Advanced wood building systems form a significant market opportunity for use of wood in taller and larger buildings, which are currently required to be of non-combustible construction in accordance with the provisions set forth in Part 3 of Division B of the National Building Code of Canada (NBCC).
The spread of flames over solid materials is a fundamental behavior influencing the fire dynamics and growth within a compartment. As such, NBCC set prescriptive limits on the use of wood products in such building types due mainly to potential fire hazard. Typically, NBCC limits surface flammability characteristics (i.e. flame spread) of combustible interior finishes, while limiting their thickness up to 25 mm when used in buildings required to be of non-combustible construction. Flame spread ratings are traditionally assessed by exposing a 19 mm specimen to the ULC S102 test method, also called the “Steiner Tunnel” test. These thin elements, usually interior finish materials, tend to heat up fast and ignite rapidly. However, with the advent of cross-laminated timber (CLT) and structural composite lumber (SCL), massive structural timber elements can be used and provide an enhanced fire performance while potentially remaining exposed. These massive elements act as a semi-infinite slab (thermally thick) and exhibit a significant different fire behavior when compared to thinner elements.
In an objective to evaluate the surface burning characteristics of massive timber assemblies such as CLT and SCL, flame spread tests on massive timber assemblies have been conducted in accordance with ULC S102 test method. This study evaluated the flame spread of fully exposed massive timber specimens (i.e. untreated/uncoated) as well as the effect on flame spread by using intumescent coating with CLT. Test results provide low flame spread ratings when compared to those of common combustible interior finish materials provided in Appendix D-3 of NBCC. Specifically, the obtained flame spread ratings of 3-ply CLT assemblies of 105 mm in thickness are 35 and 25 for a fully exposed CLT (untreated) and for a CLT panel protected by intumescent coating respectively. Fully exposed SCL of 89 mm in thickness provided ratings of 35 and 75 for parallel strand lumber (PSL) and laminated strand lumber (LSL) respectively.
Lastly, the use of materials that exhibit lower flame spread ratings than typical combustible interior finish materials would result in a reduced “risk” of ignition and potentially time to flashover conditions depending on the configuration of the room of fire origin. By doing so, such reduced risk would allow achieving the NBCC objectives and functional statements [F02 – OS1.2, OP1.2] when developing an alternative solution.