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Pathways to Building Fire Spread in the Wildland-Urban Interface
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Issue 101: Pathways to Building Fire Spread in the Wildland-Urban Interface

By Michael J. Gollner, Ph.D.

While the Wildland-Urban Interface (WUI) is not a new concept, fires in WUI communities have rapidly expanded in frequency and severity over the past few decades. The number of structures lost per year has increased significantly, from around 900 per year in the 1990’s to almost 3000 per year in the 2000’s (NIFC, 2014). This trend is the result of many factors, including increased development in rural areas, fuel management policies, and climate change, all of which are projected to increase in the future (Krawchuk et al., 2009). As Fire Protection Engineers, we are now increasingly faced with protection challenges for these communities, requiring us to draw on a diverse set of resources to preserve property and life safety.


With the advent of more extreme fires becoming the norm, a different thought process must be taken in comparison to traditional structural firefighting techniques and risk assessments. In structural firefighting, the assumption for most occupancies is that the structural design of the building, passive fire protection systems and active fire protection systems will provide sufficient protection for the occupants to escape and for the fire department to safely enter the building to provide full extinguishment. In large WUI fires, many buildings burn down tens of hours after the main fire line passes through a community due to firebrand ignition. Firebrands and other smoldering debris slowly transition to flaming from innocuous sources that are difficult to identify, while the main fire front threatens new homes and communities miles away. These firebrands can also be transported several kilometers ahead of the front depending on atmospheric conditions; therefore, a large area is affected over which no firefighting crew has sufficient resources to cover (Koo et al., 2010). A different theory or approach to firefighting and structure protection must be envisioned to prevent future large scale losses. Current strategies for exterior fire protection in the WUI (e.g. homeowner checklists, mesh coverings for vents, etc.) pale in comparison to those developed for use within buildings (e.g. fire sprinklers, smoke detectors, fire retardant materials, etc.). One concept is to limit the pathways by which firebrands or other fire sources can penetrate a property or community and destroy a structure, a problem our recent report (Gollner et al., 2015) tried to shed some light on.


The NFPA, through the Fire Protection Research Foundation, sponsored a project to perform a thorough literature review and gap analysis of Pathways to Building Fire Spread in the Wildland-Urban Interface. Faculty and students from the Department of Fire Protection Engineering at the University of Maryland, College Park compiled over 200 references to review and assess gaps in the field as part of this project.


We found responsibility for the protection of buildings in the WUI falls between both wildland and urban fire authorities, with mixed guidance available for homeowners, code officials, etc. The NFPA has begun to address this problem by instituting several standards, including NFPA 1141, 1142, 1143 and 1144, which aim to reduce structural ignitions and provide adequate firefighting infrastructure in WUI communities. Other resources available include the International Wildland-Urban Interface Code by the ICC, California Building Code Chapter 7A, the Wildfire Home and Property Checklist by the Insurance Institute for Business & Home Safety and community-wide programs such as Firewise Communities, Fire Adapted Communities and Ready, Set, Go! A necessity for improvement of these resources is technical knowledge which can be used to understand pathways for fire spread and their statistical and/or quantitative contribution to fire risk. While the general pathways for fire spread in the WUI (flame, radiative and ember exposure) are known, the exposure conditions generated by surrounding wildland fuels, nearby structures or other system-wide factors and the subsequent response of WUI structures and communities are not well known or well understood. Several key pathways into structures, such as eaves, vents, windows, roofs and decking have received attention and limited study, but no effort has been made to compile all available data quantitatively for use in an applied, risk-informed framework.

After reviewing the available literature, many areas related to pathways for fire spread in the WUI were found to still be in need of additional research. As part of a gap analysis, these areas were broken down into those related to quantification of risk and hazard and more practical and specific issues. While it is useful to identify vulnerabilities and best practices, protection of WUI communities cannot evolve without more quantitative analyses to optimize protection schemes, standards and risk and hazard analyses. Areas necessary to inform quantification of risk and hazard included pre- and post-fire data collection, improved testing of firebrands, understanding of ember and wildland fire fundamentals, and improved understanding of structural ignition mechanisms. Continued testing and pre- and post-event data collection may help to provide the input needed to perform risk-based modeling and support future performance-based design practices for the WUI environment.

There are also many other practical issues, which relate to specific areas of code and standard development and WUI community protection or firefighting that are in need of rapid research and development. These include guidance on the appropriate separation distance between homes (only one large-scale test has been conducted to date by Maranghides and Johnsson, 2008), the effectiveness of home fire sprinklers (interior or exterior) in preventing home-to-home spread, standardized criteria for decking, fences and other outdoor materials including exposure to direct flame contact and firebrands as part of testing, the effectiveness of external coatings (foam, gel, etc.) including how and when they should be applied, and the effectiveness of different community planning techniques (fuel management, defensible space, etc.). A guidebook for Fire Protection Engineers working in the field would also be useful, as current codes and standards do not provide a "best practices” guide for designers that my benefit both new and refurbished developments.

There are many means for improvement beyond direct structure protection. State laws addressing defensible space, ingress, egress, and water supply can create a safer environment for firefighters, resulting in more structures being saved (Gude et al., 2008). Many of these issues are already covered in NFPA 1141 and 1144; however, they could be improved with further knowledge including case studies and research. Data needed for quantitative risk analysis, such as wildfire exposure conditions or the reaction of components to these conditions, is severely lacking (Maranghides and Mell, 2013). Policies that address existing and future development in the WUI should be coupled with national, state, and local policies that address wildland fuel management (Gude et al., 2008).

A higher occurrence rate of extreme fires also means that it will become important to assess incident fire severity based upon the most extreme weather conditions where high wind speed, low moisture content, etc. create challenging fire scenarios. This means that relying on historical fire and weather data will only be useful if some sense of the ecological fire regimes and drought patterns are taken into account.

These categories represent a wide spectrum of subjects within possible WUI research. As protection of property in the WUI has now become an increasing firefighting priority, we cannot forget that firefighters are constantly endangered while striving to protect structures. An average of four wildland firefighters have died annually at wildland fires or prescribed burns in the years 2002-2012. In the most recent incident, the Yarnell Hill Fire killed nineteen members of a Hotshot wildland firefighting crew and huge media attention was focused toward the problem of safe WUI firefighting (Fahy et al., 2014). This event was the largest single loss of life for firefighters since the September 11, 2001 terrorist attacks on the World Trade Center in New York. Thought and planning for firefighter safety, including access to safety zones, adequate egress, etc. cannot be forgotten when performing community planning in the future (Butler, 2014).

Michael J. Gollner is with the University of Maryland’s Department of Fire Protection Engineering.


  • NIFC, 2015 National Interagency Fire Center.
  • Krawchuk, M.A., Moritz, M., Parisien, M.-A., Van Dorn, J., Hayhoe, K., 2009. Global pyrogeography: the current and future distribution of wildfire. PLoS One 4, e5102.
  • Koo, E., Pagni, P.J., Weise, D.R., Woycheese, J.P., 2010. Firebrands and spotting ignition in large-scale fires. Int. J. Wildl. Fire 19, 818.
  • Gollner, M.J., Hakes, R., Caton, S., Kohler, K., 2015, Pathways for Building Fire Spread at the Wildland Urban Interface, Fire Protection Research Foundation, National Fire Protection Association.
  • Maranghides, A., Johnsson, E., 2008. Residential Structure Separation Fire Experiments. NIST Technical Note 1600. Gaithersburg, Maryland, USA.
  • Gude, P., Rasker, R., Noort, J. Van Den, 2008. Potential for Future Development on Fire-Prone Lands, Journal of Forestry. 106(4) 198-205.
  • Maranghides, A., Mell, W., 2013. Framework for Addressing the National Wildland Urban Interface Fire Problem – Determining Fire and Ember Exposure Zones using a WUI Hazard Scale. NIST, Gaithersburg, MD.
  • Fahy, R.F., LeBlanc, P.R., Molis, J.L., 2014. Firefighter Fatalities in the United States -2013. NFPA No. FFD10.
  • Butler, B.W., 2014. Wildland firefighter safety zones: a review of past science and summary of future needs. Int. J. Wildl. Fire 23, 295–308.

Related Articles:

3rd Quarter 2013 – Framework for Addressing the National Wildland Urban Interface Fire Problem Alexander Maranghides and William Mell, National Institute of Standards and Technology

This article is an excerpt from NIST Technical Note 1748. The authors argue that a fundamental reason why Wildland Urban Interface (WUI) fires have been steadily escalating (as has the fire suppression costs associated with them) is because of the very limited consideration of potential wildland fire and ember exposures in building codes and standards. They argue that the limited information currently available does not address the full range of realistic WUI exposures and offers little context for the design of ignition-resistant landscapes and buildings. The result is a proposed WUI exposure scale that can be used to provide the boundaries where specific land use and/or building construction regulations would apply. READ MORE

3rd Quarter 2010 – Wildfire: Past, Present and Future – Ronny J. Coleman and Kate Dargan, retired California State Fire Marshals

The problem of wildfires is increasingly complex and traditional solutions are losing effectiveness. Coleman and Dargan describe how experts are redefining the way they approach wildfires and highlight five approaches for combating wildfires currently being developed or implemented in California: firesafe land use planning, building construction and design, vegetative fuels management, community education, and first responder training. READ MORE

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