By Jack Keays, MSc., P.Eng.
As architects, engineers, and builders push toward using sustainable, ‘green,’ and cost-effective building materials, National Building Code of Canada’s (NBC’s) tight framework with regard to combustible construction is coming into the spotlight.
Combustible and non-combustible construction, and combustible elements in buildings required to be of non-combustible construction, are terms with which most industry professionals are all too familiar. Materials permitted for use in construction depend on the occupancy, height, and area of the proposed structure.
Historically, constructing buildings of non-combustible materials was one of the only methods available to prevent large-scale conflagration. Densely populated areas, such as towns and cities, moved away from wood construction and built structures from brick, stone, and steel—with the intent of limiting fire spread between buildings and reducing the risk of major losses..
Building regulations have existed for centuries in various formats; changes to such regulations have been driven by issues prevalent in the industry at the time. Examples of this include the move toward non-combustible materials following major fires in towns and cities, such as the Great Fire of London in 1666 and, more recently, amendments to the Ontario Building Code (OBC) to address the issue of falling balcony glass in Toronto. (For more on laminated glass, see “Expanding Laminated Glass Performance” by Valerie L. Block, CDT, LEED AP, in the September 2012 issue of Construction Canada. )
The Canadian code
NBC is the model building code adopted or adapted, in part or fully, in provinces and territories across Canada. The first edition was published in 1941, and has been continuously updated with the intent of embracing new technologies, materials, and methodologies. While changes and revisions have been made, many of the early principles on which NBC requirements are based—such as area and height limitations and construction requirements—have remained largely unchanged.
The use of non-combustible or limited combustibility materials in construction has been an NBC requirement since its first edition. The definition of non-combustible construction was introduced in the 1960 version of the code; before this, non-combustible construction was not clearly defined.
The intent of non-combustible construction was to obtain a degree of safety from fire hazards by using only specific materials for structural members or assemblies, and by limiting the amount of combustible material incorporated into a building’s construction. The definition of non-combustible construction in the 1960 NBC is:
Non-combustible, as applied to a building construction material, means a material that falls in one of the following groups (a) through (c):
(a) Materials that are classed as non-combustible when tested in accordance with [Canadian Standard Association] CSA specification B54.1-1960, Determination of Non-combustibility of Building Materials.
(b) Materials having a structural base of non-combustible material, as defined in (a), with a surface not over 3.2 mm (1/8-in.) thick, which has a flame-spread rating not higher than 50.
(c) Materials, other than as described in (a) or (b), having a surface flame spread of not higher than 25 without evidence of continued progressive combustion and of such composition that surfaces that would be exposed by cutting through the material in any way would have a flame-spread rating higher than 25 without evidence of continued progressive combustion.
This definition allowed for certain materials with limited combustibility—such as treated wood and plywood—to be considered non-combustible. Classifying such materials in this manner was not the code’s intent, and the definition was amended in the 1965 NBC to return to the intended meaning.