Over the years, there have been many reports of fires that have burned inside and outside of log buildings without destroying the building’s structural integrity, illustrating the fire resistive nature of solid wood walls. It is a combination of the insulating response of the charred wood at the surface with the slow rate at which flame will spread along the wood surface, and the fact that there are no concealed cavities in a log wall through which the fire may travel (ultimate fireblocking!). Combined with the selection of beam and deck second floor and roof options often incorporated into log buildings, log structures are a top choice for endurance and integrity in a fire.
This paper has attempted to provide a greater understanding of wood, the implications of your building site and building design, and their impact on your decisions toward developing an understanding of the fire resistive nature of solid wood walls. The unique aspect of most log and timber homes is that the logs themselves are what make up the structural soundness of the building. Because of this, most solid wood walls can be expected to satisfy one-hour fire-resistant ratings while greater wall thickness can result in even longer ratings. The Log and Timber Homes Council contends that the performance of solid wood walls, while composed of combustible materials, is sufficient to perform satisfactorily as the bearing wall portion of the Heavy Timber type of construction recognized by building codes.
For more information, refer to these publications:
- Wood Handbook: Wood as an Engineering Material (FPL–GTR–113), Forest Products Laboratory, Madison, WI
- Structural Fire Design: Wood (FPL 450), E.L. Schaffer, Forest Products Laboratory, Oct. 1984
- Charring Rate of Wood for ASTM E119 Exposure, Robert H. White and Erik V. Nordheim, Fire Technology Vol. 28, No. 1, Feb. 1992
- 1997 Urban-Wildland Interface Code, International Fire Code Institute, 5360 Workman Mill Road, Whittier, CA 90601-2298
- The Supplement to the National Building Code of Canada, National Research Council of Canada, Ottawa, NRCC#17724
- Fire Resistance of Log Walls, Dalibor Houdek, Ph.D., LOG BUILDING NEWS, International Log Builders Association, Number 35, September, 2001 (http://logassociation.org/resources/fire_resistance.pdf)
- AF&PA American Wood Council, 1111 Nineteenth St., NW, Suite 800, Washington, D.C. 20036, www.afandpa.org and www.awc.org. Publications include ANSI/AF&PA NDS-2005 National Design Specification® for Wood Construction, Design for Code Acceptance (DCA) #1 Flame Spread Performance of Wood Products, DCA #2 Design of Fire-Resistive Exposed Wood Members, DCA #3 Fire Rated Wood Floor and Wall Assemblies, and Technical Report #10 Calculating the Fire Resistance of Exposed Wood Members.
Summary of Fire Tests & Supporting Opinions
- In a 1981 letter from E.L. Schaffer, Project Leader in Fire Design Engineering at Forest Product Laboratory, the Log and Timber Homes Council received Schaffer’s opinion that a 4” tongue & groove joint wall will last one hour. He based this on char rate data (table 2 of Res. Note FPL-0145). He also noted a test conducted by the National Bureau of Standards on load bearing and non-load bearing tests of 8-foot high walls made by stacking 2×4 lumber (3-5/8” thick) and 2×6 lumber (5-5/8” thick). In the load bearing walls, buckling occurred in 68 minutes for the 2×6 wall. Burn through occurred in 85 minutes for the 2×4 non-load bearing wall.
- In the 1980’s, a Log and Timber Homes Council Member Company, Authentic Log Homes, performed and documented their own test. A fire was ignited in a small “dog house size” log building. Fire temperatures reached 1,800oF after 90 minutes, but cotton waste material applied to the outside of the wall did not ignite, illustrating the thermal barrier capacity of the wall. At the 2-hour mark, the test was halted and the load bearing capacity was measured. At 2-1/2 hours into the test, less than 1/3 of the log thickness had burned, and the remaining wall presented sufficient surfaces to withstand the design load. It was noted that there was not a tendency of the wall to burn faster at seams or corners.
- In 1987, a log construction burn-through test was conducted at the University of Wyoming in accordance with ASTM E-119 Standard Method for Fire Testing of Building Construction. The construction, built to the specifications of Authentic Log Homes (6×8 nominal Lodgepole pine), withstood a 2-hour burn without failure. Four deviations were employed that were believed to create a more severe test situation:
1) A 4-sided log wall structure (9-ft x 3-ft x 12-ft long) provided the furnace and test specimen to simulate what would take place in an oven with consumable walls.
2) The test was run exposing the bearing log wall to ambient weather conditions with wind being the major concern (wind velocity was measured a 5-minute intervals and effects noted).
3) A 7,000-pound load was supported by the wall prior and throughout the test.
4) The hose stream test had to be altered because the structure was a closed cubicle. To compensate for this, the water pressure was increased from 30 psi to 175 psi.
- Lincoln Logs, Ltd. tested a 12-ft x 9-ft high assembly using nominal 6×8 red cedar logs with moisture content of 7%. Warnock Hersey International conducted the test (WHI 495-0925) in accordance with ASTM E119 test methods. With an equivalent of 2,000 pounds per lineal foot of load placed on the log wall, the wall passed a one-hour fire endurance test and hose stream test.
- In 1995, Honkarakenne Oy (Honka Homes USA, Inc.) complied with Finish requirements by performing fire resistance testing “on a loadbearing separating wall construction” according to standard DIN 4102 (Test Report NRO RTE11134/95:E by VTT Building Technology). This test is significantly similar to ASTM Standard E-119. The tested assembly was comprised of 14 courses of nominal 6” thick logs (137mm wide x 215mm tall). The test summary is as follows:“The tested wall construction with a width of 3000 mm and a height of 3010 mm subjected to a centric line load with an intensity of 6,1 kN/m and supported freely at the top and bottom edges met in one test during 90-minutes the requirements of loadbearing and separating wall constructions presented in the standard DIN 4102 Part 2 Edition 1977.”
- Sashco Sealants, Inc. performed E119 testing to establish the fire resistive rating of their backer rod and chinking system on a round log wall.