By Michael Schmeida, MSc, LEED AP, FASTM, GA Director of Codes, Standards and Research
Over the last ten years, wildfires have occurred in nearly all 50 states. While the media focus tends to be on the western United States, the data shows wildfires happen wherever human settlement touches undeveloped wildland or vegetative fuels. Because of this, the International Code Council has developed and maintains the International Wildland-Urban Interface Code (IWUIC).
The IWUIC prescribes in mandatory code enforceable language what construction techniques, practices and deliverables are required in an area that has adopted the IWUIC. Examples include requiring defensible space around a structure, using only noncombustible mulch, ensuring water availability for firefighting among many other stipulations.
The IWUIC also has requirements based on two very important definitions, “fire-resistance-rated” construction and “ignition-resistant” materials. Both concepts work against structure loss and fire propagation in a wildfire. However, the two ideas are often confused and misapplied.
Let’s look at the definitions of each term in the IWUIC first. Fire-resistance-rated construction is defined as follows: “The use of materials and systems in the design and construction of a building or structure to safeguard against the spread of fire within a building or structure and the spread of fire to or from buildings or structures to the wildland-urban interface area.” [Italics in the original]
The definition for an ignition-resistant building material is: “A type of building material that resists ignition or sustained flaming combustion sufficiently so as to reduce losses from wildland-urban interface conflagrations under worst-case weather and fuel conditions with wildfire exposure of burning embers and small flames.”
As a material property, ignition-resistance is applicable to many building materials. But in an IWUI situation, the focus is primarily exterior finishing materials such as roofing, cladding, and decking. The primary purpose of ignition-resistance requirements is to prevent the structure from igniting from embers. Embers can travel as much as one-half mile away from a blaze resulting in small fires. Items such as fire-retardant-treated wood decking, cement board siding and clay or metal roofing materials are often employed to meet IWUIC requirements for ignition resistance.
It should be noted that Section 503 of the IWUIC lists the appropriate ASTM tests for determining ignition-resistance and the performance criteria of materials when tested as such. Such tests include, most notably, ASTM E84 on flame spread, often referred to as the Steiner tunnel test. Importantly, these test methods evaluate materials, not assemblies or systems.
Fire-resistance-rated construction is also a significant contributor to IWUIC compliance. Fire-rated construction requires whole assemblies to be rated for the prevention of the passage of flame and hot gases. The importance of fire-rated exterior walls and roof-ceiling assemblies in WUIC settings cannot be overstated. Such assemblies act as a line of defense, preventing the structure from being consumed by fire. An effective means of achieving fire-resistance-rated construction are tested assemblies using gypsum sheathing under an appropriate cladding. Should the cladding ignite, even if ignition-resistant, the gypsum sheathing will provide a substantive barrier to flame, protecting the framing and rest of the structure.
While the ability to achieve fireproof structures remains elusive, combining fire-resistance-rated construction and ignition-resistant materials increases the possibility that a structure will survive a wildfire threat. Losses of property and potentially life can be reduced in the WUI zone.