Mass timber buildings are moving from pilot projects into mainstream construction, and fire safety in cross-laminated timber (CLT) structures has become one of the most discussed topics in modern building design. CLT panels deliver two-way spans, fast erection times, and a lighter structural footprint, but they also raise a question every owner and code official asks: how does a wood building perform in a fire? The short answer is that engineered timber behaves predictably, because it chars at a known rate and the char layer insulates the structure underneath. Reviewing fire safety in high-rise buildings guidance first gives a useful baseline before examining timber-specific behavior.
This article covers what mass timber actually is, how charring creates fire resistance, which code provisions apply, and what protection strategies designers use to meet fire-resistance ratings.
What Mass Timber Is and How It Differs from Light Framing
The two most common forms of timber construction are light timber framing and mass timber. Light framing uses small-dimension members spaced closely together, while mass timber products are engineered in factories from smaller sawn wood members, with section sizes typically 12 inches or more. Those larger sections change the fire story completely. The key facts about fire and safety features of any building system start with how much combustible material is exposed to heat.
The mass timber family of products
Mass timber solutions include glued laminated timber (glulam), cross-laminated timber (CLT), nail laminated timber (NLT), mass plywood panels (MPP), laminated veneer lumber (LVL), and dowel laminated timber (DLT). Each product has a different layup and connection method, but all share factory-controlled quality that makes fire behavior predictable.
| Product | Composition | Typical use |
|---|---|---|
| Glulam | Lumber laminations bonded with adhesive | Beams, columns, long spans |
| CLT | Boards laminated in alternating directions | Walls, floors, two-way spans |
| NLT | Lumber nailed together on edge | Floors, roofs, industrial decks |
| MPP | Veneers pressed into large panels | Walls, floors, shear panels |
| LVL | Thin veneers laminated in parallel | Beams, headers, rim board |
| DLT | Lumber joined with hardwood dowels | Floors, walls, exposed decks |
Why CLT panels lead the market
CLT panels are the most popular form of mass timber construction because of their two-way span capability and highly efficient construction for walls and floors. Laminating boards in alternating directions lets the product resist loads in both axes, which simplifies layout and cuts the number of beams required.
The benefits show up in the budget and the schedule. Lighter construction can result in savings in foundation works compared with other materials, offsite prefabrication allows highly accurate production and faster overall construction times, and smaller work crews finish the job safely and quietly. Owners and developers looking for a highly sustainable building solution also meet their goals with mass timber, because the panels store carbon and replace heavier, higher-emission materials.
How Charring Creates Fire Resistance
The fire safety requirements for any building start with the fire-resistance rating (FRR) each structural member must achieve. Light-timber construction is encapsulated within non-combustible gypsum drywall to achieve protection from fire and an FRR. Mass timber works differently: when exposed to fire, it achieves its FRR through the insulating benefits of charring. The char layer protects the load-carrying ambient-temperature wood underneath, so the member can be designed to carry applied forces for a known fire duration.
The char layer and the residual section
Char forms at a predictable rate for each species and density. For the softwoods commonly used in CLT, a typical design value is roughly 1.5 inches of char per hour of fire exposure. Char itself is a poor conductor of heat, which slows the temperature rise deeper into the member. Engineers calculate a reduced residual section that stays at ambient temperature and carries the structural load for the required duration.
Fire-resistance ratings come from standard tests such as ASTM E119, which expose assemblies to a time-temperature curve and measure how long the assembly contains the fire and carries its load. For timber, the test also captures the char profile and any fall-off of charred layers, and those results feed the calculation methods used in design.
Char rates and sacrificial layers
Designers either accept the char depth as a sacrificial layer or protect the member so no char forms. The choice depends on the required FRR, whether the fire exposes one side or two, and whether the panel is load-bearing. Layer delamination can accelerate heat transfer, which is why panel layups and adhesive performance under fire are part of the product certification.
Code Requirements and Height Limits for Timber Construction
Within the United States, each state adopts one or more model building codes, and all 50 states adopt the International Code Council (ICC) International Building Code (IBC). Some states also adopt the National Fire Protection Association (NFPA) Life Safety Code, otherwise known as NFPA 101. Many states further amend the model codes to provide the basis for construction compliance. The International Fire Code (IFC) adds requirements for construction, fire-protection systems, maintenance, and firefighting operations, and the fire safety and property protection systems a building needs depend on its construction type and height.
Construction types that allow timber
Within the IBC, timber can be utilized in Types III, IV, and V construction. Types III and IV can be constructed to a maximum building height of 85 feet under the base code, and taller allowances are available through heavy timber and mass timber provisions in newer editions. The American Wood Council and WoodWorks provide resources and technical guidance on how timber construction can be utilized to maximize height and area.
The 75-foot high-rise trigger
The IBC requires buildings with an occupied floor above 75 feet, defined as high-rise, to have an increased level of fire protection and structural performance. Compared with a mid-rise building, there is a significant increase in expected structural performance for all high-rise buildings in fire. Crossing that threshold adds fire-resistance requirements, more suppression and detection capacity, and stricter structural robustness expectations, because evacuation takes longer and fire service access is harder.
Protection Strategies: Encapsulation, Connections, and Compartmentation
Design teams choose among several protection strategies to meet the required FRR. Encapsulation covers timber surfaces with non-combustible materials such as gypsum board for a defined period, usually one or two hours, so the wood never reaches charring temperatures during the rating period. Exposed timber designs instead rely on the charring model, accepting visible wood while calculating the sacrificial layer. The same logic that makes highway agencies run safety audits before a road opens applies here: a formal review of every unprotected surface, connection, and penetration catches the details that cause failures.
Encapsulation periods and exposed surfaces
Encapsulation is the most direct way to raise an FRR: each layer of Type X gypsum adds roughly 30 to 60 minutes of protection. Fully encapsulated buildings often specify two-hour protection on structure and one-hour protection on non-load-bearing elements. Exposed timber designs keep the aesthetic but must account for char depth on every side of a member, including the underside of floors and the faces of columns.
The protection strategy falls into one of three patterns:
- Full encapsulation: every timber surface is covered with gypsum for the full rating period.
- Partial encapsulation: only critical members are covered, with char calculations on the rest.
- Exposed timber with char allowance: no covering, but the design accepts the calculated char depth.
Connections are the weak point
Steel connectors and fasteners conduct heat into the core of a timber member, so they are typically embedded, wrapped with fire-resistant materials, or sized with extra timber around them. A bolted connection that passes through a panel can fail before the panel itself, which is why connection design gets its own review on every CLT project. Fire-stopping around service penetrations is equally critical: a missing seal at a pipe sleeve can let flames travel through a wall assembly that otherwise meets its rating.
Suppression, Detection, and Performance in Real Buildings
Automatic sprinkler systems are the most effective partner for mass timber. In a sprinklered CLT building, a fire forms a char layer and then water cools the surface and stops further charring. Detection systems matter too, because an early alarm gives occupants more time to evacuate and firefighters more time to respond while the structure is still fully intact. The fire and safety features of high-rise buildings, from sprinklers to standpipes, combine with the timber structure itself to deliver the overall performance.
Compartmentation is the third line of defense. Floor and wall assemblies divide the building into fire compartments sized so occupants can evacuate before conditions become untenable, and the structure can survive burnout of the compartment of origin. CLT compartments perform well in tests because the panels are airtight and the char layer keeps the load path intact, provided every joint and penetration is sealed.
What full-scale tests show
Full-scale fire tests on CLT assemblies consistently show that panels maintain load-bearing capacity through the design fire when char rates and delamination are accounted for. Compartment tests demonstrate that the biggest risk is not the timber burning on its own but unsealed penetrations and unprotected connections that let fire spread between compartments. These findings drove the tall wood provisions that now allow timber buildings well beyond the original 85-foot limit, subject to additional engineering review.
What Owners and Designers Should Verify Before Approval
Before a CLT project moves to construction documents, run the design against a short checklist. The review should confirm the fire-resistance rating of every structural member, the char calculation method and species data, the encapsulation schedule, and the protection of connections and penetrations. Details that look minor on paper, such as fire-rated glass in an atrium wall or the seal around a pipe sleeve, become critical in a timber building.
Pre-approval checklist
- Confirm the construction type and height allowance under the adopted IBC edition.
- Verify the FRR for columns, beams, floors, and walls from the char model or the encapsulation schedule.
- Review connection details and service penetrations with the fire protection engineer.
- Confirm sprinkler coverage, detection, and standpipe requirements for the building height.
- Check the fire department access and water supply assumptions with the local authority.
- Document the maintenance plan for exposed timber and any intumescent coatings.
Mass timber changes the conversation about wood construction because its fire behavior is measurable and designable. The char layer, the code framework, and the protection details work together to give owners the same level of safety they expect from any building, with the speed and sustainability that timber provides.