In a fire, the char forms on mass timber at a predictable rate, slowing combustion, while retaining strength and allowing time to evacuate the building. | Photo: Arup

Modern building codes take fire safety and protection seriously. All building materials experience negative impacts from exposure to fire: steel buckles, concrete spalls and wood burns. Wood-framed structures have served the needs of society since the 1700s—from single- family homes, to larger, multi-story apartment buildings, townhouses, commercial and industrial properties.

In a fire, the char forms on mass timber at a predictable rate, slowing combustion, while retaining strength and allowing time to evacuate the building.

Wood also has a proven record for safety, evidenced by its use not only in 90 percent of all U.S. home construction, but in some of today’s most innovative non-residential architecture.

The International Building Code (IBC) considers fire impact management and fire ignition prevention in its guidance on fire resistance. Wood products may be responsible for load-bearing functions, separating functions or both.

As a result, the IBC considers the following factors when determining fire resistance, or how long a component or assembly can continue its function(s) during a fire event:

  • Structural resistance. The duration the assembly can support its load during a fire event.
  • Integrity. Whether the assembly can prevent the spread of high-heat flames and gasses.
  • Insulation. Whether the assembly can prevent temperatures on surfaces not directly exposed to flames from rising above its pre-fire temperature by 325° F (180° C), or 250° F (140° C) on average.

Fire safety on Construction Sites

International Building Code (IBC) Chapter 33 and the International Fire Code (IFC) provide safety precautions and requirements for fire during construction including:

  • Access to fire extinguishers. 1 portable fire extinguisher must be placed at each stairway on all floor levels with combustible materials.
  • Maintaining means of egress. Means of egress must be maintained during construction, demolition, remodeling or alterations and additions to buildings.
  • Availability of standpipes. In buildings required to have standpipes (water supply piping included in buildings to allow delivery of water to sprinklers and fire hoses on each floor) not less than one standpipe shall be available during construction for fire department use.
  • Commissioning of Sprinkler System. The sprinkler system must be tested and approved before the certificate of occupancy is awarded.

Further resources on how to reduce the frequency and severity of fires during construction are available through the Construction Fire Safety Practices website.

In occupied buildings, methods to mitigate the impact of fire on life, safety and property include:

  • Firewalls
  • Gypsum encapsulation
  • Automatic sprinklers
  • Fire detectors and evacuation plans
  • Fire department consultation and approval

Research and analysis show that wood not only meets prescriptive fire safety and protection codes, but it often surpasses them. In a fire test, a 7-inch-thick wall comprising cross-laminated timber (CLT) with 5/8-inch type X gypsum lasted 3 hours and 6 minutes. That’s one hour longer than code requirements.

(Source: American Wood Council 2016, based on ASTM E119 fire resistance test.

Mass timber products such as CLT, nail-laminated timber (NLT), dowel-laminated timber (DLT) and glue-laminated timber (glulam) offer inherent fire-resistance characteristics that are critical in structural applications.

In a fire test, a 7-inch-thick wall comprising CLT with 5/8-inch type X gypsum lasted 3 hours and 6 minutes. That’s one hour longer than code requirements.

When exposed to fire, the outer layers of thick mass timber members char to provide natural protection against fire penetration. The char layer insulates the wood, slowing combustion and delaying the rate at which heat moves into the layers of wood below. Those areas of the wood not exposed to heat or fire can retain full strength, allowing the member to continue to provide significant structural capacity for the building while it is evacuated. Mass timber’s density means there are no cracks for air or fire to enter, ensuring that the fire’s impact on the wood is gradual and predictable.

Regardless of a structure’s material composition, fire safety policies and procedures are essential to managing a fire event during construction or after occupancy.

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