1) Is mass timber fire resistant?
Heavy timber and mass timber building elements char at a slow and predictable rate, providing for inherent fire resistance. During fire exposure, mass timber chars on the outside, which forms an insulating layer protecting interior wood from damage. During a fire resistance test of a 5-ply cross-laminated timber (CLT) panel wall, the panel was subjected to temperatures exceeding 1,800 degrees Fahrenheit. The assembly sustained loads for three hours and six minutes, far more than the two-hour fire resistance rating that building codes require. Additionally, when the code requires mass timber to be protected with gypsum wall board, it can achieve nearly damage-free performance during a contents-fire burnout event.
2) How is a structural member’s fire resistance measured?
Fire resistance ratings for mass timber building elements are commonly developed from empirical models contained in the National Design Specification® for Wood Construction and TR10 – Calculating the Fire Resistance of Wood Members and Assemblies. The models use char rate data collected during ASTM E119 fire resistance test of exposed mass timber. Since char rates are predictable, it has been possible to develop structural models that account for the loss of cross section due to charring and the ability of the member to support the applied load. Like all materials, structural failure eventually occurs. For steel, failure is due to weakening of the metal. For mass timber, failure occurs when the cross section is no longer adequate.
3) What’s important to know about fire resistance rated assemblies?
Fire resistance rating is performance criteria used in the building code to ensure the structure will not collapse when exposed to fire. The fire resistance rating time assigned to a building is based on the perceived risk due to building area, height, and occupancy. Buildings that represent a greater risk are required to have a greater fire resistance rating. Any material that can achieve the required fire resistance is permitted, but there are often limits to the allowable height and area of combustible construction.
4) Does a building’s construction type determine how mass timber systems can be used?
Buildings are classified according to height and area limitations so the construction type does have an effect on how mass timber systems can be used. In the 2021 International Building Code (IBC), mass timber construction is classified as Type IV and has four subcategories, A, B, C, and Heavy Timber (HT). The code currently allows:
- Type IV-A – Maximum 18 stories, with gypsum wallboard on all mass timber elements.
- Type IV-B – Maximum 12 stories, limited-area of exposed mass timber walls and ceilings allowed.
- Type IV-C – Maximum 9 stories, all exposed mass timber designed for a two-hour fire resistance.
- Type IV-HT – Maximum 6 stories, previously the Heavy Timber construction type under the 2018 IBC.
Types III and V permit the use of light wood framing throughout much of the structure. However, it is possible to classify a building as Type III or Type V when constructed primarily of mass timber building elements due to the height and area of the building. For example, a mass timber building six stories in height could be classified as Type III-A or a four-story building as a Type IV.
5) How does mass timber’s fire performance compare to other structural materials?
Building codes require all building systems to perform to the same level of safety, regardless of material—and wood-frame construction is approved in the IBC and International Residential Code (IRC).
For mass timber specifically, the result of the years-long ICC process to develop and approve safety requirements for each of the new types of construction (Type IV-A, Type IV-B, and Type IV-C), tall mass timber buildings have fire protection requirements more robust than those required for comparable noncombustible buildings.
