Understand how potential property damage from wildfires is determined.
A property’s vulnerability refers to the amount of structural damage to a home in the event of a wildfire and considers the most significant characteristics related to a building surviving a wildfire. These estimates are based on a property’s probability of burning, the height of flames, and exposure to embers.
What building characteristics are included?
Significant building characteristics related to building survival in a wildfire were determined through historic data on buildings lost or damaged in historic fires. From this analysis significant building characteristics were identified:
- Slope of property: The steepness of a property. It refers to the number of feet the land rises, or falls, across a property. Slope is an important indicator of the ability of fire to reach a building.
- Exterior Wall Type: The material the external walls of a home are made of. Certain materials are more likely to ignite, while others may be ignition resistant or non-combustible.
- Defensible space: The cleared area between a home and surrounding vegetation such as grass, trees, shrubs or any wildland area that surrounds it. This clear area protects the home from catching fire by distancing it from vegetation and fuel sources.
- Roof material: The material the roof of a building is made out of such as concrete, slate, gravel and rock, tile, and wood. A key factor for structure survival indicates a structure's ability to prevent ember or flame penetration.
How is vulnerability calculated?
How wildfire will impact a home depends on the vulnerability of that home to the fire’s intensity and embers. The slope of the property, and distance to potential wildfire locations determine a property's risk from fire embers. The building materials such as the type of roof and siding, as well as the distance to fuel sources impact the building's likelihood of igniting.
The amount of damage a home may sustain in a wildfire is based on its unique building characteristics. These characteristics are then compared to a property’s probability of burning, the height of flames, and exposure to embers to determine its vulnerability.
“Fragility curves” developed by ARUP Corporation were applied to the First Street Foundation Wildfire Model to determine the amount of damage a home may sustain based on a property’s wildfire risk, ignition likelihood, fire intensity, and exposure to embers; and building characteristics such as the size of the structure, slope of the property, materials, and building value.