Post-fire Assessment of Interface Landscapes

Wildland fires are a natural component of ecosystems across much of North America. When residential and/or developed areas abut or intermingle with wildlands, these historically common wildfires can threaten lives and property. The 1970's marked a shift in population trends in the United States, with rural areas demonstrating a higher percentage increase in population than urban areas. The migration of people to rural, natural settings coincides with a steady expansion of urban/suburban areas into wildlands. Not only does this migration expose people to naturally occurring wildfires, it also influences the frequency with which wildfires occur. Today, the majority of interface wildfires are caused by people. Fire professionals refer to these and other related issues as the wildland-urban interface (WUI) fire problem. A critical area of research for addressing this problem focuses on the factors that influence the vulnerability of individual homes and/or entire communities to wildfire. Characteristics of the structures, such as building materials and maintenance, and landscape patterns around structures influence structural vulnerability.

Research Methods
The goal of this study was to develop a better understanding of how building materials and the arrangement and composition of landscape plants influence structural vulnerability during wildfires. We visited 9 communities where wildfires threatened 39 homes and caused damage to some of the homes. We surveyed the homes, landscaping, and surrounding vegetation communities to collect information on several factors: (1) the building materials of each house, (2) indicators of fire behavior and intensity, (3) types of vegetation in the landscape and of natural vegetation surrounding the property, and (4) proximity of the house to natural vegetation and of the house to the fire. We analyzed this information to understand the main factors contributing to home damage during wildfires and to understand more about fire spread in WUI communities.

In this study, the exterior siding was the most important building material factor for determining if a home was damaged. Vinyl siding was the most vulnerable exterior material, with stucco, brick, and block being more resistant to fire damage. Soffit material can also be important; vinyl soffits are vulnerable to fire and provide an access point for fire to enter the attic and roof when damaged. The destruction of three homes in our study was likely due to fire entering the attic through fire-damaged soffits.

Distance to burning fuels, either fuel in the surrounding natural vegetation or in the landscape, was the most important landscape factor for determining whether a home was damaged. Most homes within 16 feet of burning fuels sustained damage. These findings corroborate the firewise guidelines of maintaining a relatively open zone of defensible space between the home and highly flammable natural vegetation. It is not only necessary to maintain an area of separation between the home and natural wildland fuels, but it is also crucial to follow firewise guidelines of using low flammability plants and maintaining vertical and horizontal separation of fuels in the landscape near the home to reduce the likelihood of landscape plant ignition within the defensible space.

Fire intensity was another influential factor determining whether homes were damaged by wildfire. Most of the homes damaged in our study were directly in front of the spreading fire. Firebrands, or floating embers that can ignite spot fires ahead of the main fire, were produced by most fires that we studied and caused the destruction of two of the homes where intense fires burned through dense vegetation surrounding the communities. In these cases, firebrands ignited mulch or plant material adjacent to the home, which then carried the fire to the structure itself.

The ignition, consumption, and scorch of landscape plants were most influenced by the distance to burning fuels with fire intensity and type of plant - deciduous or evergreen - playing important roles as well. Plants less than 0.5 feet from burning fuels tended to ignite and have a high percent consumption. Plants with at least 0.5 feet clearance from burning fuels had 88 percent survival. This distance is much lower than the recommended separation of six to ten feet suggested by firewise guidelines. Plants with less than one foot clearance to burning fuels did have a high percentage of scorch, but did not ignite or continue the fire's spread. However, with higher wind speeds, the separation distance would need to be greater to prevent fire spread in the landscape. Evergreen plants tended to survive with less damage than deciduous plants; however this is likely due to the fact that most evergreen plants in the study sites were pine trees which have high crowns providing separation and protection from the burning surface fuels.

Recommendations developed as a result of this study for managing the home landscape and choosing building materials include:

  • Only use non-flammable mulch within 3-5 feet of the house;
  • Maintain a landscaped area designed with firewise principles extending about 50 feet out from the house;
  • In the area within 50-130 feet of the house, the key firewise principles should still be followed. This includes the creation and maintenance of vertical and horizontal separation. However, in this outer zone there is some flexibility with species selection and other factors, as long as the horizontal separation is maintained. Homeowners wishing to achieve other landscaping goals, such as creating wildlife habitat, conserving water or energy, or conserving the diversity of native plants, can use this outer zone to incorporate principles of those landscaping programs;
  • Do not use vinyl siding or soffits on homes in fire-prone areas.

The findings from this southern post-fire assessment illustrate that it is important to design and manage the home landscape to reduce fire risk and also to consider risk of damage when deciding on building materials. In addition, our findings suggest that current firewise recommendations may not be sufficient for protecting homes against damage during intense wildfires. In particular, the area of defensible space around structures in the WUI may need to be larger than currently recommended in some types of vegetation communities, since we documented damage to one home in excess of 94 feet away from the flaming front of an intense wildfire. Examining current recommendations for reducing wildfire risk to homeowners based on results from this study will help fire professionals identify potential improvements and supply policymakers with the empirical data required to develop and implement public policy focused on fire prevention.

Links to Additional Information about Fire in the Wildland-Urban Interface: