Understand how a wildfire can impact a building based on its unique characteristics.
Because the damages of a wildfire are greatly affected by a property’s specific features, Risk Factor allows you to adjust a building’s features to see how the building’s unique characteristics, such as building material, roof type, the slope of a property, and defensible space directly affect its risk. You have the ability to view the estimated damages a property may face from wildfire, including the estimated days to repair or rebuild.
Likelihood of wildfire reaching a property
The location and intensity of historic fires are used to determine the most likely potential ignition locations for future wildfires. The First Street Foundation Wildfire Model considers a wide variety of weather patterns that impact fuels by making them hotter and drier, as well as the weather conditions that help spread fires further such as high winds and low humidities. The model simulates the ignition of wildfires based on historic fire locations. It then observes how wildfires spread given the fuel and fire weather conditions, and tracks fires that grow to a sufficient size, noting the distance, location, and duration of these fires.
The model simulates over 100 million wildfires to see which grow and become damaging. It outputs over 8 million significant fires that are used to calculate the annual likelihood of a home being in a wildfire based on how many times the 8 million simulated fires reached the building’s location.
It’s important to note that wildfire reaching the building doesn't necessarily equate to the home being completely destroyed.
Likelihood of wildfire destroying a property
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 amount of damage a home may sustain in a wildfire is based on its unique building characteristics. 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 the 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.
These characteristics are then compared to a property’s probability of burning, the length of flames, and exposure to embers to determine its vulnerability.
Wildfire damage over time
The likelihood of a wildfire does not remain fixed from year to year, instead that risk carries over and accumulates over time. To track this, the Wildfire Model calculates a cumulative wildfire likelihood, which shows the likelihood of a building being in a wildfire at least once over 30 years.
Likelihood of complete destruction
Taking into account a home’s unique building materials and property characteristics the model determines the likelihood a wildfire will completely destroy the home over a period of time. Use the tabs in the damages over time section to understand the property’s risk of complete destruction over different time periods.
Estimated cost to rebuild
Damage estimates are based on the most significant characteristics related to a building surviving a wildfire such as the size of the structure, slope of the property, materials, and building value. These characteristics are then compared to a property’s probability of burning, the length of flames, and exposure to embers to determine its vulnerability.
“Fragility curves” developed by ARUP Corporation were applied to our 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.
Wildfire damages by flame length
Flame length is a measure of fire intensity. Flame length is the distance between the tip of the flame and the center of the flame at the base of the fire on the ground. It is measured on a slant when the flames are tilted due to the effects of wind and slope.
Wind pushing a fire tilts the flames ahead toward unburned fuel or vegetation such as leaves, branches of trees, or bushes. As the wind bends flames toward unburned fuels the heat from the flames preheats the fuels, raising its ignition temperature more quickly than with no-wind conditions. This results in more rapid fire spread and as rate of spread increases, so does flame length and fire intensity.
The model measures the flame length of all simulated wildfires that reached a property.
- Average flame length: How tall, on average, the flames that reached a home in the modeled simulations were.
- Maximum flame length: The largest flame size that reached a home in the wildfire simulations.
The amount of damage a home may sustain in a wildfire is based on its unique building characteristics. Building characteristics have different likelihoods of ignition at different flame lengths. For example, a property whose roof is made of metal may be less likely to ignite if it is in a fire with shorter flame lengths. Metal roofing is non-combustible and offers Class A protection.
While the physical land or parcel a home sits on may have a high likelihood of burning, the physical structure of the home may be less likely to sustain serious damage if it has a metal, non-combustible roof, which increases the structure’s ability to prevent ember or flame penetration to the home.
Days to repair
The Wildfire Model determines the damage from wildfire exposure at different flame lengths by comparing building attributes to historical burn data and building material properties analyzed by the Arup Corporation.
Estimated days for damage repair reflect the total number of days needed to repair the amount of wildfire damage caused based on its flame length. Repair time estimates evaluate the time needed to repair damaged building materials, as well as factors that delay the start of building repairs. Factors that impede repair start include time for floodwater recession, building restoration, contractor and engineer mobilization, and equipment long lead times. Once the impending delays are resolved, repair time estimates are calculated by averaging repair times for each damaged building component.
Breakdown of wildfire intensity
The intensity of a fire can vary from low to medium to high and different areas within one larger wildfire can burn with different intensities. Low-intensity fires occur when conditions are cool, moist, and wind speeds are low, or when there is not much vegetation to provide fuel for a wildfire. High-intensity fires occur when the fire weather is extreme and conditions are hot, dry, and windy, there is a large amount of burnable vegetation, and the topography is steep.
As the environment continues to change, temperatures will continue to rise creating a drier atmosphere, creating conditions that are prime for wildfires to spread. Rising average temperatures increase the rate of evaporation in dense wilderness areas, causing soil and vegetation to dry more quickly and become flammable. A warming planet and increased evaporation also mean changing weather patterns. Changing precipitation patterns are exacerbating dry seasons in areas prone to wildfires, causing burns to become more frequent and severe. Another consequence of changing precipitation patterns is lower air humidity. Lower humidity in the air causes plants to release their moisture to balance the environment, resulting in dry vegetation that is more susceptible to wildfire.