Building Better Boards: Increasing the Fire Resistance of Cross Laminated Timber

It’s no surprise that one of the oldest construction materials is also one of the most sustainable. For centuries, wood has provided shelter to humans, and as we strive for increasing levels of environmentally minded development in the future, there is little doubt that it will continue to play an integral role. Although a natural and renewable resource, building with wood still brings with it problems that are as old as the technology itself, one of which is the material’s susceptibility to an age-old nemesis—fire.

This CLT specimen survived almost 100 minutes of exposure in a standardized test reaching nearly 1000°C. The unexposed side of the specimen remained at less than 50°C for the entire test.

This CLT specimen survived almost 100 minutes of exposure in a standardized test reaching nearly 1000°C. The unexposed side of the specimen remained at less than 50°C for the entire test.

At the Forest Products Laboratory (FPL), researchers are hard at work developing ways to assess and improve the fire resistance of Cross Laminated Timber (CLT), a building material that has great potential in low to mid-rise construction. According to the USDA, CLT is a newcomer to North America, but has already gained a reputation in Europe as, “a renewable building material that is strong and versatile.”

CLT concept.

CLT concept.

A CLT panel consists of multiple layers of kiln-dried lumber boards stacked in alternating directions, and bonded together with structural adhesives. The end result is an inexpensive, strong, solid, rectangular panel that can be used for building walls, floors or roofs.

The Fire Research Team at FPL aims to help improve CLTs marketability by bringing cutting-edge science to the laminated timber manufacturing process. Through a partnership with Virginia Tech, Clemson University, and North Carolina State University, researchers have conducted numerous studies that have yielded clues about how CLT panels can be better made to increase their fire-resistance rating.

“Layer delamination caused by adhesive failure at high temperatures and premature smoke penetration are two of the major concerns being addressed,” said Laura Hasburgh, a fire protection engineer at FPL. According to Hasburgh, one of the main goals of the research is to determine char rate models, “so that the unburnt, full-strength thickness of a CLT can be predicted, which can, in turn, be used to predict the remaining strength of a CLT over the course of a fire.”

By measuring the remaining strength of the panel at different times during the test, the team can predict what fire resistance rating can be achieved by the CLT panel. A total of 25 CLT specimens have been tested to date following industry standards.

Researchers theorize that by changing the grade of wood used, the layout of the individual component boards, or the types of adhesives, manufacturers could optimize CLT products to be used for a wider range of applications. In a future that will simultaneously demand increased reliance on more sustainable building materials, larger structures to house a growing world population, and readily available construction materials to support victims of natural disasters, CLT panels are a good tool for developers to have at their disposal.

Fire Safety for New Wood Products FPL's History in Fire Research

A compelling issue at FPL is how we ensure that traditional and new innovative wood products do not adversely contribute to loss of life and property in fires.The Durability and Wood Protection Research at FPL is prepared to answer these questions. A part of the group’s mission is improving durability and wood protection through improved building design, advances in low-toxicity wood preservatives, and improvements in fire safety.

With unique fire research facilities, fire safety research at FPL addresses the potential contribution of wood products to the growth of a fire, the ability of structural wood elements to withstand a fire, and the chemical treatment of wood products to reduce their flammability. Fire safety is a major component of existing building codes and will be continue to be so in future developments of editions of the building codes and other regulatory documents.


This burning test structure from 1975 is at the point of flashover—the sudden spread of flames over an area when it becomes heated to the flashpoint. Results of these tests indicated that sandwich panels provided structural integrity for various lengths of time depending on the facing material used for the panels.

FPL has worked on fire safety for decades and has helped quantify the fire performance of wood products. This research has contributed to the development of treatments of wood that reduced their flammability. Research helped define the fundamentals of fire behavior and efforts to develop methodologies for fire testing of wood and composite materials to ensure proper measurements of relevant performance characteristics. More recently, contributions have been toward data and models required for fire safety engineering of forest products in a performance-based building code environment.