By Michael Schmeida, MSc, LEED AP, FASTM, GA Director of Codes, Standards and Research
The most important purpose of building codes is to ensure life safety, and secondarily, the preservation of property and minimization of loss thereof.
Fire is a significant threat to both life and property as evidenced by the recent January 2025 wildfires in the Los Angeles area. As such, building codes and subsequent construction practices focused on mitigating the effects of fire are of importance. This is why sections of the International Building Code (IBC) and International Residential Code (IRC) are dedicated to fire-related criteria and whole codes are dedicated to fire-related design such as the International Fire Code (IFC) and more recently the International Wildland-Urban Interface Code (IWUIC).
For the purposes of this article, we will focus specifically on IWUIC and the definitions (found in Section 202) included to address fire-related mitigation, and often, the confusion that exists among the following three concepts:
- Fire-Resistance
- Ignition-Resistance
- Noncombustibility
The IWUIC defines them as:
- Fire-Resistance-Rated Construction: 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.
- Ignition-Resistant Building Material: 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.
- Noncombustible: As applied to building construction material means a material that, in the form in which it is used, is either one of the following:
- Material of which no part will ignite and burn when subjected to fire. Any material conforming to ASTM E136 shall be considered noncombustible within the meaning of this section;
- Material having a structural base of noncombustible material as defined in Item 1 above, with a surfacing material not over 1/8 inch (3.2 mm) thick, which has a flame spread index of 50 or less. Flame spread index as used herein refers to a flame spread index obtained according to tests conducted as specified in ASTM E84 or UL 723.
“Noncombustible” does not apply to surface finish materials. Material required to be noncombustible for reduced clearances to flues, heating appliances or other sources of high temperature shall refer to material conforming to Item 1. No material shall be classified as a noncombustible that is subject to increase in combustibility or flame spread index, beyond the limits herein established, through the effects of age, moisture or other atmospheric conditions.
The nuanced difference among these three terms is that fire-resistance-rated construction focuses on whole assembles, while ignition-resistance and noncombustibility focus on different material properties within those assemblies.
- 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.
- 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.
Section 503 of the IWUIC lists the appropriate ASTM tests for determining ignition-resistance and the performance criteria of materials when tested. 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.
- Noncombustibility is in the context of the IWUIC primarily a material property. The easiest and most colloquial way to say something is noncombustible is that it does not contribute significantly to fire propagation; or in other words, it doesn’t burn. In the WUI application materials such as concrete, CMU, steel and gypsum panels would be considered noncombustible. Having said this, in the other codes, noncombustible construction is defined as that which is made of noncombustible materials. For example, Type I construction in the IBC.
While the ability to achieve fireproof structures remains elusive, combining fire-resistance-rated whole assembly construction with ignition-resistant and noncombustible materials increases the possibility that a structure will survive a wildfire threat, reducing the loss of property and potentially life in the wildland-urban interface (WUI) zone.