Fire Resistance of Unprotected CLT Floors & Walls Manufactured in the U.S.

Project Lead: Lech Muszynski

Despite a growing body of empirical evidence generated by European, Japanese and Canadian research on the fire endurance of cross-laminated timber (CLT), a lack of full-scale U.S. testing of structural CLT manufactured within the U.S. is often cited as a major barrier to approval of the new building material for use in tall structures.  Past testing in test furnaces suggests that CLT and glue-laminated timber can outperform both light-frame timber assemblies and steel and concrete elements. This is due, in part, to the fact that in thick CLT panels a layer of charred wood forms on the exposed surface that then serves to insulate and protect the wood behind the char layer. This project will document the flammability of Douglas-fir and spruce-pine-fir CLT panel assemblies produced in the United States. Tests are being conducted on wall and floor panel assemblies with standard overlapping connections and produced with two different types of commonly-used adhesives. Sensors placed throughout panels will provide data about how fire affects the interior and exterior of a panel. A thermal imaging camera will provide information on how the structural integrity of panels is affected by fire and fire suppression activities. When this research project was proposed in 2015, no full-scale fire performance testing of U.S.-made, structural CLT had been performed. 


Project duration: 3/1/16 – 2/28/19


The objective of the proposed research project is to investigate the fire performance of 5-layer Douglas fir CLT wall and floor assemblies made in Pacific Northwest Region. The specific objectives are:

  • Determine fire performance for unprotected and fire protected wall and floor CLT assemblies according to ASTM E119 standard.

  • Measure the out of plane deformations of wall and floor elements related to the fire and wetting events and evaluate their effect on the building stability.

  • Assess the impact of fire on the local integrity of the CLT layup.

Primary Findings:
  • All three unprotected CLT floor assemblies and two of the three wall assemblies produced by American manufacturers passed the 2-hour fire resistance test following ASTM E119 standard procedure. One assembly passed 100 min mark before the diaphragm was breached.

  • Despite the charred layers fall off observed in assemblies bonded with PUR adhesive system the differences in mean char rates for the first two layers in floor assemblies using PUR and MF systems were smaller than the variability of char rates within individual assemblies (as reflected by the standard deviation).

  • The effect of softening of the PUR bonds is most apparent as an accelerated rate of the floor assembly deflection in the final 40 minutes of the tests.

  • Within the 2-hour standard test exposure the unprotected half-lap joints provided adequate barrier against transmission of hot gases and flames through the assemblies.


Testing returned results consistent with fire testing performed in Canada and
Europe, indicating that U.S.-manufactured CLT panels perform equally well in fire
tests and meet code requirements for use in any type of construction, based on this
fire testing.


Panels made of 100 percent Douglas-fir, as well as panels made of a combination
of spruce, pine and fir, had similar char rates and passed the fire testing
performance requirements for structural integrity.
The mean surface temperature on the unexposed faces remained at room
temperature (below 24 degrees Celsius), well below the standard qualification
criteria of 139 degrees Celsius.
Panel assemblies tested were made using both melamine-formaldehyde (MF) and
polyurethane (PUR) adhesives. The char layer material on panels made with PUR
adhesive delaminated earlier, but panels using both adhesives passed the fire tests
with complete structural integrity.
The use of more than 50 thermocouples (five times the number required for
standard fire testing) in each assembly generated detailed data used to calculate
char rates and other information.
All but one of the six CLT panels passed the two-hour fire endurance test
following ASTM E119 standard procedure. One wall assembly failed at one hour
and 40 minutes, when the char front passed through the half lap joint, opening the
path for hot furnace gases. The other five assemblies passed the two-hour tests
with no major breach of their integrity (no gas or smoke passing through to the
unexposed surface).


  • Pickett B. (2016): Fire Resistance Testing of CLT Floor/Ceiling Assemblies, Western Fire Center Inc., Western Fire Center Report #16046a, 2016, pp. 25 pp.

  • Pickett B (2017): Fire Performance of ASTM E119 Evaluation of CLT Load-Bearing Wall Assemblies, Western Fire Center Inc., Western Fire Center Report #16046b, 2017, pp. 29 pp.

  • Holloway C. (2018): Paper beats rock: does wood beat steel? WSE401 project report. OSU. 46 pp (a undergraduate research project report describing the methodology of measuring char depth across the wall and floor specimens exposed to fire tests, and processing the data for the char maps. Supervised by L. Muszynski)


2019 Mass Timber Conference Poster: Fire Resistance of Unprotected Cross-laminated Timber Wall Assemblies Made in the USA (Seung Hyun Hong et al., M.S. Wood Science 2019)