr — the char layer insulates the structural core, and the IBC's tall-wood types specify the required fire protection). Hybrid systems (timber frame with concrete cores for lateral resistance) are common.
Structural design
Gravity design per the NDS (panel capacities, vibration control for CLT floors — the footfall criterion often governs), lateral design (CLT shear walls with hold-downs, or concrete/steel cores), and the fire design (char calculations per NDS Chapter 16 or encapsulation for Type IV-A). Moisture management during construction is critical — mass timber must stay dry (weather protection sequences are part of the construction plan). Acoustic design (mass timber transmits sound — buildups for STC/IIC ratings).
Code and construction
IBC Types IV-A (18 stories, protected), IV-B (12 stories, limited exposed timber), IV-C (9 stories, exposed timber with char design) — each with specific fire, height, and area limits. Erection is fast (panels crane into place — the 'flat-pack building'), with tight tolerances (CNC-fabricated panels need survey-accurate foundations). Special inspection covers connections and the fire-protection details. Cost: competitive with steel/concrete at 6-12 stories where the speed and carbon story add value.
The bottom line
Mass timber (CLT/glulam) builds 8-18+ story buildings per IBC Types IV-A/B/C — engineer connections, char-based fire design, and moisture protection. Fast erection, carbon benefits, beautiful exposed structure.
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